Plasma spectrometry in the earth sciences: techniques, applications and future trends

Chemical Geology, 95 (1992) 1-33 1 Elsevier Science Publ ishers B.V., Ams te rdam

Plasma spectrometry in the earth sciences" techniques, applications and future trends

Ian Jarvis a and Kym E. Jarvis b aSchool of Geological Sciences, Kingston Polytechnic, Penrhyn Road, Kingston-upon-Thames. Surrey KT I 2EE, UK

bNERC IUP-MS Facility, Department of Geology, Royal Holloway and Bedjord New College, Egham, Surr~3' TIi +20 OEX, UK

(Accepted for publication July 8, 199 l )

ABSTRACT

Jarvis, I. and Jarvis, K.E., 1992. Plasma spectrometry in the earth sciences: techniques, applications and future trends. In: 1. Jarvis and K.E. Jarvis (Guest-Editors), Plasma Spectrometry in the Earth Sciences. Chem. Geol., 95: 1-33.

Plasma spectrometry is one of the most popular and versatile techniques for the analysis of geological and environmental samples, including rocks and minerals, waters, dust, vegetation, soils, sewage sludges and sediments. Inductively coupled or direct current argon plasmas are used as emission sources in ICP- and DCP-atomic emission spectrometry (ICP-AES, DCP-AES); an ICP provides an ion sources in ICP-mass spectrometry ( ICP-MS) . Reviews of the two plasma sources, sample introduction systems, and the instrumental and analytical performances of emission and mass spectrometers, dem- onstrates the superiority of higher-temperature, ICP-based systems. ICP-AES and ICP-MS are characterised by wide linear responses of more than five orders of magnitude. They are rapid and highly cost-effective multi-element techniques which can theoretically determine over 70 elements in < 2 ml of sample solution in < 2 min. In practise such performance is rarely possible because detections limits, and particularly in the case of ICP-AES, spectral interferences, limit the range of elements which may be quantified. Plasma spectrometry is primarily a solution-based technique, and the dissolution step ultimately controls both the range of elements quantifiable and the limits of determination which may be achieved. Limits of quantitative analysis for solid samples are typically in the order of a few/zg g ~ for ICP-AES and a few hundred ng g ~ for ICP-MS. However, chemical separation and preconcentration procedures are described for the rare-earth elements, precious metals and several other elemental groups, which enable determinations to be made at sub-ng g ~ levels. The better precision of ICP-AES and the greater sensitivity, near-complete freedom from interferences, and isotopic capabilities of ICP-MS, mean that ICP-AES is best used for major- and minor-element determinations, while ICP-MS is reserved for trace- and ultra-trace-element work. Comparisons with atomic absorption, X-ray fluorescence and instrumental neutron activation demonstrate that plasma-based techniques compete well with more established instrumental methods. Further developments in sample preparation and presentation procedures, particularly in the area of solid sample analysis, will increase further the potential applications of plasma spectrometry in the earth sciences.

1. Introduction

Inductively coupled plasma (ICP) sources combined with atomic emission spectrometers (AES) were first introduced into geoanalytical laboratories in the late 1970's. These instruments are based on the principle that by view- ing the appropriate region in an Ar plasma tail flame, the atomic and ionic emission lines of analytes can be measured against very low background emission intensities. The very high

temperatures (up to 10,000 K) in the ICP ensure far more efficient atomisation than for lower-temperature flames such as those used in atomic absorption spectrometry (AAS), and chemical interferences are consequently very small. The closely allied technique of direct current plasma-atomic emission spectrometry (DCP-AES) uses a lower-temperature (6000 K) plasma source which exhibits inherently poorer analytical characteristics. Finally, inductively coupled plasma-mass spectrometry

0009 -2541 /92 /$05 .00 © 1992 Elsevier Science Publ ishers B.V. All rights reserved.

2 I. JARVIS AND K.E. JARVIS

(ICP-MS) was developed from work on exist- ing plasma systems, the first commercial instruments appearing in 1983. ICP-MS employs an Ar plasma as an ion rather than emission source, analyte ions being extracted via a series of vacuum stages into a quadropole mass spectrometer for measurement.

1CP-AES, DCP-AES and ICP-MS all ex- hibit very wide linear response ranges of more than 5 orders of magnitude, making it possible to determine major, minor, trace and ultra- trace elements in a single sample preparation. Potentially, up to 70 elements (and in ICP- MS, their isotopes) may be measured simultaneously in < 2 min. using < 2 ml of solution. Clearly, such powerful analytical techniques have almost limitless applications in the earth sciences ranging, for example, from petrogenetic studies of ore bodies (Hall and Plant, 1992b in this issue), to investigations of rare- earth element (REE) patterns in sediments (Roelandts and Deblonde, 1992 in this issue), and from the analysis of ostracod shells to re- construct ancient lake environments (Holmes et al., 1992 in this issue), to studying the potential effects of acid rain on soils (Beau- chemin et al., 1992 in this issue).

In this review, we summarise the characteristics of plasma spectrometry and describe how it may be applied to the analysis of earth science materials. The reader is referred elsewhere for more detailed descriptions of plasma emission spectrometry (Boumans, 1987a, b; Montaser and Golightly, 1987; Moore, 1989; Thompson and Walsh, 1989; I. Jarvis and Jar- vis, 1992) and ICP-MS (Riddle et al., 1988; Date and Gray, 1989; Hall, 1989; K.E. Jarvis et al., 1990, 1991 ) instrumentation and methods. Our aim is to emphasise the strengths and weaknesses of the two techniques so that users might better appreciate the possible applications and limitations of the geochemical data which they obtain. We regard a clear under- standing of these aspects as an essential prerequisite (cf. Hall and Plant, 1992a in this is-

sue) for the interpretation of any analytical data set.

2. The ICP source

A plasma may be defined as any luminous volume of partially ionised gas, although in spectroscopy the term is normally restricted to electrically excited discharges. In ICP spectrometry the plasma is generated from radiofrequency (RF) magnetic fields induced by a copper coil which is wound around the top of a glass torch, as shown schematically in Fig. 1. A sample is generally introduced as a solution which is first nebulised to form a fine aerosol. The aerosol is transported into the centre of the ICP where it rapidly undergoes desolvation (removal of the solvent from the liquid sam- pie), vaporisation to molecular level and dissociation into atoms, some of which are ionised. Both atoms and ions become excited in the plasma and as they revert to their ground states in the tail flame they emit light (pho- tons). In ICP-AES, their characteristic emission in the tail flame is measured using an optical spectrometer. In ICP-MS, ions are extracted from the plasma into a mass spectrometer for analysis.

2.1. Torches

ICP torches generally consist of three concentric glass tubes fabricated from quartz. The torch is designed to support a stable Ar plasma at its top while allowing a sample to be injected into its core. The plasma is supported by an Ar "coolant gas" which for the industry standard Fassel torch is supplied at 10-181 min- ~ to the outer concentric tube. This gas flow is introduced tangentially to create a vortex which sta- bilises the plasma in the centre of the torch, and cools the walls of the torch, preventing it from melting. The inner-most tube consists of a capillary through which the sample is injected into the plasma by a 0.5-1.5 1 min-~ stream of Ar. The intermediate tube is often not used for

I'LASM.\ SPECTROMETRY IN THE EARTH SCIENCES 3

tailflame

fireball ~

induction coil ~

radiofrequency generator

coolant gas

\

o o

l j / torch auxiliary gas ~ J sample capilliary ~ I- nebuliser

P e;iuStaplt ' ~ j

~ j/saml?le t~o drain ~ solution

spectrometer

spray chamber

Fig. 1. Schematic representation of the sample introduction system used in many ICP-AES instruments. (After I. Jarvis and Jarvis, 1992. ) The sample solution is pumped up a flexible capillary tube by a peristaltic pump into a concentric glass (Meinhard ®) nebuliser. The sample aerosol is sorted in a spray chamber, and the finest droplets are carried by the Ar injector gas into the central capillary of a Fassel quartz glass torch. The aerosol is desolvated, vaporised, dissociated, ionised and excited in the Ar plasma fireball, which is maintained by a radiofrequency magnetic field. The wavelengths and intensities of the emission signal generated in the tail flame are measured by a conventional emission spectrometer.

routine analysis of aqueous solutions, but it may be employed to introduce a small (0-1.5 1 min -~) auxiliary tangential Ar flow which raises the plasma away from the injector tube. This prevents overheating and sooting-up of the injector tube when running organic solvents.

2.2. Plasma formation

A water-cooled copper tube is coiled around the upper part of the plasma torch. The coil is connected to a RF generator, which creates an oscillating RF magnetic field within the Ar flowing through the torch. ICP RF generators operate within the industrial frequency bands of 27.12 or 40.68 MHz and most systems are operated at forward powers of 0.9-1.5 kW;

higher powers are generally used when running organic solvents.

The Ar stream passing through the torch is initially seeded with electrons, generally using a high-voltage spark. The electrons are sub- jected to intense oscillations of the RF magnetic fields generated by the induction coil and collide with Ar atoms causing ionisation. Each charged particle formed in the gas stream is controlled by the magnetic fields which force it to flow in a closed annular path within the torch. Resistance to the induced motion of the charged particles causes ohmic heating of the gas stream which rapidly rises in temperature to ~ 10,000 K. This transfer of energy by in- ductive coupling (which avoids the use of electrodes) to form a high-temperature plasma is the definitive characteristic of an ICP.

4 l JARVIS AND K.E. JARVIS

2.3. Plasma gases

Ar is the preferred plasma gas because: (a) it is inert and therefore does not readily react chemically with samples; (b) it has a high first ionisation energy of 15.75 eV, causing effective ionisation of, and emission by, almost all other elements; (c) it is optically transparent; and (d) it has a moderately low thermal con- ductivity, so that heat is retained in the plasma fireball enabling stable operation at moderate power inputs. Nitrogen has been investigated as an alternative to At, but produces increased spectral and isobaric interferences when used alone. However, work on mixed-gas plasmas using Ar with subordinate amounts of N2 or He is continuing in several laboratories (Evans and Ebdon, 1989; Murillo and Mermet, 1989; Beauchemin and Craig, 1990), and may have specific applications in both ICP-AES and ICP-MS.

2.4. Characteristics o f an Ar plasma

The combination of torch, induction coil and gas flows are designed to support a stable to- roidal plasma centred at the open end of the quartz glass coolant tube. As shown diagram- matically in Fig. 2, the plasma has three discrete regions: the fireball; secondary region; and tail flame, each of which has very different physical and chemical characteristics (Kal- nicky et al., 1975; Fassel, 1977; Boulos and Barnes, 1981, 1987). The fireball or "induction region" is a doughnut-shaped area into which energy is transferred into the plasma by the interaction between the Ar flow and the RF magnetic field (Fig. 2c). The highest temperatures ( > 10,000 K) are developed in this region ( Fig. 2a and b ) which contains large pop- ulations of highly excited Ar ions. The fireball is an intense light source dominated by Ar emission lines and a broad background continuum, making it unsuitable for use as an analytical source.

The fireball is surrounded by a secondary re-

(a)

0 0

/ coolant gas

6000 K

7000 8000

9000 t~->10000 HO toooo

i~ l i i~nduction coil (b)

eddy currents /" torch ~ ..."

injector gas / .~-- \7"~5 _--./ ~, ,sample .. . . . ol {/ rl ~ (/(. . . . . . ;.-/~' R ~)

induction ~ ~ / coi, / tl /

magnetic f~el/~H J" J~ ~l~ ~rch (C) argon flow

Fig. 2. Generation and characteristics of an inductively coupled plasma. (After I. Jarvis and Jarvis, 1992. ) a and b. The plasma is maintained by a 10-18 I ra in- flow of Ar coolant gas. The sample aerosol is carried by 0.5-1.5 1 min -~ Ar injector gas into the core of the ICP, passing through the central annulus of a > 10,000-K fireball. Emission is measured in the 6200-6500-K area of the tail flame, 12-18 mm above the induction coil. c. The plasma is induced by radiofrequency magnetic fields generated by currents passing through a water-cooled copper induction coil. Electrons and ions produced in the plasma form circular eddy currents; resistance to this induced motion maintains a high-temperature plasma by ohmic heating.

gion with temperatures of ~ 8000 K, which is also luminous but slightly transparent. Beyond this, the tail flame (Fig. 2b) or "normal analytical zone" is characterised by an incredibly low intensity of background emission and is consequently nearly invisible until a sample is injected. Here, the emission spectrum exhibits a cont inuum with a relatively small number of discrete lines produced predominantly by Ar atoms. At the normal observation height for spectroscopic measurements of 12-18 m m above the induction coil, tail flame tempera-

PI.ASMA S P ECTROMETRY IN THE EARTH SCIENCES 5

tures remain between 6200 and 6500 K. In ICP-MS, ions are extracted from the central channel of the tail flame ~ 10 mm above the load coil, where a similar range of temperatures appertain. Sampling the higher-temperature portions of the tail flame ensures that cooling-related physical and chemical effects, such as the formation of secondary molecular species (e.g., polyatomic ions), remain minimal.

Samples are injected into the centre of the plasma toroid, and so pass through the centre of what is effectively a very high-temperature tube-furnace. There is little inter-mixing of the sample with gases in the plasma fireball. The sample is heated rapidly to ~ 8000 K by con- duction, convection and radiation effects, and sample molecules undergo nearly instanta- neous desolvation, vaporisation, dissociation, ionisation and excitation. The high temperature of an ICP ensures that dissociation is highly efficient and, unlike flame atomic absorption spectrometry (FAAS) and DCP-AES, chemical interferences (caused by the recombination of atoms and the formation or reten- tion of stable molecular species in the flame) are negligible. Similarly, ionisation interferences are also generally insignificant.

3. Sample introduction

Sample introduction for plasma spectrometry is generally accomplished using solution nebulisation. Sample solutions are aspirated by a nebuliser (Fig. 1 ) which shatters the liquid into fine droplets using an Ar gas stream of ~ 1 I min - I These droplets are directed into a spray chamber which removes the unsuitable larger ( > 10/tm ) material, and allows only the finest spray to pass into the plasma. The low injector Ar gas flows required in ICP spectrometry result in the efficiency of this nebulisation and sorting process being very low, typically only ~2% of the sample solution aspirated being transferred into the plasma.

3. I. Nebulisers

Three main types of nebuliser are commonly employed in ICP systems: concentric, cross-flow and Babington-type nebulisers. In the concentric glass nebuliser designed by J.E. Meinhard (Meinhard, 1976, 1987; Fig. 1 ) the sample is introduced along a fine glass capillary positioned axially within a larger glass tube with a side-arm. Ar is injected through the side- arm and flows to a 10-20-~tm annular gap which surrounds the sample capillary. The Venturi effect produced by the gas stream as it is forced through the annulus is sufficient to suck the sample through the tube and blast the liquid into droplets. Meinhard ® nebulisers provide a highly reproducible and reliable performance for the analysis of dilute aqueous solutions, and are probably the best nebuliser design for that purpose. However, their borosilicate glass construction and the delicate nature of the capillary tube at the nebuliser tip prevent their use for aspirating solutions containing HF or concentrated alkalis. Even samples including HF neutralised with boric acid may seriously damage the nebuliser. Addition- ally, they are very prone to blockage by particulate material and are intolerant of high levels of total dissolved solids (TDS). They will generally only perform at their op t imum with solutions containing < 0.5% TDS.

Cross-flow nebulisers (Kniseley et al., 1974; Anderson et al., 1981; Browner and Boorn, 1984) operate by the injector gas being fed through an axial capillary tube which is positioned at right angles to a second capillary car- rying the sample solution. The shearing action of the gas jet over the end of the sample capillary causes the solution to be drawn through the tube and shattered into an aerosol. Cross- flow nebulisers use larger capillary sizes and are easier to manufacture from inert materials such as PTFE (polytetrafluoroethylene) with sap- phire or plat inum-ir idium capillaries. They are less-prone to salting-up than concentric glass

6 I. JARVIS AND K.E. JARVIS

designs, but may not produce the high precision of the latter.

Babington-type nebulisers (Dresner, 1981 ) are capable of nebulising samples containing high TDS contents and /o r particulate matter, and are uniquely suited to the aspiration of slurried samples (see K.E. Jarvis, 1992 in this issue). In one commercial design (McKinnon and Giess, 1981 ; Walton and Goulter, 1985 ), a peristaltic pump is used to introduce the sample solution through a wide-bore capillary which feeds the top of a vertical V-groove cut into a ceramic or other corrosion-resistant material (e.g., Ryton ® ). The solution then flows down the groove under the influence of grav- ity. An Ar gas stream is introduced through the inside of the nebuliser, blasting out of a small orifice drilled at the base of the groove, and shattering the liquid flowing down the groove into an aerosol. Poorer precision and enhanced memory effects generally make such nebulisers less suitable than pneumatic designs for the analysis of simple dilute aqueous solutions, but their versatility enables them to be employed with a much greater variety of sample types.

3.2. Spray chambers

The main purpose of a spray chamber (Fig. 1 ) is to ensure that only the finest ( < 10/~m) droplets produced by a nebuliser reach the plasma, since larger droplets are incompletely dissociated in the plasma and contribute noise to the analyte signal. Spray chambers contain baffles and /o r impact surfaces which cause larger particles to condense and flow to drain. However, elements such as B, Hg and Au are notoriously difficult to remove from the sample introduction system, and care must be ex- ercised to ensure that wash-out times are sufficiently long to avoid inter-sample memory effects. The inherent sensitivity of ICP-MS makes the technique particularly prone to such effects, and washout times are necessarily several times those typically employed ( 1-2 min.

as opposed to 15-30 s) in plasma emission spectrometry. On most ICP-MS systems, the spray chamber is surrounded by a water-cooled jacket operating at ~ 5°C for inorganic samples. Cooling the spray chamber not only helps to maintain a constant temperature, and thereby improve signal stability, but also re- duces the amount of water vapour entering the ICP.

4. Inductively coupled plasma-atomic emission spectrometry (ICP-AES)

Quantification of elemental abundances in samples by ICP-AES is based on the ability to separate a complex emission spectrum into its component wavelengths, with sufficient sensitivity and resolution to precisely measure light intensities at the characteristic wavelength of each analyte. Schematics of typical ICP-AES systems are shown in Figs. 1 and 3.

4.1. Spectrometer design

There are two major categories of ICP-AES spectrometer (Fig. 3 ): polychromator (simultaneous), and monochromator (sequential) systems [see Moore ( 1 9 8 9 ) a n d Thompson and Walsh ( 1989 ) for further details ]. Both are capable of measuring the full range of wavelengths used in AES, typically from the upper part of the vacuum ultraviolet ( 170 nm ) to the limit of visible light ( 780 nm) , although some low-cost instruments may have more restricted wavelength ranges.

Simultaneous spectrometers are direct read- ing designs (Fig. 3a). Light is focussed onto a fixed diffraction grating which splits the light into its component wavelengths. Exit slits lo- cated at pre-determined positions around a Roland circle are used to focus light of specific wavelengths onto individual photomultiplier tubes for measurement. The number of elements which can be determined simultaneously is limited by the physical space required to position photomultiplier tubes for adjacent

PL.~SMA SPECTROMETRY IN THE EARTH SCIENCES 7

(a) Paschen-Runge photomultiplier polychromator

mirror exit slits tubes \ \ I

\ ~ % ~ ~ __ I ----. ---. ° " ~ \

/ \ V ~ : \ <,~. riot tie¢~ / . \ , ' ~ " y " x , \ 4 % I ...... , . . / ,,v ~ , / - K . 4 \ O , -

/ / ." ~" ~ \ .... slits . //" .- .-% \\ J

/ / ~,' / _ / .." . % \~/

/ / ~,,, / / z . I ~ t~-~ ~),.c, \e

/ "I / /" " -- " i ~\ photomultip er .... .... .;tubes

Y - - / ICP emission

l/"<, 5W--@ % / z \

/ ~ / \ l e n s fixed ~ / ent rance

diffraction gratings ~ Rowland circle ~-" (primary) ~ ~ _ ~ ~ / slit

(b) exit slit x\

\

~ l ~ _ ~ ; - J _ - / / i." I i~ 1'

mirrors

/ \ \

Czerny-Turner rotatable e n t r a n c e

monochromator diffraction grating slit

pholomultiplier tube

ICP emission source

l e n s

Fig. 3. Schematic representations of two major types of ICP-AES spectrometers. After I. Jarvis and Jarvis, 1992. ) a. A Paschen-Runge polychromator, contains a fixed array of photomultiplier tubes set at predetermined analyte wavelengths. The limited spectral range in the first order (typically 175-500 nm) of the conventional Paschen-Runge mounting, is complimented by a flat-field mounting positioned with the Rowland circle. This additional mounting enables the determination of the alkali metals (K, Li, Na) which have their most sensitive lines in the 500-800-nm range. Such systems allow the simultaneous determination of up to 60 elements in < 2 rain. b. A Czerny-Turner monochromator, contains only one (or two inter-changeable) photomultiplier tube(s) combined with a movable diffraction grating. Rotation of the grating enables the system to scan through the entire 175-800-nm wavelength range or to drive sequentially to each analyte wavelength chosen. The inherent flexibility and lower capital cost of such systems is offset by extended analysis times ( ~ 10 min. for a typical trace-element programme ).

8 I. JARVIS AN[) K.E. JARVIS

wavelengths, most systems allowing 20-60 elements to be measured simultaneously. For the determination of elements such as P and S which have their op t imum analytical wavelengths in the vacuum ultraviolet ( < 200 nm ), the spectrometer and its associated optics must be sealed under vacuum or purged with Ar or N , .

One major disadvantage of polychromators is their high cost, because they require a separate suite of electronics for every analytical wavelength installed. They are also relatively inflexible, since analytical wavelengths are best chosen before the instrument is assembled. These limitations are not as great as might be expected, because in reality there is often only limited choice of analytical wavelengths. The main advantages of polychromator systems is their speed of analysis and low running costs, since they potentially enable the determination of 60 elements in 1-2 min., using only 1- 2 ml of sample solution. In practise the number of elements which can be quantified is limited by the sample type and the sample preparation procedure employed, and a maximum of 25-30 elements is a more realistic expecta- tion for simultaneous analysis. Finally, the simplicity of polychromators ensures reliable operation and op t imum short and long-term reproducibility.

Sequential systems are based on computer- controlled scanning monochromators (Fig. 3b), which can be pre-programmed to drive rapidly from line-to-line, measuring sequentially at each analytical wavelength chosen. Such systems are inherently cheaper and more flexible than polychromators because they can measure any wavelength required. The main disadvantage of the monochromator is speed, since the analysis t ime is directly proportional to the number of elements being determined. A 20-30-element programme which requires 1-2 min. per sample on a polychromator will typically take 5-10 min. on a monochromator , depending on the integration times and number of background and inter-element correc-

tions employed. The volume of sample solution consumed will also be increased accordingly. Longer analysis times significantly enhance operating costs and invariably cause some compromise in analytical precision and accuracy.

Combined spectrometers employing both a polychromator and a monochromator provide op t imum performance and flexibility but are correspondingly expensive.

4.2. Emission spectra, interferences and line selection

ICP-AES is less susceptible to interferences than most other spectrometric techniques, but the coincidence or overlap of emission lines remains a serious analytical problem. Many elements have large numbers of individual lines, Fe alone, for example, exhibiting > 1000 separate atomic and ionic lines between 200 and 300 nm (Michaud and Mermet, 1982 ). Com- pilations of ICP-AES lines and spectra (Par- sons et al., 1980; Phelps, 1982; Boumans, 1984: Nygaard and Leighty, 1985; Winge et al., 1985: Wohlers, 1985; Brenner and Eldad, 1986; Bou- mans and Vrakking, 1987 ) provide vital information on potential spectral overlaps.

ICP emission spectra are also characterised by background continua. The background spectrum is not only due to emission from the plasma itself but also originates from stray light within the spectrometer, recombination radiation and line broadening effects. Background enhancements caused by A1, Mg and Ca are particularly problematic in the analysis of geological samples and require correction.

Line selection and the identification and correction for spectral interferences is one of the most critical aspects of successful ICP-AES analysis. Three main types of spectral interference (Fig. 4) may be distinguished: (a) line coincidence; (b) wing overlaps; and (c) con- t inuum interferences. Happily, such interferences need not cause major analytical problems. Direct line coincidences can generally be avoided or minimised by choosing an alterna-

PI.~SMA SPECTROMETRY IN THE EARTH SCIENCES 9

(a) . Co II (b) A I I 220.467

I Pb II ~ 228 616

co 1 ,,9~C' It 220~6~ II

" ' I! ~, iII -} :

g :

] 1/!1 228618 ~ 0 . g mL -1

Ti 200 ug mL -1 , , : t ; L'

line c o i n c i d e n c e lap i i . . . . . i i

226 5 228 6 228 7 220 3 220 4

Wavelength (nm)

(c)

. ~ i / " ~ ' ~'~" '

Au I 197 819

-1 Au 10 ug mL

1 AI 1000 ug mL

t ,

continuum interference

197 7 197 8 197 !)

Fig. 4. Examples of spectral interference found in ICP-AES. (After I. Jarvis and Jarvis, 1992. ) a. Line coincidence, an unresolvable near-coincident Ti line occurs with the best available Co line. Emission from samples containing both elements will be the sum of both signals, requiring subtraction of the Ti component prior to quantification of Co. b. Wing overlap, a significant wing overlap from a major AI line on the most sensitive Pb line necessitates an interference correction to be made for many samples. The realistic limit of determination for Pb is degraded by around one order of magnitude (to ~ 20 ~g g ~ ) because of this interference in lithogeochemical samples. c. Cont inuum interference, one of the most interference-free Au lines lies within an area (190-220 nm) of increased background emission caused by recombinat ion radiation from AI. The contribution of such enhanced backgrounds to the emission signal are readily removed by off-peak background correction, but as with all such procedures this will cause some deterioration in analytical precision and limits of detection.

tive analytical line, and wing overlaps can often be eliminated or minimised by increasing the resolution of the spectrometer by using nar- rower slits or higher spectral orders. Contin- uum interferences may be estimated from background measurements taken on either side of the analyte peak, and can be subtracted from the analyte signal.

Where alternative strategies prove impossi- ble, the contributions of each concomitant element to the analyte signal can be estimated by running single-element interference standards [typically 1000/tg ml -I (parts per million) solutions], and correction factors can be cal- culated. Fortunately in ICP-AES, interfer-

ences from concomitants generally increase in a linear manner with concentration, and are relatively simply to correct. Therefore, corrections can be applied if the concentration of each interfering element is measured at the same time as the analyte signal. Such corrections work well if the interference factor is small ( < 5-10% of the analyte signal ) but larger corrections may seriously compromise the precision and accuracy of determinations, and are best avoided.

Despite a seemingly endless number of potential emission lines, in practise choosing an analyte line is relatively straight-forward since, generally, one of the half-a-dozen most sensi-

I 0 I. JARVIS A N D K.E. JARVIS

rive lines will be sufficiently free from likely interferences to be employed. Clearly, the analyst must take into account the sensitivity of the analyte signal relative to any potential interferences. If the intensity of a peak is large enough, corrections will yield no significant improvement in accuracy, and are unnecessary. Inter-element corrections, therefore, are rarely required for major-element determinations. The actual lines chosen by the analyst will depend on a number of factors including the resolution, sensitivity and other characteristics of the ICP-AES instrument employed, and the composition of the samples being analysed.

5. Direct current plasma-atomic emission spectrometry (DCP-AES)

DCP-AES employs a DCP rather than an ICP as an emission source. DCP-AES has found less widespread acceptance in academic and other research establishments than ICP- AES, but nevertheless is favoured by some analytical service companies who regard it as being a more cost-effective alternative. Much of the initial development work on plasma- mass spectrometry used a DCP source (Gray, 1989), although ultimately an ICP has proven more suitable.

5.1. Instrumentation

Commercial DCP-AES instruments are generally three electrode systems which gener- ate an inverted Y-shaped plasma (Ebdon and Sparkes, 1985; Potts, 1987; Moore, 1989; Thompson and Walsh, 1989), the emission from which is measured using a high-resolution Echelle spectrometer. The plasma is struck by a high-voltage spark between two ceramic- sleeved graphite anodes positioned at an angle of ~ 60 ° and a small ceramic-sleeved tungsten cathode. The plasma is maintained by passing a low-voltage (40-50 V) current regulated at ~7 A through each of the anodes. Argon sheathing gases are introduced at 1-2 1 min-

around the three electrodes and carry a stabi- lised DC-arc discharge from the anodes to the cathode block. The sample aerosol is introduced with a Ar gas flow of 2-4 1 min ~ via a separate injector tube set below the intersec- tion of the two anodes, and is carried into the inverted-Y of the plasma. The excitation region produced is characterised by low background emission but is relatively small ( ~ 0.5 mm across), and so requires good instrument stability to avoid signal drift.

5.2. Analytical characteristics

Maximum temperatures generated in the excitation region of a DCP are much lower than in an ICP, attaining only 5700-6000 K (Decker, 1980). The lower temperatures result in some chemical interferences, enhanced matrix effects and fairly severe ionisation interferences when compared with ICP-AES (Zander, 1986; Routh, 1987 ), although spectral interferences are marginally less significant because of the simpler DCP spectrum. The use of an ionisation buffer such as Cs, can be used to compensate for differential ionisation effects.

Despite the inherent limitations of the lower- temperature DCP source, many of the advantages of the ICP are retained, including wide linear calibration range and high sample throughput for simultaneous instruments, although a maximum of only 20 simultaneous lines are available on most commercial systems.

6. Inductively coupled plasma-mass spectrometry (ICP-MS)

The principles of ICP-MS are relatively straightforward. For nearly all elements present in a sample, ions may be generated in a suitable ionising source such as an ICP. Ions are physically extracted from the plasma into a mass spectrometer and measured using an ion detector. This process of mass selection is rapid, and the instrument is able to obtain a

PLAS'MA. SPECTROMETRY IN THE EARTH SCIENCES I 1

spectrum for the entire mass range of 7Li to 2~8U in ~ 1 min.

6. I. Systern operation

A schematic of a typical ICP-MS system is shown in Fig. 5. The sample, typically in the form of a solution, is introduced into the plasma with the injector Ar flow. As the sample enters the region of higher temperature, it is rapidly volatilised, dissociated, excited and finally ionised. The analyte emerges from the mouth of the torch as a mixture of atoms, ions, undissociated molecular fragments and unvo- latilised particles. The ions are extracted from the central channel through a sampling cone aperture. The cone is normally fabricated from high-purity nickel or has a nickel body with a plat inum tip. In both cases, nickel is the preferred material, principally because it is dura- ble and available at reasonable cost. However, the type and concentrat ion of acid used during sample introduction must be considered carefully, since strong acids ( > 5% v / v ) increase the rate of erosion of the cone surface. In addition, certain acids such a s H 2 S O 4 , for example, are particularly aggressive even in rela-

tively dilute form and will considerably shorten the life of a sampling cone.

Behind the sampling cone is a region of low pressure ( ~ 5 mbar) . The temperature de- creases rapidly as the ions enter this region, so the composit ion of the extracted gas is effectively frozen. A "sk immer" or second aperture is mounted behind the sampling cone. The physical condit ion of the skimmer (also con- structed of nickel) is important, and a smooth outer surface and sharp orifice help to reduce the formation of polyatomic ions and generally improve short-term signal precision.

An electrostatic lens system is placed behind the skimmer in the region of high vacuum (5.10 -6 mbar) . The function of the lens stack is to focus the ions which then pass into the mass spectrometer, where a quadrupole system acts as a mass filter. A stable ion path ex- ists along the axis of the four quadrupole rods for ions of only one mass at a time, so by vary- ing the RF and DC potentials on these rods, ions of selected masses are allowed through to the detection system in a sequential mode. This operation is carried out very rapidly and the quadrupole is usually "scanned" through a range of masses between 100 and 1600 cpm. The resolution of such systems is sufficient to

I Quadrupole 11 Lens I Power Unit ISupplies

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Fig. 5. Schematic of a typical I C P - M S ins t rument . (After Gray, 1989.) The sample in t roduct ion system is essentially identical to that in ICP-AES (Fig. 1 ), a l though in I C P - M S the torch is or iented horizontal ly ra ther than vertically. Ions are extracted from the plasma through a nickel sampling cone into an area of low pressure ( 1 ). Focussing of the ion beam is accomplished using a nickel sk immer and a series of electrostatic lenses in an area of in te rmedia te vacuum (2) . The focussed ion beam ult imately enters the high vacuum (3) of a quadrupole mass spectrometer , where ions are rapidly sorted and counted using an electron mult ipl ier detector.

12 I JARVIS A N D K.E. JARVIS

Rh

Ru [ / Ru

100 102 104 106

Pd

A Pd , / 5 ,

108 110 i

112 i

114

i

m/z 188

IF

Au

190 192 194 196 198

Fig. 6. Mass spectra for some platinum-group elements (Ru, Rh, Pd, Ir, Pt) and Au for the regions 99-114 and 188-198 m/z. The sample contains 200 ng ml 1 (parts per billion) of each element and has been prepared in a 0.5 M HC1 matrix. Note the clear resolution of each isotope from its neighbours, and the different responses of different isotopes of each element, which are consistent with their natural relative abundances.

completely separate adjacent isotopes (Fig. 6), but is inadequate to resolve the small differences in mass which separate polyatomic ions from near-coincident elemental peaks. High- resolution ICP-MS instruments employing magnetic sector spectrometers have recently been developed for this purpose, but sacrifice the rapid scanning multi-element capability of quadrupole systems.

Ion detection is usually accomplished using electron multiplier detectors. The ability to count individual ions, coupled with very low background signals, result in excellent sensitivity for nearly all elements in the periodic table. Unfortunately, at very high count rates the response of these detectors becomes non-linear due to counting losses in the electronics. Variable dead times (a factor which corrects for non-linearity in response) also occur depending on the age of the detector. Data handling is carried out using a PC, and facilities are normally available for reporting of data and

for transmission of data into third-party software.

6.2. Interferences

ICP-MS system optimisation is an important prerequisite for accurate and precise elemental determination. The major interferences which occur in ICP-MS fall into three categories: (a) polyatomic ions; (b) doubly charged ions; and (c) refractory oxides. In addition, "non-spectroscopic matrix effects" (see p. 13) have also been observed (e.g., Gillson et al., 1988).

Polyatomic ions form in the plasma and result from the reaction between the most abun- dant ions in the plasma, At, H, O and N. Many different combinations of these molecules form but perhaps the most common (and abun- dant) are those involving Ar, for example ArO + and ArH +. In addition, the acids used either for preparation of samples or to stabilise them, may contain high levels of elements which form polyatomic species. Sulphur from H2SO4 or chlorine from HC1 or HCIO4, for example, form combinations of ArCI +, CIO + and ArS + which cause significant interferences. Perhaps the most serious of these are 4°Ar32S+ on 72Ge, 328160+ on 48Ti, 4°Ar35Cl+ on 75As, 4°Ar37C1+ o n 778e and 35Cl l60+ o n 51V. In the case of V and As, no isotopes are available which are free from interference and their determination is compromised in samples prepared with Cl-based acids or in samples which naturally contain high levels of C1. The choice of acid matrix is therefore a critical one and HNO3 is used in preference whenever possible.

Most of the ions which form in the plasma are singly charged (M ÷). The scale of measurement of mass, m/z or amu, is that for a singly charged ion of each isotope. Using the same measurement scale, if an element forms a proportion of doubly charged ions (M 2 + ), then the signal from them is seen at a mass to charge ratio of two, i.e. at half the mass of the parent ion. Some doubly charged ions form in

PLASM a, S P E C T R O M E T R Y IN THE EARTH SCIENCES I 3

the plasma if the second ionisation energy of the element is lower than the first ionisation energy of Ar (the plasma support gas). How- ever, ICP-MS systems can be optimised to give low levels of M 2 + ions, typically between only 0.2-0.5% of the parent ion.

Incomplete dissociation in the plasma and/ or recombination in the plasma tail flame, can lead to the occurrence of oxide species, e.g. MO+. These species occur 16, 17 or 18 amu above the parent ion and may cause an interference. During REE analysis, for example, there is a potential interference problem from the light REE (LREE) -oxides on some of the heavy REE's (HREE's). In addition, the seven naturally occurring isotopes of Ba occur adjacent to the LREE's ( 130-138 re~z), so barium oxide species are also a potential source of interference on some of the REE's.

The interferences discussed above can usually be predicted from a knowledge of the sample matrix and preparation method. Tables listing many of the possible polyatomic, doubly charged and oxide ion species which form may also be consulted (e.g., Tan and Horlick, 1986 ), although such lists give little indication of the magnitude or seriousness of each potential interference. Despite the range of possible spectroscopic interferences which can occur, few actually cause serious problems in routine analysis. In most cases alternative isotopes can be chosen which are free from any interference effects, and in general terms ICP-MS is far less prone to insoluble spectral interference than plasma emission spectrometry.

7. Non-spectroscopic matrix effects

In addition to spectral interferences, analytical accuracy may be compromised in plasma spectrometry by non-spectroscopic matrix effects. The most important of these relate to the behaviour of solutions during sample introduction, rather than during their excitation in the plasma and analysis. Nebulisation efficiency, droplet size and fractionation, and aer-

osol transport are all affected by the physical characteristics of a sample solution. In particular, differences in acid concentration and TDS will significantly affect sensitivity and often precision.

Some matrix effects do occur during excitation, and in both ICP-AES and ICP-MS, large excesses (> 1000 /~g ml ~ ) of single matrix elements can cause suppression or enhance- ment of analyte signals. In ICP-MS, the effects are most extreme in a heavy matrix such as that of uranium, where suppression of the analyte may be as much as 60% of the "normal" signal. The mechanism by which these processes occur is poorly understood, but may result from scattering of "light" analyte ions by the "heavy" matrix as the ion cloud enters the extraction lenses. However, sodium matrices (> 1000 /~g ml ~) also cause significant suppression of analyte signals in ICP-MS (Beauchemin et al., 1987), so the cause ofan- alyte signal modification may be complex. Such matrix effects are not usually a problem in geological samples since they do not generally contain any one single element at a high concentration. Limestones (Ca), brines (Na) and many ore minerals are notable exceptions.

In general, non-spectroscopic matrix effects are best overcome by using matrix-matched standards in ICP-AES and more dilute solutions in ICP-MS.

8. System optimisation

In optimising the performance of an ICP system an analyst may vary: (a) Ar gas flows; (b) forward power being delivered to the plasma; and (c) observation height or sampling distance in the tail flame, all of which can critically affect performance. A range of exper- iments may be undertaken to obtain optimum performance for a system and this is well worth pursuing in ICP-MS, where optimum conditions are usually similar for most elements. This is not the case in ICP-AES, where each

14 1. JARVIS A N D K.E. JARV[S

emission line exhibits unique behaviour in the plasma. Consequently, systems are rarely optimised for any single element because this will adversely affect the measurement of other analytes in a multi-element procedure. It is more important to establish a series of compromise conditions which allow adequate excitation of a wide range of elements, with greater atten- tion being given to improving the sensitivities of trace elements that are close to detection limits, than to higher abundance elements which may be readily determined under non- optimum conditions. Such conditions are invariably close to those recommended by ICP- AES manufacturers for their particular instruments, and these will prove perfectly satisfactory in most analytical situations.

9. Instrument calibration

One of the greatest strengths of plasma spectrometry is its ability to produce multi-element linear responses over calibration ranges of more than 5 orders of magnitude, enabling the simultaneous determination of major, mi- nor and trace elements in a single sample preparation. In ICP-AES and DCP-AES linear responses can be obtained for most analytes from detection limits up to solution concentrations of ~ 1000/tg m l - 1 but significant curvature occurs for some lines in the range 1,000-10,000 /tg ml ~ (Ramsey and Coles, 1992 in this issue). Such high levels may be attained in some samples prepared using routine dissolution procedures, so calibrations and dilution factors should be assessed critically prior to analysis. In ICP-MS, linear responses are obtained for concentrations of up to ~ 10/tg ml- 1 for most elements operating the ion detector in pulse-counting mode. For the measurement of higher concentrations ( 10-500/tg ml- i ), the ion beam may be attenuated or the detector operated in a less sensitive analogue mode, and a linear response maintained.

In all cases, the instrumental response must first be calibrated against standards of known

composition which contain the analytes sought. For plasma emission spectrometry, matrix matching between samples and standards may be used to remove variations in nebulisation and sample transport efficiency caused by differences in TDS contents and acid concentrations. By contrast, the higher dilution factors employed in ICP-MS mean that exact matrix matching is unnecessary although parity of acid compositions and concentrations is always advisable.

Two different categories of standards can be used for calibration: (a) synthetic multi-element solutions; and (b) standard reference materials.

9.1. Synthetic standards

The principle in producing a synthetic standard solution is simple enough: a known weight of an element or compound is dissolved in a pre-selected volume of reagent (generally a dilute acid ) to prepare a single-element stock solution of known concentration (typically 1,000 or 10,000/tg ml- ~ of the analyte). Aliquots of each single-element stock solution are then mixed and diluted to produce a series of multi- element working solutions which are used for calibration.

The production of multi-element standards introduces a number of potential analytical hazards: (a) the large number of single-element solutions typically added to the final standards necessitates that individual standards are relatively pure, so an assessment of the suitability of potential commercial or in- house standards may be necessary; and (b) the compatibility and stability of elemental standard solutions should be considered; the formation of insoluble compounds (e.g., many sulphate salts) must be avoided, while some salts polymerise in multi-element mixtures and are consequently unsuited to long-term stor- age. Mo, Nb, Ta and W are particularly prone to such problems.

Fortunately, most elements are stable in di-

I'LASMA SPECTROMETRY IN THE EARTH SCIENCES ] 5

lute (0.5-1 M) HNO3, the preferred acid me- dium for ICP-MS, and standards are now commercially available in this form. Excep- tions to this are the platinum-group elements (PGE's: Au, Ir, Os, Pd, Pt, Rh, Ru) , Sn and possibly Te which are most stable in 1 M HC1, and the incompatible elements (HI', Nb, Ta) and Ge, Mo, P, S, Si, W and Ti which are generally prepared in H20. Standards with dilute HCI matrices, therefore, are required to successful calibrate for the PGE, and special standards may be required for a few other elements. In ICP-MS, the introduction of polyatomic chloride ion interferences (e.g., ArC! +, C10 +, CIOH + ) which compromise the determination of some trace elements, does not cause a problem if only high mass elements (i.e. the PGE's) are sought. A trace of HF is commonly used to improve the stability of some elements (Ge, Nb, Si, Ta, Ti) in aqueous solutions, and the addition of HF should also be considered in the preparation of both samples and multi-element standards, although such solutions are not suitable for systems employing Meinhard ® nebulisers.

Standards should ideally be prepared in the same acid matrix as their corresponding samples, but beyond this, the degree of matrix matching advisable depends on the technique used and the accuracy required. In plasma emission spectrometry, better accuracy may be obtained using standards which are effectively synthetic sample matrices, containing all of the major elements in their correct proportions, in addition to any minor or trace elements sought. For ICP-MS, accurate data may be obtained without such rigourous matrix-matching.

9.2. Standard reference materials

A wide range of rock, mineral, soil, plant, sewage sludge and other environmental international standard reference materials (SRM's) are now available from producers such as the Canada Centre for Mineral and Energy Tech- nology (CANMET), U.S. National Institute

for Standards and Technology (NIST), South African Bureau of Standards (SABS) and U.S. Geological Survey (USGS). Suitable reference materials may be prepared in an identical manner to samples and can be used for two purposes: ( 1 ) instrument calibration; and (2) to assess analytical accuracy.

Most SRM's are well characterised for their major constituents. If a sufficiently large number (min imum 5-10) of standards are available for the matrix being investigated, SRM's (or preferably second generation in-house equivalents) may be used for instrument calibration, ensuring a good match in the physical and chemical behaviour of both samples and standards. Such an approach works very well in ICP-AES for the analysis of major elements and selected trace elements in rock samples digested using li thium metaborate fusions (Walsh and Howie, 1980; Walsh, 1982: Tot- land et al., 1992 in this issue). Unfortunately most SRM's remain poorly characterised for many trace elements (Abbey, 1992 in this issue), so calibrations are limited by the availa- bility and quality of SRM data. The ,vide range of elements determinable by ICP-MS, in particular, means that such an approach is rarely satisfactory, and trace-element calibrations using synthetic standard mixtures are almost always preferable. However, the accuracy of results may be reasonably assessed using a far smaller number of materials and it is in this application that SRM's find their greatest use. Even here, however, it may not be possible to find a suitable matrix with certified or pro- posed values for the full range of elements under investigation, so no independent check of analytical accuracy may be possible.

10. Internal standards

Spiking of samples with identical concentrations of an internal standard element has been used extensively in both ICP-AES (Moore, 1989; Ramsey and Coles, 1992 in this issue; Walsh, 1992 in this issue) and ICP-MS (Date

I 6 I. J A R V I S A N D K.E. J A R V I S

and Gray, 1989; K.E. Jarvis et al., 1991 ), in an attempt to compensate for variations in instrument sensitivity caused by electronic drift, nebuliser and aerosol transport efficiency, TDS content, and other matrix effects (Belchamber and Horlick, 1982; Myers and Tracy, 1983). To correct for short-term fluctuations, the internal standard element must be determined simultaneously with the analyte, so in ICP- AES internal standardisation is only viable for polychromator and dual monochromator systems. The scan rate of ICP-MS instruments, on the other hand, is sufficiently rapid to regard them as being capable of "simultaneous" measurement, so internal standardisation is possible with all available commercial instruments.

Internal standards must be selected with great care since they must: (a) behave in the same way as the elements being sought; (b) not be one of the analytes required; (c) not con- taminate or destabilise sample solutions; and (d) occur only at low and preferably constant levels in samples. For practical reasons, the internal standard element should also: (e) not be highly toxic; and ( f ) be available in a suitable high-purity form at realistic cost.

The ratio of all elements to the internal standard in both calibration standards and in sample solutions is determined and then used to compensate for sensitivity differences during an analytical run. Several different internal standard elements have been used, including Cd, La, Lu and Sc (Brenner et al., 1980, 1981; Walsh and Howie, 1980; Crock and Lichte, 1982; Crock et al., 1983 ) for ICP-AES, and In and Rh (Stotesbury et al., 1989; Amarasiri- wardena et al., 1990; K.E. Jarvis et al., 1991 ) for ICP-MS. However, there are two serious limitations on the use of internal standards: (a) the range of possible elements which may be used is very restricted: and (b) different elements behave differently during analysis. In ICP-AES, small fluctuations in forward power and injector gas flow, for example, may produce large changes in signal for some elements

while others are relatively unaffected. In ICP- MS, progressive blocking of the sampling cone during prolonged aspiration of samples containing elevated levels of TDS, will lead to different degrees of sensitivity loss for different elements.

Despite the above limitations, Walsh ( 1992 in this issue) has demonstrated that three separate internal standards (Ga for Si, AI, Mg; ki for Na, K; Cd for Ca, Fe) can be used successfully to improve the precision of major-element determinations for silicate rocks by ICP- AES, obtaining a routine precision and accuracy of ~ _+ 0.5%. A more complex procedure has been described by Ramsey and Thompson (1985, 1987) and Ramsey and Coles (1992 in this issue) who have developed a parameter- related internal standard method (PRISM), which employs corrections based on the experimentally determined behaviour of analytes relative to two independent internal standards (e.g., Rb, Hg), chosen to monitor different aspects of plasma response. Notable improvements in analytical precision and accuracy can also be demonstrated using this approach.

In and Rh are almost routinely employed as internal standards in ICP-MS, yet there is clear evidence that neither element is suitable for compensating for sensitivity changes in all analytes (I. Jarvis et al., 1991 ). In some cases, the application of an unsuitable internal standard may lead to severe analytical bias, and external drift correction provides a far more viable alternative.

11. Sample preparation

Plasma spectrometry is primarily a solution technique, and although slurry nebulisation (K.E. Jarvis, 1992 in this issue), laser ablation (Moenke-Blankenburg and Gtinther, 1992 in this issue) and other methods are being developed, solid samples are still generally dissolved prior to routine analysis. A detailed dis- cussion of sample preparation and dissolution techniques is outside the scope of this paper,

PL-kSMA SPECTROMETRY IN THE EARTH SCIENCES l 7

but general procedures are outlined in Johnson and Maxwell (1981), Potts (1987), Moore (1989), Thompson and Walsh (1989), Van Loon and Barefoot (1989), I. Jarvis ( 1991 ) and Totland et al. ( 1992 in this issue). More comprehensive reviews are provided by Bock (1979) and Sulcek and Povondra ( 1989 ).

It should be emphasised that although the necessity for sample dissolution can lead to many analytical pitfalls, it also has a number of inherent advantages: (a) dissolution of relatively large samples of heterogeneous solid materials means that a chemically uniform and homogeneous subsample is presented to the instrument, minimising the representability, grain-size and other matrix interference effects encountered in solid sample techniques; (b) dissolution ensures the decomposition of refractory materials (which might be incompletely dissociated in the plasma) prior to analysis, thereby reinforcing the relative freedom from matrix interferences which charac- terise the method in general; (c) partial disso- lutions, such as the aqua regia-based leaches used in mineral exploration and environmental studies, allow the rapid determination of element concentrations in labile phases, while avoiding the incorporation of data from refractory minerals which might distort or hide trends in the elements of interest; (d) sample solutions are well suited to chemical separation and preconcentration procedures such as ion-exchange chromatography (Roelandts and Deblonde, 1992 in this issue; Watkins and No- lan, 1992 in this issue) and hydride generation, and to alternative sample-introduction methods such as flow injection (Eaton et al., 1992 in this issue) which allow the determination of selected elements down to ultra-trace levels: (e) samples may be analysed against ar- tificial standard solutions, enabling exact matching between samples and standards for an almost infinite range of sample types and element combinations.

Difficulties in preparing and handling small sample masses and volumes, and the necessity

to produce solutions which are sufficiently concentrated (0.5-1% TDS for ICP-AES; < 0.2% for ICP-MS) to maximise the number of trace elements which are determinable, mean that most methods published use 0.1-1 -

g sample weights, although much smaller samples may be analysed if required.

11.1. Water samples

Natural waters are close to neutral (6.5-8.0 pH ) and therefore carry only very low levels of dissolved transition metals, so although B, Ba, Ca, Fe, K, Mg, Na, S, Si and Sr may be determined routinely (Thompson and Walsh, 1989), concentrations of other elements are generally below the detection limits of plasma emission spectrometry. Environmentally sensitive elements including A1, Cr, Mn, Co, Cu, Sn, Se, Mo, Hg, T1 and Pb may be determined directly by ICP-MS (Taylor, 1989), but even here detection limits are insufficiently low to allow most other elements to be quantified. Consequently, pre-concentration procedures such as evaporation, chelation followed by solvent extraction, or ion-exchange chromatography are generally used for such studies, and are invariably necessary for trace-element analysis of seawaters and brines (McClaren et al., 1985: Beauchemin et al., 1988 ).

11.2. Biological samples

Biogeochemical surveys are being used increasingly in both environmental studies and mineral exploration. Botanical and other biological samples must first be ashed to ensure their complete decomposition. Two procedures are employed: (a) wet ashing; and (b) dry ashing. Wet ashing uses oxidising acid mixtures to destroy organic matter. Mixtures of HNO3 and HC104 are generally employed, although HNO3 alone, possibly in combination with a microwave digestion procedure (1. Jarvis, 1991 ), is preferred for ICP-MS work. Alternative methods involving H2SO4 and

I 8 I. J~,RVIS AND K.E. JARVIS

H202 are more time consuming and the final sulphuric acid matrix may cause solubility, sample transport and interference problems. Dry ashing is rapid and cheap since it merely involves the oxidation of samples in an oven at temperatures of 450-600°C. Even at such low temperatures, however, volatile elements including As, Hg, Se will be lost and at higher temperatures Cd, Pb, Zn will also be partially volatilised. Following ashing, samples may be dissolved in 1 M HNO3.

11.3. Geological and environmental samples'

Partial (aqua regia leaches) and complete digestions (HF-HC104 acid digests and alkali fusions) are both regularly employed in the analysis of geological and environmental samples by plasma spectrometry.

11.3.1. Aqua regia leaches. In most environmental studies and mineral surveys it is generally not necessary or desirable to totally de- compose samples, since the trace elements of interest are typically adsorbed onto particle surfaces, loosely bound by clay minerals, trap- ped in Mn- or Fe-oxides and -oxyhydroxides, and associated with organic matter. In addition, to maintain cost effectiveness, the analytical precision and accuracy required for such work need not be better than _+ 5%.

"Aqua regia" leaches are commonly favoured for such work, digests typically consist- ing of leaching samples with a refluxed HC1- HNO3 mixture at 95°C for several hours, followed by dilution as required prior to analysis. Near-total recovery of base metals (including Cu, Cd, Co, Mn, Ni, Pb) is achieved by this procedure (Foster, 1973) and loss of potentially volatile elements (As, Hg, Se) is minimal. The use of HC1 in this method will compromise the determination of As by ICP-MS due to the 4°Ar35C1+ polyatomic ion interference on 75As, and alternative reagents may be preferred if this element is sought (I. Jarvis, 1991 ). In any case, the major silicate-associ-

ated elements will only be partially recovered, and elements such as Be, Cr, Sn, W, Zr, which are generally associated with refractory minerals, will remain undissolved. Solubility limits may also be reached for Ag, Pb and possibly some other elements in high-grade ore samples. Such limitations will ultimately limit the accuracy of the analysis.

11.3.2. HF-HC[04 digests. The complete dissolution of samples generally requires the use of stronger-acid combinations or fusion with alkali fluxes (I. Jarvis, 1991 ). Digestion of samples in open Teflon ~ PTFE vessels using concentrated HNO3, HF and HC104 acids is a procedure favoured by many geochemists. Samples are typically attacked with HNO3- HF-HCIO4 mixtures which are then evaporated to incipient dryness. The residues are digested in a further aliquot of the acid mixture and a second evaporation undertaken (silica is volatilised as silicon tetrafluoride and some other elements such as Se and B will also be lost), followed by final dissolution of samples in 1 M HNO3. This preparation is well suited to analysis by plasma emission spectrometry, or ICP-MS. However, for the latter, residual chloride ions from remnant HC104 may cause interference problems with the determination of As and V, and to a much lesser extent Cr, Fe, Ga, Ge, Se, Ti, Zn; potential interferences must be carefully assessed in samples prior to final validation of the analytical data.

Repeated attacks with HNO3-HF-HCIO4 may be required to dissolve some materials, and even then minerals such as barite, cassiter- ire, chromite, magnetite, tourmaline and wol- framite may remain only partially digested. Refractory phases including beryl, columbite, corundum, chromite, garnet, ilmenite, kya- nite, rutile, staurolite, tantalite and tourmaline may be more effectively digested if dissolution in undertaken in sealed high-pressure Teflon (")- lined vessels or "bombs". However, such procedures, are time consuming and labour inten- sive. Despite their limitations, open vessel

PLASM \ SPE( 'TROMETRY IN THE EARTH SCIENCES 1 9

digestions will yield fully quantitative recovery of most elements and have the major advantage that since Si is removed during the evaporation stage, the TDS of the final solutions is reduced, ensuring op t imum lower limits of determination.

11.3.3. Alkali fusions. If Si or elements which concentrate in refractory minerals (e.g., Cr, HI', Sn, Zr) are to be determined, fusion with an alkali flux is the preferred method (I. Jarvis, 1991, Totland et al., 1992 in this issue). Fu- sion of samples with lithium metaborate (kiBO2) at 1050°C, followed by dissolution in 0.5 M HNO3 is commonly used for this purpose and effectively digests most refractory minerals. The use of sodium peroxide (Na202) or sodium carbonate (Na2CO3) fluxes are preferred by some workers and is necessary ifB is sought.

The major disadvantage of all fusion procedures, however, is that they introduce large quantities of TDS which necessitate increased dilution (typically 500- to 1000-fold for plasma emission spectrometry and 5000-fold for ICP- MS) of samples for analysis. These increased dilution factors will invariably push some trace-element concentrations in solutions below the limits of quantitative analysis. The fusion procedures may also lead to increased loss of volatile elements (including Cd, Pb, Sb, Sn, Zn from LiBOe fusions), thereby preventing their determination.

I 1.4. Overview

It must be emphasised that sample preparation is an essential aspect of all plasma spectrometry and other solution-based analyses. Sample preparation, and particularly sample dissolution, procedures will ultimately limit the range of elements which can be accurately quantified. Clearly, methods which result in the incomplete dissolution of refractory minerals and /o r the loss of volatile species will preclude the accurate determination of some elements.

Less obviously, the introduction of contami- nants during digestions and /o r the addition of high levels of TDS (as in the case of alkali fusion procedures) will invariably lead to higher limits for quantitative analysis, and may prevent the determination of a further suite of elements.

Sample preparation ultimately limits the range of elements and the precision and accuracy of analytical data which can be obtained from a sample. No single preparation procedure is suited to the digestion of all materials or the determination of all elements, but for rocks and soils, for example, combined data from both open-vessel digestions and alkali fusions enables the accurate and precise determination of a wide range of elements in most sample types.

12. Separation and preconcentration procedures

Despite low instrumental detection limits, spectral interferences and /o r other matrix effects preclude the determination of several important suites of trace elements in some geological and environmental samples. These difficulties may be overcome by using chemical procedures which separate groups of analytes from their matrix. Elemental groups which are routinely treated in this way include the: (a) REE's: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; (b) hydride- forming elements: As, Bi, Ge, Sb, Se, Sn and Te; (c) organometallic halide-forming metals: Ag, As, Au, Bi, Cd, Cu, Mo, Pb, Sb and Zn; (d) precious metals: Au and Ag, and the PGE's: Ir, Os, Pd, Pt, Rh and Ru: (e) petrogenetic discriminators: Hf, Nb, Ta and Zr.

12.1. Rare-earth elements (REE's)

REE distributions have become a routine requirement for most petrogenetic studies, and both ICP-AES and ICP-MS provide rapid, reliable and cost-effective methods for the deter-

2 0 I. JARVISAND K.E. JARVIS

mination of the REE's in samples. Spectral interferences from major and minor elements limit the range ofREE's (La, Ce, Y) which can be determined in untreated samples by ICP- AES, but these problems may be overcome using a cation-exchange separation procedure. Simple mass spectra and considerably greater sensitivity of ICP-MS for high-mass elements allows REE determinations to be made directly on many materials (K.E. Jarvis, 1988, 1989; Totland et al., 1992 in this issue) to ~ 0.1

1 #g g Sample preparation can be a major problem

with REE determinations since the bulk of these elements, and particularly the heavier REE's (Dy-Lu) , commonly reside in refractory accessory minerals such as zircon, sphene and garnet, phases which may be incompletely digested using conventional open-vessel mixed- acid attacks. It is essential, therefore, that complete dissolution is ensured, either using an acid attack followed by a "mini-fusion" of the insoluble residue with NaOH, KHF2 or LiBO2 fluxes (Walsh et al., 1981; I. Jarvis and Jarvis, 1985 ), or fusion of the complete sample (Hall, 1990; Sholkovitz, 1990; Watkins and Nolan, 1990. 1992 in this issue).

Details of cation-exchange separation procedures may be found elsewhere (e.g., Thomp- son and Walsh, 1989; I. Jarvis, 1991; Watkins and Nolan, 1992 in this issue; Roelandts and Deblonde, 1992 in this issue). Typically, samples are fused with LiBO2 and dissolved in 0.5 M HC1. Solutions are loaded onto Bio Rad ® AG 50W-X8 (200-400 mesh) cationic resin and the major and most trace elements are eluted with 1.70 M HC1, leaving the REE's, Y, Sc, Ba and some Hf, Sr, Zr adsorbed onto the resin. These elements are then eluted using 4 M HC1 which is evaporated to dryness, residues being taken up in 0.5 M HNO3.

Calibrations are best achieved with synthetic multi-element mixtures of the REE's and Y, concentrations of which reflect their natural relative abundances. In both ICP-AES and ICP-MS, matrix matching precludes the ne-

cessity for complex inter-REE interference corrections except where extreme REE distributions are found in samples. Small amounts ofBa, Zr and some Ca, Fe, Sr, Ti, also occur in final solutions and in ICP-AES interferences from these elements must be corrected (K.E. Jarvis and Jarvis, 1988). Barium oxide interferences may also prove problematic in the analysis of the REE's in Ba-rich samples by ICP-MS, and also require correction. Limits of quantitation for separated samples range from 0.1/tg g - ~ Pr to 0.003/tg g - 1 Lu by ICP- AES (I. Jarvis and Jarvis, 1992), and from 4 ngg ~ (parts per billion ) N d t o 0 . 5 ngg -~ Tb by ICP-MS (K.E. Jarvis~ 1989).

12.2. Hydride elements

A number of elements (As, Bi, Ge, Pb, Sb, Se, Sn, Te) will readily form gaseous hydrides at ambient temperatures. Many of these so- called "hydride elements" are toxic in the en- vironment and /o r are important mineral exploration targets or pathfinders. In general, however, they occur at very low concentrations in samples and are subject to major spectral interferences from other matrix elements in ICP-AES. Their determination by ICP-MS is less problematic since much better detection limits may be achieved. Their routine determination by ICP-AES in particular, however, requires some form of separation procedure.

Hydride formation is easily achieved by re- acting samples with aqueous sodium tetrahy- droborate (NaBH4) as the reducing agent, and effectively removes this select group of trace elements from their matrix. The hydrides can then be introduced directly into the plasma without the need for nebulisation, thereby avoiding the inefficiency inherent in that part of the sample introduction system. Poten- tially, therefore, hydride generation allows 100% efficient injection of select trace elements into a dry plasma. Hydrides are best produced using a continuous flow system which ensures continual mixing of sample solutions

171 ASMA SPECTROMETRY IN THE EARTH SCIENCES 2 1

with the NaBH4 and the introduction of a constant flow of H_~ and hydrides into the plasma (Thompson et al., 1978a, b).

The chemistry of hydride formation is very complex (see I. Jarvis and Jarvis, 1992 for dis- cussion) and conditions for op t imum per- tbrmance vary considerably between different elements and between different sample matrices. Consequently, fully quantitative simultaneous determination of all hydride-forming elements does not appear to be possible. The problem is compounded by sample preparation requirements, since no single sample de- composit ion method appears to be suitable for the entire suite of elements. Nevertheless, the high efficiency of analyte transfer via hydride generation and the removal of interferences caused by matrix elements, yields ICP-AES limits ofquanti tat ion (0.01-0.4/tgg l ) forthe hydride elements which are at least 1 to 2 orders of magnitude better than those achievable lbr unseparated samples. By comparison, ICP- MS can attain a similar performance for many of these elements without the need for a hydride system, although again improved sensitivity may be gained by its use (Powell et al., 1986: Wang et al., 1988 ).

12.3. Organometallic halide-forming metals"

Solvent extraction techniques have been used to separate and preconcentrate metals from complex sample matrices for analysis by FAAS for over 25 years. Identical techniques may be applied to plasma emission spectrometry or 1CP-MS analysis (Clark and Viets, 1981 a, b; Motooka, 1988 ). The methods typically involve the preconcentration of metals by tricaprylmethyl a m m o n i u m chloride (Ali- quat ~- 336)-methyl isobutyl ketone (MIBK) or Aliquat ® 336-diisobutyl ketone (DIBK), which extract the metals as metal chelate com- plexes, concentrating them in the organic phase and removing them from interferences caused by major elements such as A1, Ca, Fe, Mg and Mn. The Aliquat ® 336-DIBK mixture (Mo-

tooka, 1988) can be analysed directly by ICP- AES with minimal instrumental adjustment, enabling the quantitative determination of Ag, As, Au, Bi, Cd, Cu, Mo, Pb, Sb and Zn in HC1 extracts from geological and environmental samples. Improved determination limits of 0.03-1.5/zg g-~ may be achieved for these elements using this procedure. The technique has not yet been generally applied in ICP-MS studies, but useful improvements in determination limits would also be achieved here.

12.4. Precious metals"

The precious metals display only moderate sensitivity in plasma emission spectrometry but their high masses (%Ru -~ ~°Pd and ~84Os- ~SPt) make them particularly well-suited to determination by ICP-MS (Fig. 6). Unfortu- nately, they occur in most natural materials at very low concentrations ( 10-100 ng g- i Ag; 1 - 10 ng g-~ Au, Pd, Pt, Ru; < 1 ng g-~ lr, Os, Rh) and are very heterogeneously distributed, commonly being concentrated in discrete particles or phases, so their determination almost invariably requires a separation and preconcentration step.

Fire assay remains the commonest sample preparation procedure for precious metal analysis (Bacon et al., 1989; Van Loon and Bare- foot, 1991 ). One basic reason for the contin- ued use of this technique is the relatively large sample size which can be treated, typically one assay ton (29.167 g), which to some extend compensates for nugget effects during subsam- pling. Smaller sample sizes are generally considered to be inadequate for assessing low- grade ores. Fire assay procedures vary considerably from laboratory to laboratory and their reliability depends to a large extent on the skill of the analyst, sample composition, and the fusion flux mixture used.

Nickel sulphide fire assay is generally preferred to classical lead fire assay (Beamish, 1966: Van Loon and Barefoot, 1991 ) for multi- element analysis because it provides efficient

22 I. JARVIS A N D K.E. JARVIS

collection for most of the noble metals (it is less effective for Au), although special techniques are necessary to avoid the loss of volatile Os compounds (e.g., OsO4), particularly at the button dissolution stage. Nickel sulphide fire assay typically involves the fusion of 30-g samples at 1050°C with nickel powder, flowers of sulphur, borax, sodium carbonate and silica, the precious metals being concentrated and collected in a nickel sulphide button. The cooled NiS button is separated from the slag, ground and dissolved in HC1 to leave insoluble noble metals which are then removed by filtration. The residue may be dissolved in aqua regia for analysis by GFAAS, ICP-AES, DCP-AES or ICP-MS.

Plasma emission techniques are routinely used in combination with fire assay in some commercial geochemical exploration analysis companies, but the determination limits attainable are only sufficient to quantify the precious metals in highly mineralised samples. The use of a nickel sulphide fire assay as a sample preparation technique for ICP-MS has been described by Date et al. (1987), Gregoire (1988) and Jackson et al. (1990), and yields determination limits which are similar to or better than those achievable (Denoyer et al., 1989) using instrumental neutron activation analysis (INAA) or graphite furnace AAS (GFAAS). The ICP-MS technique has the advantage of rapidity of analysis, and avoids the need for sophisticated hardware (i.e. the nuclear reactor of INAA) and complex solution chemistry (necessary for GFAA$); sample preparation procedures are summarised in Van Loon and Barefoot ( 1991 ). Essentially, plasma spectrometry methods employ a standard NiS fire assay with dissolution of the button in 12 M HC1, and separation of the noble metals by coprecipitation with a tellurium collector. The precipitate is dissolved in warm HNO3-HC1. Routine ICP-MS limits of quantitation obtained using this procedure (Jackson et al., 1990) are: 29 ng g l Au; 2-6 ng g- 1 Pd and Pt, Ru: and < 1 ng g- l Ir, Os and Rh.

As an alternative to fire assay, the feasibility of ion-exchange separation of the noble metals has been demonstrated by Chung and Barnes (1988) who determined Ag, Au, Pd and Pt with reasonable accuracy by ICP-AES, following fusion with 4:1 lithium metaborate-lithium tetraborate, chelation of the precious metals by a polydithiocarbamate resin, decomposition of the resin with hydrogen peroxide, and digestion of the residue in 1 M HNO3. Sen Gupta (1989) adopted the alternative approach of separating matrix elements by absorption onto a cationic resin (Dowex ~ 50W- X8), and successfully determined all of the noble metals excluding Os in a range of SRM's by this method.

12.5. Petrogenetic discriminators: Hf Nb, Ta and Zr

The accurate determination of low abundances of Hf, Nb, Ta and Zr in geological materials has been found to highly informative in petrogenetic studies (Pearce et al., 1984; Hall and Plant, 1992b in this issue). Alkali fusions are necessary to ensure the complete dissolution of Hf- and Zr-bearing minerals (e.g., zircon ZrSiO4, baddeleyite ZrO~), and although Zr can be routinely determined by either ICP- AES or ICP-MS, even the low limits of quantitation for the latter (4/,g g- ~ for a LiBO2 fusion) are insufficient to enable the determination of the complete suite of elements in many sample types. Low levels of Nb and Ta (0.2- 0.3 #g g- 1 ) may be determined successfully by ICP-MS in open acid digestions, but again the quantities present in some materials may be too small to quantify.

These limitations have been addressed recently by Hall and Pelchat (1990) and Hall et al. (1990) who described a procedure for the determination of Hf, Nb, Ta and Zr in rocks, soils, sediments and plants, utilising an analyte separation procedure: a standard LiBO2 fusion is dissolved in 1.5 MHC1 containing 4-6% HF; cupferron (the ammonium salt of nitroso-

PI ~.SMA SPE( 'TROMETRY IN THE EARTH SCIENCES 23

phenyl-hydroxylamine) solution is added to coprecipitate the analytes, and the precipitate is separated and dissolved in 16 M HNO3 with the aid of H202. The final solution is analysed by ICP-MS, yielding quantitation limits of 1.2 ltg g J for Zr, and 0.06-0.08/tg g- 1 for Hf, Nb and Ta.

13. Analytical performance

Plasma spectrometry is one of the most powerful and cost effective multi-element techniques available to the geoanalyst. How- ever, ICP-AES, DCP-AES and ICP-MS have different strengths and weaknesses, which at least in the case of the two ICP techniques, make the methods complimentary rather than competitive.

13. I. ICP-AES

ICP-AES is theoretically capable of determining the majority ( > 70) of elements in the periodic table with detection limits of 0.2-25 ng ml- J for most elements. In practise, 35-40 elements can be routinely quantified in unmineralised geological samples, and this capability is extended considerably where abundances rise above background levels, as in mineralised or contaminated materials. Detection limits translate to limits of quantitation in samples of a few/tg g- ~, and even lower levels may be determined using separation procedures.

ICP-AES has now been used for the routine analysis of geological samples for over a dec- ade. Signal drift is minimal on modern instruments and may be readily corrected using data from external drift monitors as an indication of long-term sensitivity changes. A routine analytical precision of _+0.5-1% relative standard deviation (RSD) and an accuracy of + 2- 5% RSD can be expected for major- and mi- nor-element analyses of most samples, with trace elements being marginally poorer, depending on concentration and matrix type. The use of multiple internal standard methods

(Walsh, 1992 in this issue) may improve this performance further, yielding routine RSD of better than + 0.5% for many elements.

Taking into account sample preparation and instrumental limitations, the main elemental capabilities of ICP-AES in geoanalysis may be summarised:

(1) all 10 major elements (Si, Ti, A1, Fe, Mn, Mg, Ca, Na, K, P), plus Be, Cr and Zr may be determined with good precision and accuracy in solutions prepared following LiBO2 fusions, but Ge, Sn and W (also requiring fusion-based sample preparations) can be measured only in mineralised samples;

(2) determination of B requires a fusion procedure with Na2CO3;

( 3 ) at least 13 additional trace elements may be routinely determined in mixed-acid digestions employing HF-HCIO4, particularly Ba, Ce, Co, Cu, La, Li, Ni, Sc, Sr, V, Y and Zn, and in some cases Ga and Nb;

(4) aqua regia leaches of mineralised samples enable the determination of extractable portions of the above major and trace elements, plus Ag, As, Bi, Cd, Hg, Mo, Pb, U and Th;

(5) determination of the REE's (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Lu ) is readily accomplished following a LiBO2 fusion and simple cation-exchange separation procedure:

(6) As, Bi, Ge, Sb, Se, Te and Sn may be determined individually or as small groups in suitably prepared solutions by hydride generation;

(7) solvent extraction may be used to separate and preconcentrate Ag, As, Au, Bi, Cd, Cu, Mo, Pb, Sb and Zn from HC1 extracts, enabling the determination of these elements down to background levels in unmineralised or un- contaminated materials.

13.2. DCP-AES

DCP-AES shares many of the analytical characteristics of ICP-AES and has the same limitations imposed by sample preparation procedures. Detection limits are better than

24 1. JARVIS AND K.E. JARVIS

ICP-AES for the alkali metals and some alkali earths, although the refractory elements such as AI, Cr, Si, Zr are considerably less sensitive. Long-term drift over an 8-hr. period is typically 2-5% in DCP-AES, compared with 1-2% for ICP-AES. More importantly, enhanced chemical interferences caused by the lower temperature of the DCP source require the use of ionisation buffers to control ionisation effects and consequently the technique is generally characterised by poorer precision and accuracy than ICP-AES.

Despite its limitations, DCP spectrometry is inherently more robust, tolerating a wide range of sample types including slurries and organic solvents with minimal adjustment. As a result, DCP-AES is favoured in some commercial situations where low costs combined with less stringent analytical requirements justify their use.

13.3. ICP-MS

ICP-MS can theoretically be used to determine all elements in the periodic table with the exceptions of He, F and Ne, although in prac- tice this list also extends to H, C, N and O. ICP- MS spectra are simple (Fig. 6) and peaks are readily identified from mass tables. The sensitivity of an element is controlled by its degree of ionisation in the ICP and the abundance of the isotope being measured. A plot of sensitivity against mass (when corrected for degree of ionisation and isotopic abundance) exhibits a relatively smooth response curve across the entire mass range, with slightly poorer values for lighter elements such as Li and B. Never- theless, detection limits are low, typically < 1 ng ml - I for the lightest elements, while for heavy elements, such as the REE's, PGE's, Th and U, values are better than 0.05 ng ml - 1.

An upper limit of 2000 #g m l - ~ TDS is generally imposed on any solution or slurry presented for analysis, since exceeding this level causes blocking of the sampling cone orifice, and leads to rapid loss in sensitivity. Conse-

quently, dilution factors are highly significant in evaluating analytical performance. Low detection limits translate into quantitation limits in the solid of between 10 and 500 ng g-J, taking into account the limitations imposed by dissolved solids. Therefore, quantitation limits in samples prepared by mixed HF-HC104 acid digestion are generally considerably lower than those obtained by plasma emission spectrometry. However, if samples are prepared using an alkali fusion, the dilution factors required are larger (typically - 5 0 0 0 ) and as a result, quantitation limits for many light elements are comparable or poorer than those achievable by ICP-AES.

Short-term precision is usually better than + 2% RSD for most elements in "real" samples. Long-term reproducibility is highly de- pendent on sample matrix, even when TDS levels are < 2000 ~tg m1-1, since minor depo- sition on the sampling cone orifice can cause significant signal loss over short periods of < 1 hr. However, reproducibility may be improved considerably, to better than + 5%, by using external drift correction procedures or by employing carefully selected internal standards.

Although ICP-MS instrumentation is capable of determining most elements in the periodic table at ultra-trace levels, sample preparation still remains the limiting factor for most analyses:

( 1 ) alkali fusions are best avoided because of the high dilution factors required, but are essential for elements such as Zr and Cr, as well as Ta, Nb and REE's in certain rock types. Ma- jor elements may be determined in these preparations, but the relatively modest precision attainable limits the application of such data for many geological purposes.

(2) Many trace elements, including Li, Sc, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Nb, Mo, Sn, Sb, Cs, Ba, Ta, W, TI, Pb, Bi, Th and U, can be successfully determined after a mixed-acid digestion in many sample types.

( 3 ) Elements such as Be, Ge, Cd, Te and Hg, which tend to occur at very low levels ( <0.5

PLASM & SPECTROMETRY IN THE EARTH SCIENCES 25

/~g g - ' ) in unmineralised geological samples, cannot be determined routinely without a prior separation and preconcentration procedure.

(4) The REE's are straightforward to determine and for most rock types require no separation from their matrix. They may be determined routinely in either alkali fusion solutions or acid digests. However, at the very low levels found in ultrabasic rocks, for example, separation and preconcentration may be required.

( 5 ) The sensitivity of ICP-MS makes it ideal for the determination of the PGE's and Au. However, with the exception of some ore con- centrates, most samples are not sufficiently en- riched with respect to these elements to enable their determination in normal sample preparations, so separation of the group using fire assay or ion exchange is normally required prior to analysis.

14. Overview and comparison with other techniques

Plasma spectrometry displays the following analytical characteristics:

14.1. ICP-AES

14.1.1. Advantages. The main positive features of ICP-AES are:

( 1 ) very rapid ( 1-2 min.) simultaneous multi-element analysis;

(2) low detection limits of 0.2-25 ng ml -J for most elements;

(3) linear calibration curves over 4-6 orders of magnitude;

(4) suitable for the simultaneous determination of major and trace elements using a single solution;

( 5 ) good precision ( + 0.5-1% ), reproducibility and accuracy;

(6) capable of routinely determining 35-40 elements;

(7) freedom from chemical interferences and relative freedom from other matrix effects;

( 8 ) not prone to serious analytical bias when

unusual samples are run in a routine analytical programme;

(9) good performance for elements difficult to determine by many other techniques (B, Be, S, P, Ti, V, REE's);

(10) ability to analyse small sample sizes and small solution volumes (0.01 g in 1-2 ml is feasible);

( 11 ) ease of use, good reliability and inherent safety of modern instrumentation.

14.1.2. Disadvantages. Negative aspects are: ( 1 ) the technique is destructive; (2) some elements (notably Cs, Rb) have

very poor sensitivities which preclude their routine determination;

(3) spectral interferences limit the range of trace elements determinable in many sample types;

(4) analytes are best determined in solution (this can be an advantage in some cases);

( 5 ) sample preparation is time-consuming; (6) relatively high running costs (offset by

rapid sample throughput) and high capital costs for large simultaneous systems.

14.2. DCP-AES

Advantages and disadvantages of DCP-AES are broadly comparable with ICP-AES, although the number of elements available for simultaneous determination is limited to ~ 20 on current commercial systems. However, notably poorer sensitivity for refractory elements, enhanced chemical interferences and significant instrumental drift lead to generally poorer overall precision and accuracy for DCP-AES data. Consequently, DCP-AES has generally only found widespread use in commercial situations where lower capital and consumable costs (Ar consumption is lower) combined with less stringent analytical requirements, merit the use of DCP-AES as opposed to ICP-AES methods.

26 1. JARVIS a N D K.E. JARVIS

14.3. ICP-MS

Advantages and disadvantages relating to sample preparation and introduction, and some aspects of the ICP source are shared between ICP-AES and ICP-MS.

14.3.1. Advantages. The main positive features of ICP-MS are:

(1) rapid (3-5 min.) simultaneous multi- element analysis;

( 2 ) ultra-low detection limits of 0.05-0. l ng ml-~ for most elements;

(3) linear calibration curves over at least 6 orders of magnitude;

(4) negligible backgrounds, virtual freedom from spectral and chemical interferences, and other matrix effects;

( 5 ) matrix-matched standards not required for accurate results;

(6) adequate precision ( _+ 2-5%) and reproducibility, good accuracy;

(7) capable of routinely determining > 65 elements;

(8) rapid isotope ratio determinations are possible, enabling stable-isotope tracer studies and the use of isotope-dilution techniques for optimum precision and accuracy;

(9) excellent performance for "difficult" elements (REE's, Ag, Au, PGE's, incompatible elements, Th, U ).

14.3.2. Disadvantages. Negative aspects are: ( 1 ) intolerant of high TDS ( < 2000/~g ml-

preferred); ultra-low detection limits may not translate to low limits of quantitation;

(2) precision inadequate for many applications of major-element data;

(3) extended washout times required for some elements;

(4) instrumental drift significant (typically > 5% per hour), requiring correction and reducing sample throughput;

( 5 ) high capital and running costs. In most geological situations, therefore, ICP-

MS is best reserved for determination of trace

and ultra-trace elements, while plasma emission spectrometry, preferably ICP-AES, is used for major- and minor-element determination. However, continuing improvements in instrumentation will undoubtedly strengthen the challenge of ICP-MS to ICP-AES for many applications.

14.4. Comparison with other techniques

Like any instrumental technique, plasma spectrometry must be viewed in the light of available alternative and complimentary instrumentation. The rapid simultaneous multi- element capability is probably the strongest feature of ICP-AES, DCP-AES and ICP-MS, allowing them to compete favourably with a wide range of techniques, including FAAS, GFAAS, X-ray fluorescence (XRF) and INAA. As discussed above, DCP-AES provides a performance which is comparable to but significantly poorer than ICP-AES, and to avoid rep- etition, only the two 1CP techniques will be considered further.

14.4.1. Atomic absorption spectrometry (AAS). ICP-AES enables the determination of a larger number of elements than FAAS although in many cases detection limits are similar. The alkali metals are more sensitive by FAAS while Cs and Rb cannot be determined by ICP-AES. However, the latter yields considerably better limits of detection for many "difficult" elements, such as those of low atomic number (B, P, S), refractories (A1, Mo, Nb, Ti, Zr) and the REE's. For many elements, GFAAS will provide order of magnitude better detection limits than ICP-AES, competing well with ICP-MS.

The main disadvantage of AAS is that it is essentially a single-element technique. Poor linearity and prominent chemical and ionisation interferences generally necessitate a number of sample dilutions, the addition of ionisation buffers, and different instrumental conditions for different element groups. De-

PLASMA S P E C T R O M E T R Y IN THE EARTH SCIENCES 27

spite these limitation, AAS instruments are in- expensive, robust and are capable of even more rapid single-element determination than plasma spectrometry. FAAS can often compete well with ICP-AES where only a few ( 1- 5) elements are required, while GFAAS provides an alternative to ICP-MS where selected elements are sought at very low levels or in very small sample volumes. However, atomic absorption techniques do not provide viable al- ternatives for large-scale multi-element studies.

14.4.2. X-ray fluorescence (XRF) . XRF has long been the instrumental method of choice for most igneous geochemists. XRF is a fully multi-element technique which is theoretically capable of determining a comparable range of elements to ICP-MS, including the halogens. In practise, elements lighter than F are highly insensitive and ICP-AES is a better method for B, Be and Li, and gives improved detection limits for Na, Mg and A1. XRF is more sensitive than ICP-AES for some high-mass elements such as Nb, Rb, Sn, Th and U but cannot compete with ICP-MS in this area.

Unlike plasma spectrometry, XRF is principally a solid sample-based technique, so direct comparison of detection limits is difficult, although in principle the performances of the two methods are broadly similar. XRF is capable of better precision (typically < _+0.5% RSD) than plasma spectrometry, but because of large matrix interference corrections the accuracy of the two methods is approximately the same. XRF is generally regarded, therefore, as being better than ICP-AES for major-element determinations, particularly where narrow compositional ranges are to be analysed and standards can be closely matched to samples. However, variable and possibly unpredictable sample suites may result in considerable analytical bias, which is generally not introduced using plasma-based techniques. Additionally, ICP-MS provides considerably better detection limits and isotopic capabilities which XRF cannot match.

Long counting times and background measurements mean that XRF is slower than plasma spectrometry for trace-element determinations, although rapid analysis is possible if only major and minor elements are sought. Sample preparation requirements are very different and XRF is certainly better suited to the characterisation of highly refractory and homogeneous materials, but the situation is less clear cut for general rock and mineral analysis. Lastly, XRF instruments are expensive, capital costs being comparable to ICP-MS, although running costs are generally lower.

14.4.3. Instrumental neutron activation (INAA). Comparisons between INAA and plasma spectrometry is more difficult than for other techniques since the requirement of hav- ing access to a nuclear reactor precludes the application of INAA in many laboratories. A range of multi-element ( > 30) packages are commercially available by INAA which like XRF, is essentially a solid sample method. Sample preparation is minimal, merely requiring the loading of a representative powdered sample into a plastic vial. INAA, like ICP-MS, is particularly sensitive for the REE's, Sc, Co, Cr, Cs, Hf, Ta, Th and U, and is virtually free from matrix effects, although overlap interferences occur from complex gamma spectra.

Historically, the precision and accuracy of INAA data have been highly variable and have not compared well with other instrumental techniques. Improved procedures now ensure that acceptable results ( _+ 2-5% RSD) can be obtained for many elements (e.g., REE's), although some others will be much poorer ( > -2-_ 10% RSD). Research applications have generally been confined to determination of the REE's, Ta, Hf and Th, although much larger multi-element packages are in routine use by the mineral exploration industry. Following cation-exchange preconcentration, ICP-AES is capable of producing as good or better REE data than INAA, while ICP-MS produces a similar performance to INAA for the determi-

2 8 1. JARVIS AND K.E. JARVIS

nation of REE's, Cs, Ta, Hf, Th, U, and is also capable of providing data on a very wide range of other elements in unseparated samples.

INAA is slower (although sample preparation is much faster) and more expensive than plasma spectrometry, requiring access to very sophisticated equipment (a nuclear reactor), and with the advent of ICP-MS and the increasing unpopularity of the nuclear industry, its use is declining.

15. Future trends

Plasma spectrometry is undoubtedly well established as a solution-based routine analytical technique. However, considerable research effort is continuing on expanding the capabilities of plasma systems to include solid samples by using such methods as laser ablation microanalysis (Th, "pson and Walsh, 1989; Moenke-Blankenburg, 1989; Moenke-Blan- kenburg and Giinther, 1992 in this issue) and slurry nebulisation (Halicz and Brenner, 1987; K.E. Jarvis, 1992 in this issue) methods. To date, these approaches have led to encourag- ing, if somewhat limited, success, and although they are clearly capable of producing qualitative data in a wide range of potentially difficult sample matrices, they as yet provide little challenge for solution-based analysis in terms of analytical quality. Microanalytical applications bear more promise for ICP-MS than emission techniques because of the greater sensitivity and therefore potential for trace- element analysis of the former.

Detection limits for trace elements are commonly compromised by spectral interferences from major concomitant elements in plasma emission spectrometry, and by TDS levels imposed by sample preparation schemes in ICP- MS. The removal of major elements by cation exchange prior to the determination of the REE's exemplifies the degree to which limits of quantitation can be improved if sample pre- treatment is undertaken. A range of separation methods including on-line solvent extraction,

ion chromatography and high-performance liquid chromatography (HPLC) are currently being investigated (Moore, 1989; K.E. Jarvis et al., 1991 ) to increase further the range of elements determinable. Methods of reducing detection limits and improving the ability to analyse small sample sizes are being developed using electrothermal vaporisation (ETV; Ma- tusiewicz, 1986) and flow injection (FI; La- Freniere et al., 1985; Christian and Ruzicka, 1987; Eaton et al., 1992 in this issue). Finally, much work is continuing on both liquid sample introduction systems and data manipula- tion to further improve detection limits and analytical precision.

The speed of plasma analysis has led to sample preparation increasingly becoming a bot- tle-neck to sample throughput. The development of alternative sample preparation methodologies such as microwave digestion (Kingston and Jassie, 1988; I. Jarvis, 1992: Totland et al., 1992 in this issue), and increasing automation of sample preparation and laboratory management systems (Borsier, 1992 in this issue) are providing improvements in this area, although considerable scope for increasing productivity remains. There is little doubt that plasma spectrometry will become an increasingly popular, versatile and highly cost effective geoanalytical technique.

Acknowledgements

Our knowledge and experience of plasma spectrometry has been gained over the last l0 years working with many enthusiastic ana- lysts. We would like to acknowledge the help of Jan Barker, Simon Chenery and Nick Walsh while using their Natural Environmental Re- search Council (NERC) funded ICP-AES instrument in the Department of Geology King's College, London. We also acknowledge the support of Instruments SA (U.K.) Ltd., RTZ Mining and Exploration Ltd. and Kingston Polytechnic, and the assistance of Simon DeMars, Tim Pearce, Vince Phelan and Mar-

PI.ASMA SPECTROMETRY IN THE EARTH SCIENCES 29

ina Totland with our current ICP-AES instruments. The ICP-MS Facility at Royal Hollo- way and Bedford New College is funded by NERC and the U.K. Ministry of Defence. Sup- port from VG Elemental is gratefully acknowledged.

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Plasma spectrometry in the earth sciences: techniques, applications and future trends

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