Determination of the platinum-group elements and gold in solid samples by slurry nebulisation ICP-MS

Chemical Geology, 104 ( 1993 ) 175-188 Elsevier Science Publishers B.V., Amsterdam

[MB]

Determination of the platinum-group elements and gold in solid samples by slurry nebulisation ICP-MS

175

Mar ina Tot land a, Ian Jarvis a and K y m E. Jarvis b aSchool of Geological Sciences, Kingston University, Penrhyn Road, Kingston-upon-Thames, Surrey KT I 2EE, UK

bNERC ICP-MS Facility, Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 OEX, UK

(Received July 31, 1992; revised and accepted October 1, 1992)

ABSTRACT

Totland, M., Jarvis, I. and Jarvis, K.E., 1993. Determination of the platinum-group elements and gold in solid samples by slurry nebulisation ICP-MS. Chem. Geol., 104:175-188.

Inductively coupled plasma-mass spectrometry (ICP-MS) is one of the most sensitive techniques for the determination of the platinum-group elements (PGE's: Ru, Rh, Pd, Os, Ir, Pt) and Au. Published methods have demonstrated the appli- cation of ICP-MS to the determination of these precious metals in a wide range of geological materials following acid digestion with or without fire-assay preconcentration. However, existing methods require complex chemical manipulation to ensure the complete digestion of refractory minerals and the retention of volatile elements, particularly Os. In this paper we describe a new approach to the determination of the PGE's and Au by ICP-MS involving the introduction of solid samples as suspended slurries. Samples are ground to < 5 ~m using zirconia beads in polypropylene bottles. An acidified dispersing agent (tetrasodium pyrophosphate Na4P207 and aqua regia) ensures that grains are maintained in suspension during grinding and leads to partial digestion of the material. Subsamples are introduced into the ICP-MS as dilute ( < 2000 ltg ml - ~ ) aqueous slurries suspended in Na4P2OT. Modified instrumental conditions including increased forward power and nebuliser gas flow-rate, combined with a 3-mm torch injector tube, ensure no loss in analytical performance when compared to the analysis of simple dilute acid media. Calibration is achieved using a blank and a Na4P207 standard spiked with appropriate amounts of commercial single-element aqueous standard solutions. Limits of quantitative analysis for the method are 0.04-0.2/tg g- ~ in samples. Estimates of analytical reproducibility indicate 2-30% relative standard deviations (RSD's) for most elements in most materials. Analytical accuracy has been assessed by the determination ofRu, Rh, Pd, Os, Ir, Pt and Au in a range of well-characterised international PGE reference materials, and results obtained are in good agreement with reference values for all elements in all samples. The technique offers a means for the rapid determination of the precious metals in mineral separates, PGE ores and allied industrial materials.

1. Introduction

Inductively coupled plasma-mass spectrometry (ICP-MS) is a sensitive, multi-element technique which is particularly well- suited to the determination of the platinum- group elements (PGE's; Ru, Rh, Pd, Os, Ir, Pt) and Au. Several procedures have been published for determining the PGE's and Au in

Correspondence to." I. Jarvis, School of Geological Sci- ences, Kingston University, Penrhyn Road, Kingston- upon-Thames, Surrey KT1 2EE, UK.

rocks and minerals by ICP-MS (Date et al., 1987; Gregoire, 1988; Sen Gupta and Gre- goire, 1989; Jackson et al., 1990; Gowing and Potts, 1991 ), but all of these involve the preparation and analysis of sample solutions.

The requirement for a dissolution step has a number of disadvantages. Acid digestions are known to result in incomplete dissolution of the PGE's if they are present in the sample as native metal (Beamish, 1966), and acid attacks may also not totally dissolve other refractory phases (Totland et al., 1992 ), including many PGE minerals (Gowing and Potts, 1991 ). Al-

0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

176 M. TOTLAND ET AL.

kali fusions may be employed to ensure complete digestion of samples but relatively large amounts of flux are required, which increases the possibility of contamination from reagents and also necessitates further dilution of samples before analysis. Such dilutions raise considerably the lower concentration limit that can be quantified by the entire procedure. Subse- quent chemical separation of the PGE's and Au from their matrix using Te co-precipitation or ion-exchange procedures (e.g., Sen Gupta, 1989) may overcome this limitation, but add significantly to preparation times and lead to the partial loss of some analytes. In both acid digestion and fusion methods, particular care must be taken to ensure that Os, and to a lesser extent Ru, are not lost from the sample as volatile tetroxides (OSO4, RuO4). A separate preparation procedure involving distillation is typically recommended (Van Loon and Bare- foot, 1991 ) for the accurate determination of Os.

Fire assay, the method most commonly used to concentrate the PGE's (Van Loon and Bare- foot, 1991 ), is a complex procedure requiring experienced staff and specialised equipment. The large amount of flux added to samples during fire assay introduces contamination and necessitates the use of subsequent wet chemical techniques to further separate and precon- centrate the precious metals prior to analysis. Such procedures may also lead to analyte losses. Nonetheless, fire assay remains the in- dustry-standard method for precious-metal determination, favoured for its ability to deal with large sample masses (25-50 g) which reduce mineral heterogeneity ("nugget") effects. The method is amenable to a range of analytical finishes, including ICP-atomic emission spectroscopy (ICP-AES), ICP-MS, atomic absorption spectroscopy (AAS) and instrumental neutron activation analysis (INAA), which provide ng g-~ limits of determination.

Sample digestion and fusion methods are time-consuming, labour intensive and require

stringent safety precautions, and generally rep- resent a bottleneck in the modern geochemical laboratory (I. Jarvis and Jarvis, 1992). An analytical method that permits the direct analysis of solids would clearly have many advantages. Minimal sample handling, reduced sources of contamination, lower reagent blank levels, the retention of volatile species and fewer requirements for special equipment should all promote significant analytical gains. Sample dilution may also be reduced when compared to fusion methods, thus taking bet- ter advantage of the inherent sensitivity of ICP-MS.

It should be emphasised that there are a number of potential difficulties when under- taking the direct analysis of solids. Obtaining a homogeneous sample from an originally het- erogeneous material may be difficult, even when the sample is reduced to a very small particle size. Rock samples are particularly prob- lematic since they are composed of a variety of minerals that generally have widely different grain sizes, hardnesses and friabilities. The malleability of native gold is well known, mak- ing it difficult to reduce in grain size using conventional techniques and leading to nugget effects during subsampling. Fractionation can easily occur during transport of the dispersed solid sample into the ICP (Ebdon and Collier, 1988a), resulting in poor analytical data for samples in which composition varies with particle size. One further major limitation of solid analysis is calibration; in most cases, a standard with a similar matrix and chemical form to the sample is necessary to obtain fully quantitative data.

A number of different methods of solid sample introduction have been evaluated for ICP- MS. These include laser ablation, slurry nebulisation, electrothermal vaporisation, direct sample insertion, powdered solids introduction and arc nebulisation. A detailed review of these procedures is given in K.E. Jarvis et al. (1992). In this paper we describe a modified slurry nebulisation ICP-MS method to quan-

DETERMINATION OF PGE'S AND Au IN SOLID SAMPLES BY SLURRY NEBULISATION ICP-MS 177

titatively determine the PGE's and Au in geological materials.

2. Sample preparation

A slurry may be defined as a uniform suspension of fine particles which is sufficiently fluid to flow viscously. If the viscosity of the slurry is low, its behaviour in the spray chamber, torch and plasma will be similar to that of an aqueous solution. This similarity in physical properties should enable synthetic aqueous standards to be used, thereby overcoming one of the main problems of solid sample analysis, that of calibration.

A number of studies (e.g., Ebdon and Wil- kinson, 1987a, b; Ebdon and Collier, 1988a, b) have been undertaken on the slurry nebulisation of certain geological materials (coal and kaolin) into an ICP-AES. Many of the factors affecting the efficiency of slurry nebulisation (e.g., particle size, slurry viscosity, sample uptake) are similar for ICP-AES and ICP-MS because of their common sample introduction systems. Ebdon and Wilkinson (1987a) dis- cussed the optimisation of whole coal analysis by ICP-AES which was then used to analyse five coal samples for Cu, Fe, Mn, Ni and V (Ebdon and Wilkinson, 1987b). The quality of the results was assessed by comparison with certified values and results obtained by ashing and acid digestion followed by AAS analysis. The accuracy of the slurry nebulisation results varied considerable between samples and can only be considered to be semi-quantitative at best. Ebdon and Collier (1988a) investigated the effect of a range of dispersing agent concentrations on the emission signal of Mg by slurry nebulisation ICP-AES, and concluded that the dispersing agent concentration must be matched in the samples, standards and blanks to minimise differences in transport efficien- cies. Later, based on a study of five elements in a kaolin sample, Ebdon and Collier (1988b) demonstrated that the analysis of a slurry containing particles < 8 / lm could be calibrated

successfully against aqueous standards. A simple method for preparing slurries for

ICP-MS was first described by Williams et al. ( 1987 ). The applicability of this method to the analysis of geological samples was explored by K.E. Jarvis and Williams (1989), K.E. Jarvis (1992) and K.E. Jarvis et al. (1992), who demonstrated that, provided a maximum particle size of < 5-10 #m and a total suspended and dissolved solids content of < 2000 #g ml- l were achieved, slurry nebulisation ICP-MS is a viable method for the determination of trace elements in geological samples. The method holds particular promise for the determination of volatile elements and/or the analysis of refractory minerals, applications which commonly yield unsatisfactory results by conventional solution-based techniques.

2.1. Grinding

Grinding samples for slurry analysis is a critical step in the sample preparation procedure. Atomisation efficiency in the plasma is biased towards small particles and a mean grain size of < 5-10 #m is essential to ensure that the slurry has similar transport properties to an aqueous solution. Geological samples are rou- tinely ground to pass a 200 mesh (<75 ~m) sieve before bulk geochemical analysis, but further grinding is required to reduce the particle size to an acceptable limit for slurry analysis.

Various grinding methods have been used for the preparation of slurries including: micron- ising mills (Fuller et al., 1981 ); rotary disc mills (Halicz and Brenner, 1987); the "bottle and bead" method (Williams et al., 1987; K.E. Jarvis and Williams, 1989; K.E. Jarvis, 1992 ); and rotary grinders with tungsten carbide utensils (Verbeek and Brenner, 1989). K.E. Jarvis and Williams ( 1989 ) demonstrated the general effectiveness of the "bottle and bead" method for regrinding a range of geological


materials to a maximum particle size of < 5 #m (samples had already been ground to < 75/tm and homogenised using conventional techniques). Samples were weighed into a plastic bottle, zirconia beads and a small amount of dispersing agent were added, the bottles were sealed, and then shaken on laboratory flask shakers (each of which can accommodate up to 8 samples simultaneously) for the desired length of time. Measurement of the grain-size distribution after 2, 6 and 15 hr showed high variability between sample types for the shorter grinding times, and it was recommended that a minimum of 15 hr (or overnight) grinding should be used for all samples. A modified "bottle and bead" method (Section 2.3) was adopted for this study.

2.2. Dispersing agents

A dispersing agent is required to keep the ground particles suspended and to prevent

flocculation. An ideal dispersing agent for slurry nebulisation ICP-MS would introduce minimal contamination, would have a viscosity close to water and would not form polyatomic ions that might interfere with isotopes of interest. A wide range of dispersing agents has been used [see K.E. Jarvis (1992) for a review]. However, to date there has been no systematic study to evaluate their relative mer- its, and the choice seems to be based on per- sonal preference. Tetrasodium pyrophosphate (Na4P207) was used successfully by Williams et al. ( 1987 ) in their initial feasibility study of slurry nebulisation ICP-MS, has yielded good results for trace-element determinations of geological materials (K.E. Jarvis and Wil- liams, 1989; K.E. Jarvis, 1992), and consequently was chosen for the present work.

Fig. 1 compares the spectrum of a 0.05% m / v (Merck BDH ® Analar grade) Na4P207 solution in the mass region of the precious metals with that of 0.5 M (Merck BDH ® Aristar ®

300

0 C I I °31 - GO

o

"~ 300

I I I

I I

0.05% Na4P2Os blank

I I I I I I I

0.5 M HCI b lank

I I I I I

Rh Ru Ru i,. Pd

1 O0 lt02 104 106 168 1110

I ng ml "f PGE's

Ir

Ir ~ Pt Pt Pt Au

I I I I I I I

186 188 190 192 194 196 198

( m/z ) Fig. 1. I C P - M S mass spectra for 0.05% Na4P205 and 0.5 M HCl solutions compared with a 1 ng ml ~ mixed precious- metal s tandard.

DETERMINATION OF PGE'S AND Au 1N SOLID SAMPLES BY SLURRY NEBULISATION ICP-MS 179

grade) HC1 obtained under identical conditions. The comparison indicates that there is no significant additional contamination from the Na4P207 and that there are also no polyatomic ions formed by the dispersing agent in this region of the mass spectrum. The spectrum for a mixed precious-metal standard ( 1 ng ml - l Ru, Rh, Pd, Ir, Pt, Au) demonstrates the sensitivity of the instrumentation and clearly displays the relative natural abun- dances of each isotope.

2. 3. Methods

Three methods of sample preparation (A, B, C) using different reagent mixtures during grinding (Table 1 ) were evaluated for determining the PGE's by slurry nebulisation ICP- MS. In each case, the following procedure was used: (1) 0.200-g subsamples were weighed into 30-ml polypropylene bottles; (2) an appropriate mixture (Table 1 ) of acid (Merck BDH ® Aristar ®-grade) and dispersing agent (Merck BDH * Analar NaaP2OT) was added; ( 3 ) ~ 10 g of zirconia beads ( 1.65-mm-diameter Glen Creston ® ) were added; (4) bottles were sealed and shaken on a mechanical laboratory shaker for a minimum of 15 hr; (5) samples were washed from the zirconia beads into 100-ml volumetric flasks using 0.05% m / v Na4P207; (6) solutions were diluted to 100 ml with 0.05% m / v Na4P207 and transferred to 100-ml polypropylene bottles; (7) bottles were placed in an ultrasonic bath before and during analysis to maintain the samples as a

TABLE 1

Reagents used during grinding for the preparation of slurries

Method Reagent

Na4P207 Na4P207 HC1 ( 1 2 M ) aqua regia (%) (ml) (ml) (ml)

A 0.05 2 0 0 B 0.05 1.5 0.5 0 C 0.1 1 0 1

slurry. K.E. Jarvis and Williams (1989) found that a magnetic stirrer [as used by Ebdon and Collier (1988b) ] is unsuitable for geological materials because magnetic mineral phases (e.g., magnetite) are preferentially removed during mixing. The overall procedure resulted in a final dilution factor of 500, which is the minimum required for ICP-MS analysis of these preparations.

Problems specific to PGE-bearing materials were encountered when grinding samples to small particle sizes using the bottle-and-bead method. Some of the mineral phases present have layered structures and did not readily reduce in size, but instead coated the surfaces of the grinding apparatus. It was decided, there- fore, to investigate the addition of acid during the grinding step (Table 1 ) in an attempt to overcome this problem. The acid should par- tially digest the sample, including some precious-metal grains, and should result in a more homogeneous distribution of these elements in the slurry. Furthermore, the increased "wetta- bility" of sample powders caused by the presence of acid (particularly aqua regia) might improve grinding efficiency due to changes in surface tension. The use of a cold acid attack in sealed vessels should prevent the formation and loss of volatiles, particularly OSO4.

Consequently, in addition to control samples prepared using NaaP207 alone (method A), duplicate preparations were produced in

TABLE 2

Reference materials studied

Name Material Source

PTA- 1 Tulameen sand (iron sulphide ) CCRMP PTC- 1 sulphide flotation concentrate CCRMP PTM-I nickel-copper matte CCRMP SARM 7 platinum ore MINTEK 2/77 chromitite in-house standard MINTEK

CCRMP=Canad ian Certified Reference Materials Project, Energy Mines and Resources, Canada; MINTEK= Council for Mineral Technology, distributed by the South African Bureau of Standards.


TABLE 3

Reference data (/~g g- ~ ) compiled from recommended, suggested and literature values for the five reference materials studied

Material Element Recommended Suggested Sen Gupta Parry et al. (1989) (1988)

PTA- I Ru n.d. < 0.2 n.d. Rh n.d. 0.02 n.d. Pd n.d. 0.2 n.d. Os n.d. n.d. n.d. Ir n.d. 0.1 n.d. Pt 3.05 _+ 0.09 3.2 n.d. Au n.d. 0.3 n.d.

PTC-1

PTM- 1

SARM7

2/77

Ru 0.65 0.6 0.40_+0.12 Rh 0.62 +_ 0.07 0.6 0.36_+ 0.06 Pd 12.7_+0.7 11 10.6_+ 1.4 Os 0.24 n.d. 0.28_+0.04 lr 0.17 0.2 0.16_+0.02 Pt 3.0-+0.2 3.0 2.6-+0.7 Au 0.65_+0.1 0.7 0.5_+0.5

Ru 0.65 0.63 n.d. Rh 0.9 + 0.2 0.9 n.d. Pd 8.1 +0.7 7 n.d. Os n.d. n.d. n.d. Ir 0.34 0.2 n.d. Pt 5.8 _+ 0.4 6 n.d. Au 1.8 _+ 0.2 1.8 n.d.

Ru 0.43_+0.057 0.3 0.40_+0.13 Rh 0.24_+0.013 0.2 0.16_+0.02 Pd 1.53 + 0.032 1.2 1.58 -+ 0.10 Os 0.063 -+ 0.068 n.d. 0.070 -+ 0.028 Ir 0.074-+ 0.012 0.04 0.077 _+ 0.019 Pt 3.74+0.045 3.7 3.4_+0.4 Au 0.31 _+0.015 0.3 0.31 _+0.08

Ru 0.93 n.d. 0.70_+0.36 Rh 0.61 n.d. 0.51 +_0.13 Pd 1.67 n.d. 1.60-+ 0.22 Os n.d. n.d. 0.1 l -+ 0.03 Ir <0.2 n.d. 0.18-+0.03 Pt 3.11 n.d. 3.5-+0.9 Au n.d. n.d. 0.04_+0.05

Recommended and suggested values provided by originators; 95% confidence intervals are quoted for the former; determinations from Parry et al. ( 1988 ) are means and an-1 standard deviations of replicate measurements; n.d. = not determined.

which 12 M hydrochloric acid (method B) or aqua regia (method C) were added to the dispersing agent during grinding. The concentration of the dispersing agent during grinding was also varied. Solutions were analysed in 0.05% m/v NaaP207, calibration standards were also prepared in 0.05% m / v NaaP207.

2.4. Reference materials

Very few reference materials have recommended values for a significant number of the precious metals. Four relatively well-characterised international reference materials (Ta- ble 2 ) plus an "in house" standard chromitite

DETERMINATION OF PGE'S AND Au IN SOLID SAMPLES BY SLURRY NEBUL1SATION ICP-MS 18 1

supplied by the Council for Mineral Technol- ogy (MINTEK) South Africa, were used to as- sess the analytical data obtained in this study. Nevertheless, although only the most widely used precious-metal reference materials were chosen for this work, "recommended" values were not available for many elements. "Sug- gested" values were supplied for some elements, but an incomplete set of data remained, particularly for difficult elements such as Os. Table 3 summarises the "recommended" and "suggested" (or "preferred" ) PGE concentrations for the five materials studied and lists selected literature values. The combined data set allowed a more complete evaluation of our method.

3. Instrumentation

ICP-MS determinations were carried out using a Fisons Instruments VG Elemental PQ2 Plus ® spectrometer at the NERC ICP-MS Fa- cility, Royal Holloway University of London. A summary of the operating parameters used is given in Table 4. A number of modifications were made to the operating conditions rou- tinely used for solution nebulisation. To minimise transport effects (cf. Williams et al., 1987; Ebdon and Collier, 1988a), the standard 1.5-mm-diameter torch injector was replaced by a 3-mm injector tube. Following optimisation studies for the 3-mm-diameter injector (Wills et al., 1991 ), the plasma forward ra- diofrequency power was increased to 1700 W and a higher nebuliser gas flow rate of 1.25 1 rain-1 was employed. The ICP-MS ion lens settings were optimised on 59Co and 238U to give maximum sensitivity whilst minimising polyatomic, oxide and doubly charged ion levels.

The PGE's were determined using isotopes (Table 5 ) which exhibit no interferences from isobaric overlaps. For example, 99Ru or l°lRu must be chosen to avoid potential isobaric overlap interferences from Mo (at m/z 96, 98, 100) and Pd (m / z 102, 104). Along with the

corresponding interferences from Ru, Pd also has interferences from Cd (m/z 106, 108, 110). In addition, the use of zirconia beads in the "bottle and bead" grinding procedure resulted in high concentrations of both Zr and ZrO in the sample solutions. Large peaks were observed at m/z 106, 108 and 110 corresponding to ZrO polyatomic ions which cause additional serious interferences on Pd. The isotope chosen, l°SPd, is apparently free from isobaric, polyatomic and doubly charged ion interferences, although a molecular ion interference

TABLE4

ICP-MS operating parameters for slurry nebulisation

Instrument Plasma gas Plasma power Reflected power Generator Coolant gas flow Auxiliary gas flow Nebuliser gas flow Nebuliser Sample uptake rate Torch injector diameter Spray chamber

Ion lens settings Scan regions

VG PlasmaQuad PQ2 Plus ® Ar 1700 w < l W 27.12 MHz 14 1 min- 0.5 1 min 1.25 1 rain- De Galan ® high dissolved solids 0.5 ml min -1 3.0 mm water-cooled single-pass spray;

chamber maintained at 4 ° C optimised on 59Co and 238U 97-199 m / z with skip mass

region 111-177 m/z

TABLE5

Comparison of ICP-MS detection limits in 0.5 M HC1 and 0.05% m/v Na4P207, and quantitation limits for the slurry nebulisation procedure

Element Mass Detectionlimits Quantitationlimits (ng m1-1 ) (/~g g-i )

HCI *t Na4P207 .2 Na4P207 *z

Ru 99 0.22 0.11 0,19 Rh 103 0.031 0.025 0.041 Pd 105 0.17 0.087 0.14 Os 189 0.18 0.034 0.057 Ir 191 0.073 0.022 0.037 Pt 195 0.11 0.090 0.15 Au 197 0.065 0.12 0.19

*~Standard operating conditions; *2modified operating conditions.


from copper argide (65Cu4°Ar+) might be pre- dicted in Cu-rich materials. However, analyses of a nickel-copper matte (PTM-1 ) containing 30.2% Cu displayed no evidence of anoma- lously high Pd values (Section 4.5 ).

There are isobaric interferences on Os from W (m/z 184, 186), Re (m/z 187) and Pt (m/ z 190, 192), leaving 188Os and 189Os available for analysis. Pt has additional overlaps with Hg (m/z 196, 198) but 194pt or ~95pt maybe used. Au and Rh are both monoisotopic with no significant interferences (no 63Cu4°Ar+ interference on ~°3Rh was observed). 191Ir is also free from isobaric overlaps, but determinations at 193Ir were compromised by a small t77Hf160 interference caused by contamination from the zirconia beads, which contain high levels of Hf. Interferences caused by 178Hf~60, 179Hf160 on 194pt, 195pt were insignificant because of the much higher concentrations of Pt in samples. In the cases of Ru, Os and Pt where two isotopes are potentially available, the isotopes chosen were those which yielded the best reproducibility and accuracy during this and related studies.

A calibration standard was prepared by di- luting 1000/tg ml-1 stock PGE and Au solutions (Johnson Matthey ®) in 0.05% m / v Na4P207 to produce a solution containing 200 ng ml-1 of each analyte. It has been shown (K.E. Jarvis and Williams, 1989) that no further matrix matching should be necessary. The calibration standard was run prior to the start of sample analysis and after every two samples to monitor signal drift. An initial sharp drop in sensitivity due to sample condensation on the sampling cone was avoided (cf. K.E. Jarvis et al., 1988; Williams and Gray, 1988) by as- pirating a slurry solution continuously for 10 rain prior to calibration and analysis. Further loss of signal was compensated for by using the standard as an external drift moni tor and em- ploying a simple linear drift correction. This enabled slurry nebulisation of samples to pro- ceed over several consecutive days with no significant blockage of the sampling cone orifice

or contamination of the torch. To ensure that the slurries remained well mixed before and during analysis, sample bottles were placed in a small ultrasonic tank.

4. Results and discussion

The applicability of the technique for determining the PGE's and Au in geological materials was assessed on measured detection and quantitation limits, as well as the precision and accuracy of data obtained for the five reference materials.

4.1. Detection limits

Detection limits provide a measure of the lowest level at which the presence of an analyte may be qualitatively identified; quantitative analysis is not possible at such levels. How- ever, instrumental detection limits may be used to evaluate the sensitivity of an analytical technique and are generally calculated from measurements of the variability of a background signal. In this work, detection limits were calculated as three times the standard deviations of eleven replicate analyses of a reagent blank, and were expressed as concentrations in solution (ng ml-~) . Table 5 lists the calculated precious-metal detection limits for a 0.05% m / v Na4P207 blank solution and the corresponding detection limits for a 0.5M HC1 blank analysed using the standard ICP-MS operating conditions. It can be concluded that the opti- m u m operating conditions for slurry analysis ( 3-mm-diameter torch injector with increased plasma power and nebuliser flow rate) provide a similar instrumental performance to that observed during the analysis of dilute acid solutions under standard operating conditions. The presence of Na4P207 has no detrimental effect on detection limits, enabling < 0.1 ng ml-1 of most of the PGE's to be detected.

DETERMINATION OF PGE'S AND Au IN SOLID SAMPLES BY SLURRY NEBULISATION ICP-MS 18 3

4.2. Comparison between grinding methods

The three sample preparation procedures were evaluated based on results obtained for the three best characterised reference materials: SARM 7, PTM-1 and PTC-I. Each material was prepared in duplicate by methods A and B, and in triplicate by method C. The av- erages and standard deviations (based on triplicate analyses of each preparation ) are given in Table 6 and are compared to the reference values for Ru, Rh, Pd, Ir and Pt. Os was not included in this study due to initial difficulties experience in its determination by ICP-MS, but was successfully incorporated later. Au was also excluded from the comparison because an erratic signal, and both increasing and high background levels (probably caused by carry over between samples) prevented accurate determinations of Au in materials prepared in Na4P2Ov alone (method A ) or in the Na4P207-

HC1 mixture (method B). This problem was not observed for samples containing aqua regia (method C).

The best agreements (Table 6) with reference values are shown by method C for SARM 7 and PTM-I, while methods A and C proved comparable for PTC-I. In contrast, grinding the sample in the presence of 0.05% Na4P207 and HC1 (method B) produced low results for all elements. In addition, method A resulted in material coating the plastic bottle and zirconia beads during the grinding stage, and this coating could not be removed with repeated wash- ing. This problem was reduced by the addition of HC1 (method B) but was prevented entirely in method C by the addition of aqua regia, which avoided possible sample loss. It was concluded that only method C produced acceptable data for a range of materials, and consequently this method was adopted for the re- mainder of the study.

TABLE 6

Comparison ofRu, Rh, Pd, Ir and Pt results (~g g-~ ) for three reference materials prepared by different methods

Element Method

A B C

Reference

PTC-I: Ru 0.41 +_0.04 0.13 _+0.04 0.44_+0.03 (0.65) Rh 0.55 _+0.07 0.15 _+0.03 0.46+_0.14 0.62 +_0,07 Pd 11.2 +_0.66 2.9 _+0.5 10.2 _+0.89 12.7 _+0,7 lr 0.17 +_0.07 0.050+_0.04 0.17+_0.06 (0.17) Pt 2.28 +_0.03 0.58 +_0.09 1.86+0.56 3.0 _+0.2

PTM-I: Ru 0.22 +-0.03 0.39 _+0.02 0.64+-0.03 (0.65) Rh 0.43 _+0.06 0.66 _+0.06 1.08+-0.03 0.9 _+0.2 Pd 4.38 +_0.68 5.79 +_0.02 7.92_+0.02 8.1 _+0.7 lr 0.10 _+0.02 0.20 +_0.07 0.27-+0.01 (0.34) Pt 1.89 +_0.38 3.20 +_0.61 4.02_+0.06 5.8 +_0.4

SARM 7: Ru 0.19 +_0.08 0.16 +_0.04 0.38+_0.15 0.43 _+0.057 Rh 0.13 +_0.05 0.12 +_0.02 0.26+_0.04 0.24 +_0.013 Pd 1.18 +_0.08 1.01 +_0.04 1.75+_0.38 1.53 +0.032 Ir 0.084 +_ 0.039 0.066 +_ 0.022 0.11 +_ 0.03 0.074 +_ 0.012 Pt 3.19 +0.83 2.47 +-0.80 3.37+0.19 3.74 _+0.045

Values are means of duplicate (methodsA and B) or triplicate (method C) sample preparations: errors quoted as (rn-~ standard deviations.


4.3. Quantitation limits

While detection limits illustrate the sensitivity of an analytical instrument, quantitation limits provide a more realistic assessment of the minimum concentrations quantifiable, taking into account both sample preparation and analysis (I. Jarvis, 1992; Totland et al., 1992). Limits of quantitation (ACS-CEI, 1980) express the level above which there is > 90% confidence that the analysis are quantitative (Ports, 1987), and may be calculated as ten times the standard deviation for eleven determinations of a method blank, results being expressed as concentrations in the solid. Table 5 lists the quantitation limits obtained for slurries prepared by the adopted aqua regia- N a 4 P 2 0 7 procedure (method C). The val, des range from 0.04 to 0.2 mg g-1 for the PGE's and Au.

4.4. Precision

TABLE7

Intra-sample precision of three sample preparations and inter-sample precision for three reference materials prepared using method C

Element Intra-sample Inter-sample Reference precision precision concentration (% RSD) (% RSD) (/~g g-~ )

1 2 3

PTC-I: Ru 3.5 9.0 18 6.1 (0.65 Rh 11 11 27 3 l 0.62 Pd 3.8 0.8 4.0 8.6 12.7 Os 18 6.2 6.9 24 (0.24 Ir 12 21 16 52 (0.17 Pt 2.9 1.0 7.2 33 3.0 Au 5.7 5.1 10 59 0.65

PTM-I: Ru 6.7 12 l l 5.1 (0.65 Rh 3.3 4.7 3.7 2.9 0.9 Pd 6.5 4.2 4.4 0.2 8.1 Os 3.3 5.8 5.2 9.1 nd Ir 3.3 3.3 4.2 1.7 (0.34) Pt 2.5 1.0 1.6 1.6 5.8 Au 12 10 1.9 9.8 1.8

One limitation of solid sample analysis is the difficulty associated with the introduction of a representative sample into the plasma. This is particularly true for precious-metal determinations because the elements are typically very heterogeneously distributed in geological materials. Two measurements of precision were used here. The reproducibility of multiple analyses of a single slurry (intra-sample precision) was assessed, as well as the reproducibility of the slurry preparation and analysis steps (inter-sample precision). For this study SARM7, PTM-1 and PTC-I were selected, prepared in triplicate and each preparation was analysed three times. Table 7 lists both the intra- and inter-sample precision of the adopted aqua regia-Na4P207 procedure (method C), expressed as percent relative standard deviation (% RSD), for these three reference materials. The reference values for the materials are included as a guide to the relative concentrations of each element in the samples.

The reproducibility of multiple analyses of

SARM 7: Ru 5.9 20 25 40 0.43 Rh 20 6.9 1t 16 0.24 Pd 1.6 2.9 8.1 22 1.53 Os 13 7.4 11 16 0.063 Ir 27 11 18 26 0.074 Pt 2.3 5.1 10 5.6 3.74 Au 10 3.3 3.8 39 0.31

Reference concentrations are recommended or suggested (in parentheses) values.

each preparation ranged from 3-5% RSD for elements occurring at relatively high concentrations, to 27% RSD for those present at low levels. High intra-sample variability may be caused by inhomogeneity in the slurry or may be an artefact of the sample introduction sys- tem. However, the error introduced from the resulting signal fluctuations may be minimised by averaging multiple analyses. The precision of the lowest level determinations (Os, Ir) was limited by the low count rates for these elements. A separation of the PGE's from matrix elements would be required before the concen-


tration of these elements could be increased. The dilution factor used in this study was the min imum possible while maintaining the level of total solids below the max imum (2000 ¢tg ml-~ ) which can be used in ICP-MS analysis.

Inter-sample precision varied considerably (Table 7). Here, the sample preparation reproducibility seems to be predominant ly de- pendent on the material rather than on the concentration of individual elements. The RSD's measured for all elements in PTM-1 are

mended values for reference materials. The data obtained from slurry nebulisation ICP- MS (method C) of five reference materials are given in Table 8 and are compared to reference values. Standard deviations are based on the analysis of triplicate slurry preparations, val-

TABLE 8

Reference material results (Izg g ~ ) obta ined by slurry nebu- l isation 1CP-MS

below 10%, agreeing well with values obtained Material Element Slurry Reference

for intra-sample variations. In contrast, RSD's PTA- 1 Ru 0.013 _+ 0.006 ( < 0.2 )

of up to 40% were obtained for SARM7, and Rh 0.006_+0.004 (0.02) 59% RSD was measured for Au in PTC-1, Pd 0.23 +_0.01 (0.2)

Os 0.04 _+0.027 n.d. which are considerably poorer than the re- lr 0.050_+0.001 (0.~) producibility of individual sample prepara- pt 1.27 -0.04 y05 +_0.(i19 tions. This is not surprising given that the sam- Au 0.53 _+ 0.05 (0.3)

ple size used for each preparation was 0.2 g, PTC-I Ru 0.44 ±0.03 ((11.651 which is well below the recommended mini- Rh 0.46 20.14 (].62 +_0.7 m u m needed to ensure a representative Pd 10.2 _+(].89 12.7 +0.7 subsample. Os 0.26 ÷0.06 (0 .24)

lr 0.17 k0 .06 (0 .17) The effects of poor inter-sample precision on Pt 1.86 _+0.56 3.0 +_0.2

accuracy may be minimised by averaging three Au 0.78 _+ 0.46 0.65 ~ 0. I preparations of a single material, thus increas-

P T M - / Ru 0.64 ±0 .03 (0 .65) ing the overall sub-sample size. However, it is Rh 1.08 ± 0.03 0.9 ± 0.2 worth noting that all of the reference materials Pd 7.92 -+ 0.02 8. l ± 0.7 studied had been carefully prepared by their Os 0.19 +0.02 n.d. originators to ensure a high degree of sample Ir 0.27 _+0.005 (0.34)

Pt 4.02 +0 .06 5.8 _+0.4 homogeneity. Routine sample grinding is un- Au 1.52 _+0.79 1.8 _+0.7 likely to achieve such uniform compositions and would inevitably lead to a further deterio- SARM 7 Ru 0.38 _+ 0.15

Rh 0.26 _+ 0.04 ration in analytical reproducibility. In such Pd 1.75 _+0.38 cases, samples of up to 50 g could usefully be Os 0.063_+0.01 reground to a smaller particle size prior to lr 0.11 _+0.03

Pt 3.37 _+0.19 slurry preparation, so that small sub-samples Au 0.54 ±0.2~ are more representative of the bulk material. Ensuring adequate homogeneity would un- 2/77 Ru 0.67 ±0.09 (0.93)

Rh 0.46 ±0 .03 (0.61) doubtedly be difficult for some sample types Pd 1.38 +0.23 (1.67) and these would require the analysis of multi- Os 0.069 -+ 0.012 (0.11) ple preparations to obtain acceptable data. lr 0.16 ± 0.008 ( < 0.2)

Pt 3.62 -+0.04 (3 .11) Au 0.31 -+0.06 (0.04)

4.5. Accuracy of the slurry procedure

The accuracy of a method may be assessed by comparison of measured and recom-

0.43 ±0 .057 0.24 _+0.013 1.53 _+0.032 0.063_+0.0068 0.074 _% 0.012 3.74 _+. 0.045 0.31 ~- 0.015

Slurry values are m e a n s o f triplicate sample preparat ions; errors quoted as a n - ~ s tandard deviations. Reference data from Table 3, r e c o m m e n d e d values; suggested or l i terature values in parentheses .


ues for which were the means of three separate determinations. Good agreement between measured and reference values was obtained for Ru, Rh, Pd, Os, Ir and Au for all five materials. Platinum results ranged from higher than the preferred value in 2/77 [although our data agree with results by Parry et al. (1988), see Table 3], reasonable agreement for SARM 7, and lower than recommended values for PTA-1, PTC-1 and PTM-I. However, a final experiment which involved doubling the concentration of the dispersing agent (1 ml 0 . 2 % m / v N a a P 2 0 7 + 1 ml aqua regia) during grinding and analysis produced good Pt results (2.45 + _ 1.6/~g g - i ) for PTC-1. The increased concentration of NaaP207 did not increase signal loss due to cone blockage, and possible signal suppression due to higher levels of Na was compensated for by calibrating against standards in the same concentration of dispersing agent.

5. Conclusions

A modified aqua regia-based slurry nebulisation ICP-MS procedure has been described for the quantitative determination of Ru, Rh, Pd, Os, Ir, Pt and Au in geological materials. Only a simple sample preparation involving regrinding samples in an aqua regia-Na4P2Ov mixture is needed to determine all seven elements, including Ru and Os. The method displays significant advantages over other analytical procedures available for the precious metals, particularly for the determination of Os where a lengthy distillation step is usually recommended. The simplicity and low cost of sample preparation enables our method to be easily adopted in any ICP-MS laboratory. A single analyst may achieve a sample through- put of more than 20 samples per day (including re-grinding and analysis), resulting in a highly economical and rapid method for precious-metal determination.

The levels that can confidently be quantified range between 0.04 and 0.2 #g g- 1, enabling the

routine determination of the PGE's and Au in mineralised samples. The absence of a dissolution step avoids the problems of incomplete digestion of refractory minerals and/or the loss of volatile species. The small sample size of 0.2 g necessitates a high degree of homogenisation during the initial stages of sample preparation, and adequate homogeneity might be difficult to achieve for some geological materials. The analysis of multiple slurry preparations would be necessary to obtain accurate data in such cases .

Slurry nebulisation ICP-MS provides an ex- citing new tool for the exploration geochemist. The small minimum sample size required will allow the rapid quantification of the PGE's and Au in heavy-mineral concentrates or separates and in a variety of industrial materials. Such data would usefully augment existing bulk- sample assays which provide little information concerning the partitioning of elements between different mineral phases.

Acknowledgements

This work was funded via an RTZ Mining and Exploration Research Studentship to M.T., held at Kingston University. The encourage- ment and support of Drs. Nick Badham and Chris Carlon (RTZ) are gratefully acknowl- edged. Mr. Robbie Robert (MINTEK) kindly provided us with the in-house reference chromitite 2/77. The ICP-MS Facility at Royal Holloway University of London is supported by NERC and the Ministry of Defence. Con- structive criticism by three anonymous refer- ees provided us with a well-balanced view of the analytical difficulties associated with the routine determination of the precious metals. This paper was first presented at the Third In- ternational Symposium on Analytical Chem- istry in the Exploration, Mining and Process- ing of Materials, Johannesburg, August 1992.


References

ACS-CEI (American Chemical Society, Committee on Environmental Improvements), 1980. Guidelines for data acquisition and data quality evaluation in environmental chemistry. Anal. Chem., 52: 2242-2249.

Beamish, F.E., 1966. The Analytical Chemistry of the No- ble Metals. Pergamon, Oxford, 609 pp.

Date, A.R., Davis, A.E. and Cheung, Y.Y., 1987. The potential of fire assay and inductively coupled plasma source mass spectrometry for the determination of platinum group elements in geological materials. An- alyst (London), 112: 1217-1222.

Ebdon, L. and Collier, A.R., 1988a. Direct atomic spectrometric analysis by slurry atomisation, Part 5. Anal- ysis of kaolin using inductively coupled plasma atomic emission spectrometry. J. Anal. At. Spectrom., 3: 557- 561.

Ebdon, L. and Collier, A.R., 1988b. Particle size effects on kaolin slurry analysis by inductively coupled plasma-atomic emission spectrometry. Spectrochim. Acta, 43B: 355-369.

Ebdon, L. and Wilkinson, J.R., 1987a, Direct atomic spectrometric analysis by slurry atomisation, Part 1. Optimisation of whole coal analysis by inductively coupled plasma atomic emission spectrometry. J. Anal. At. Spectrom., 2: 39-44.

Ebdon, L. and Wilkinson, J.R., 1987b. Direct atomic spectrometric analysis by slurry atomisation, Part 3. Whole coal analysis by inductively coupled plasma atomic emission spectrometry. J. Anal. At. Spectrom., 2: 325-328.

Fuller, C.W., Hutton, R.C. and Preston, B., 1981. Com- parison of flame, electrothermal and inductively coupled plasma atomisation techniques for the direct analysis of slurries. Analyst (London), 106:913-920.

Gowing, C.J.B. and Ports, P.J., 1991. Evaluation of a rapid technique for the determination of precious metals in geological samples based on a selective aqua regia leach. Analyst (London), 116: 773-779.

Gregoire, D.C., 1988. Determination ofPt, Pd, Ru and lr in geological materials by ICP-MS with sample introduction by electrothermal vaporisation. J. Anal. At. Spectrom., 3: 309-314.

Halicz, L. and Brenner, 1.B., 1987. Nebulization of slurries and suspensions of geological materials for inductively coupled plasma-atomic emission spectrometry. Spectrochim. Acta, 42B: 207-217.

Jackson, S.E., Fryer, B.J., Gosse, W., Healey, D.C., Lon- gerich, H.P. and Strong, D.F., 1990. Determination of the precious metals in geological materials by inductively coupled plasma-mass spectrometry ( ICP-MS) with nickel sulphide fire-assay collection and tellurium coprecipitation. In: P.J. Ports, C. Dupuy and J.F.W. Bowles (Guest-Editors), Microanalytical

Methods in Mineralogy and Geochemistry. Chem. Geol., 83:119-132 (special issue).

Jarvis, 1., 1992. Sample preparation for ICP-MS. In: K.E. Jarvis, A.L. Gray and R.S. Houk (Editors), Hand- book of Inductively Coupled Plasma Mass Spectrom- etry. Blackie, Glasgow pp. 172-224.

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

Jarvis, K.E., 1992. Role of slurry nebulisation for the analysis of geological samples by inductively coupled plasma spectrometry. In: 1. Jarvis and K.E. Jarvis (Guest-Editors), Plasma Spectrometry in the Earth Sciences. Chem. Geol., 95:73-84 (special issue).

Jarvis, K.E. and Williams, J.G., 1989. The analysis of geological samples by slurry nebulisation inductively coupled plasma-mass spectrometry, (ICP-MS). Chem. Geol., 77: 53-63.

Jarvis, K.E., Williams, J.G., Totland, M.M. and Gray, A.L., 1988. The introduction of high dissolved solids into ICP-MS: an assessment of potential problems. 1st Int. Conf. on Plasma Source Mass Spectrometry, Dur- ham 1988, Poster Abstr.

Jarvis, K.E., Gray, A.L. and Houk, R.S., 1992. Handbook of Inductively Coupled Plasma Mass Spectrometry. Blackie, London, 380 pp.

Parry, S.J., Sinclair, I.W. and Asif, M., 1988. Evaluation of the nickel sulphide bead method of fire-assay for the platinum-group elements using neutron activation analysis. In: H.M. Prichard, P.J. Ports, J.F.W. Bowles and S.J. Cribb (Editors), Geo-Platinum '87. Elsevier Science, Barking, pp. 21-27.

Ports, P.J., 1987. A Handbook of Silicate Rock Analysis. Blackie, London, 622 pp.

Sen Gupta, J.G., 1989. Determination of trace and ultra- trace amounts of noble metals in geological and related materials by graphite-furnace atomic-absorption spectrometry after separation by ion-exchange or co-precipitation with tellurium. Talanta, 36:651-656.

Sen Gupta, J.G. and Gregoire, D.C., 1989. Determina- tion of ruthenium, palladium and iridium in 27 international reference silicates and iron-formation rocks, ores and related materials by isotope dilution inductively coupled plasma-mass spectrometry. Geostand. Newsl., 13: 197-204.

Totland, M.M., Jarvis, I. and Jarvis, K.E., 1992. An assessment of dissolution techniques for the analysis of geological samples by plasma spectrometry. In: I. Jar- vis and K.E. Jarvis (Guest-Editors), Plasma Spec- trometry in the Earth Sciences. Chem. Geol., 95: 35- 62 (special issue).

Van Loon, J.C. and Barefoot, R.R., 1991. Determination of the Precious Metals--- Selected Instrumental Meth- ods. Wiley, Chichester, 276 pp.


Verbeek. A.A. and Brenner, I.B., 1989. Slurry nebulisation of geological materials into argon, argon-nitrogen and argon-oxygen inductively coupled plasmas. J. Anal. At. Spectrom. 4: 23-26.

Williams, J.G. and Gray, A.L., 1988. High dissolved solids and ICP-MS: Are they compatible? Anal. Proc., 25: 385-388.

Williams. J.G., Gray, A.L., Norman, P. and Ebdon, k.,

1987. Feasibility of solid sample introduction by slurry nebulisation for inductively coupled plasma mass spectrometry. J. Anal. At. Spectrom., 2: 469-472.

Wills, J.D., Jarvis, K.E. and Williams, J.G., 1991. A method for reducing the physical effects of high dissolved solids in ICP-MS analysis. 27th Colloq. Spec- trosc. Int., Bergen, June 1991, Abstr. A-3.3.

Determination of the platinum-group elements and gold in solid samples by slurry nebulisation ICP-MS

Documents

Transcript of Determination of the platinum-group elements and gold in solid samples by slurry nebulisation ICP-MS