Solid sampling by electrothermal vaporization in combination with electrostatic particle deposition...

Ž .Spectrochimica Acta Part B 54 1999 657]668

Solid sampling by electrothermal vaporization incombination with electrostatic particle deposition forelectrothermal atomization multi-element analysis q

Thomas Buchkamp, Gerd HermannU

Physikalisches Institut, Justus-Liebig-Uni ersitat Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany¨

Received 5 October 1998; accepted 5 February 1999

Abstract

A novel combination of electrothermal sample vaporization from one furnace and electrostatic deposition of theaerosol on a L’vov platform in a second graphite furnace used for subsequent electrothermal atomization multi-ele-ment analysis is described. The aerosol generated by vaporization of liquid as well as solid primary samples istransported to the graphite furnace by an Ar gas flow and is piped into the furnace through the dosing hole via aglass capillary mounted on the autosampler arm of a continuum source coherent forward scattering spectrometer.The deposition on the graphite platform is obtained electrostatically by a corona-like discharge. The near quantita-tive deposition of the produced and transported aerosol allows optimal direct determination of the transportefficiencies by comparing the signals obtained by measuring liquid samples directly with the spectrometer with signalsobtained with samples transferred with electrothermal vaporization and electrostatic deposition. Over all transferefficiencies up to 30% are observed with liquid primary samples. Results obtained with solid sampling of BCR CRM189 Wholemeal flour are in good agreement with the certified values. Q 1999 Elsevier Science B.V. All rightsreserved.

Keywords: Electrothermal vaporization; Electrostatic deposition; Transport efficiency; Solid sampling; Simultaneous multi-elementdetermination

1. Introduction

In the areas of environmental research, toxi-cology, physiology and biochemistry, the quantita-

q This paper was published in the Special Issue of the ThirdEuropean Furnace Symposium, Prague, Czech Republic, June1998.

U Corresponding author. Tel.: q49-641-9933120; fax: q49-641-9933119; e-mail: [email protected]

tive determination of trace elements in difficultmatrices has become increasingly important. Thetime-consuming procedure of wet digestion withthe risk of contamination and analyte losses andthe use of concentrated acids that must be dis-posed of can be avoided by using direct samplingtechniques. In combination with inductively cou-pled plasma atomic emission and mass spec-

Ž .trometry ICP-AES and ICP-MS , several directsampling techniques have been established such

w xas laser ablation 1]3 and electrothermal vapor-

0584-8547r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 5 8 4 - 8 5 4 7 9 9 0 0 0 3 1 - 2

( )T. Buchkamp, G. Hermann r Spectrochimica Acta Part B: Atomic Spectroscopy 54 1999 657]668658

Ž . w xization ETV using metal filaments 4,5 andw xgraphite furnaces 6]10 . In comparison with

other direct sampling techniques, ETV offers es-sential advantages. The sample can be introducedinto the vaporizer directly as a solid and thematrix can be removed with an appropriate ther-mal pretreatment step. Several authors have in-vestigated the transport efficiency obtained withETV-ICP-MS and -AES and improved the latterby using different modifiers as physical carriersw x7,11]17 .

For determining trace concentrations, elec-trothermal atomization atomic absorption spec-

Ž . w xtrometry ETAAS and solid sampling 18 withw xslurries, e.g. Miller-Ihli 19 , direct sample intro-

w xduction with cup or boat techniques 20]26 orŽ . w xwith laser ablation LA 27 are employed.

Multi-element capability is achieved with aŽ . w xcontinuum source CS in CS-AAS 28,29 or CS

Žcoherent forward scattering spectroscopy CS-. w xCFS 30]34 . Employing LA in combination with

CS-CFS allows solid sampling simultaneousw xmulti-element determinations 35,36 .Ž .With electrostatic deposition ED in graphite

tubes, high sampling efficiencies are found forairborne particulates as well as for artificially

w xgenerated aerosols 35]42 . Sampling efficiencies)99.9% in a GF are reported for airborne parti-cles with aerodynamic diameters down to df0.3

w xmm 37 and 99.7% for aerosols generated by LAw xwith d-100 nm 35 deposited on a platform.

In this work, a novel combination of elec-trothermal vaporization of a sample from one

Ž .ETV with electrostatic deposition ETV-ED ofthe aerosol on a L’vov platform in a secondgraphite furnace for subsequent analysis with CS-CFS is reported. This combination employs theadvantages of the ETV technique with the multi-element capability of CS-CFS. Furthermore, themulti-line capability of CS-CFS, especially theability to make measurements with extremelyweak atomic lines, allows one to extend the ana-lytical working ranges by many orders of magni-tude, and to determine major constituentstogether with traces. ETV-ED-CS-CFS allowsanalyses of liquid as well as solid samples.

This ETV technique also allows determinationof the transport efficiencies directly by compar-

ison of signals, obtained by measurements of liq-uid samples with signals achieved by employingelectrothermal vaporization and electrostatic de-position, assuming almost quantitative elec-trostatic deposition of the transported aerosol.

The particle distribution on the platform sur-face and the sizes of the electrostatically de-posited particles are obtained with scanning elec-

Ž .tron micrographs SEM for a vaporized liquidprimary sample.

2. Experimental

2.1. Reagents

ŽNitric acid standard stock solutions 1.000". Ž .0.002 grl Merck KGaA, Darmstadt, Germany

were used for the preparation of single andmulti-element standard solutions. Double dis-tilled water was used for dilution of nitric acidŽ .Suprapur, Merck KGaA, Darmstadt, Germanyin order to obtain a final concentration of 2%nitric acid. All solutions were stored in ethylenepropylene copolymer containers. BCR CRM 189

ŽWholemeal Flour Institute of Reference Materi-.als and Measurements IRMM, Geel, Belgium

was used as a solid sample.

2.2. Apparatus

The experimental arrangement is shown in Fig.1. For the ETV system a graphite furnace from

Žan AAS instrument SM-30 with power supply,Grun Analytische Mess-Systeme GmbH, Ehring-¨

.shausen, Germany was used which is designedfor solid sampling Zeeman atomic absorption

Ž .spectrometry SS-ZAAS employing the boatw xtechnique 20,22]24 . The ETV system consists of

the graphite furnace with a longitudinally heatedŽand pyrolytically coated graphite tube length,

outer and inner diameters 53, 10, and 8 mm,.respectively without sampling hole and a pair of

tweezers mounted on a sledge for reproducibletransfer of the pyrolytically coated boat as a sam-ple carrier into and out of the furnace.

The furnace has two gas inlets, one on eachside of the graphite tube. With few modificationsthis furnace is used as a ‘flow-through’ vaporizer.

( )T. Buchkamp, G. Hermann r Spectrochimica Acta Part B: Atomic Spectroscopy 54 1999 657]668 659

Fig. 1. Schematic diagram of the experimental arrangement.Ž .a Electrostatic precipitator with glass capillary and inserted

Ž .wire. b ETV unit with loaded platform in the first furnaceŽ . Ž .GF , connections for carrier gas Ar and secondary gas1 1Ž .Ar and manual shutter for boat introduction as well as2

Ž .pyrolysis gas outlet. c CFS-spectrometer with lamp, crossedŽ .polarizers P and P , magnet, the second graphite furnace1 2

Ž .GF glass capillary introduced and detector system.2

One side of the furnace is equipped with a screwas a manual shutter to seal the furnace during thevaporization step. On this side the carrier gasentered the tube. The secondary gas flow enteredthe tube on the other side. A slim cone shaped

Žglass adapter length 150 mm, width and narrower.inner diameters 24 and 4 mm is fitted to thisŽside, connecting a Tygon tube Norton, Akron,

.OH, USA to the furnace. The produced aerosolis piped to a second GF via the Tygon tubeŽ .length 100 cm and inner diameter 4 mm .

For aerosol introduction through the samplinghole of the graphite tube of the analytical CFSinstrument, a new dosing arm for the autosamplerŽ .AS-60 Bodenseewerk Perkin-Elmer GmbH wasconstructed. Instead of the normal spring anchorfor the sampling capillary, a laboratory made,rotatable, pre-adjusted holding device wasmounted on the top of the arm. This device heldthe standard liquid sampling capillary and a

Žlaboratory made glass capillary inner diameter.approx. 0.6 mm mounted in the opposite direc-

Ž .tion for aerosol sampling Fig. 2 . In order toswitch between liquid and aerosol sampling theholding device can be easily rotated by 1808between the two pre-adjusted positions.

To introduce the aerosol particles, the glasscapillary is fed through the sampling hole by theautosampler and the aerosol is blown against the

Fig. 2. Photograph of the laboratory made autosampler armŽ .a rotatable holding device with two pre-adjusted positions

Ž . Ž .for, b normal liquid sample capillary, c laboratory madeglass capillary for aerosol samples.

platform inside the graphite furnace. For elec-Žtrostatic deposition, a HV electrode 50-mm stain-

.less steel wire is inserted into the glass capillarywhich is equipped with a Y-formed end-piececombining the electric and gas flow connections.The distance between the electrode and thegraphite platform is approximately 1.5 mm. Byapplying a negative voltage of 1.2 kV via a resistorof 30 MV a corona-like discharge is obtainedwith a mean current of approximately 10 mA.

High-temperature calibration of the ETV fur-nace was carried out with an optical pyrometer

Ž .PB06AF2 Keller GmbH, Ibbenburen, Germany¨while the drying temperature was measured with

Ža digital thermometer GTH 1200 A Greisinger.electronic GmbH, Regenstauf, Germany . Table

1a shows the temperature program for the ETV.The light source of the CS-CFS spectrometer is

Ža Xe short-arc lamp XBO 450Wr4, Osram.GmbH, Germany , the atomizer is a model HGA-

Ž600 with an AS-60 autosampler Bodenseewerk


¨ .Perkin-Elmer GmbH, Uberlingen, Germany , andthe detector is an optical multi-channel analyzerŽOMA, Model 1420 UV, EG and G, Princeton

.Applied Research coupled to a 0.25 m spectro-Ž .graph Jarell-Ash Europe S.A. that allows the

simultaneous detection of line intensities over awavelength interval of 50 nm. The CFS approachfor generating positive transmission line intensi-

w xties on the atomic absorption lines 30]36,43]46 ,as well as further details of the spectrometer, are

w xreported elsewhere 30,31,35,36 . Table 1b showsthe temperature program for the HGA-600.

2.3. Procedures

For optimization in the limit of low samplemass, nitric acid aqueous solutions prepared fromstandard stock solutions were used. An appropri-ate sample volume of 10]40 ml was pipettedŽReference Pipette, Eppendorf-Netheler-Hinz

.GmbH, Hamburg, Germany into the graphiteboat which was then inserted into the ETV. Aftera drying step, the sample was vaporized and elec-trostatically deposited on the platform inside the

graphite furnace of the CFS spectrometer. Thegraphite furnace was at room temperature duringthe ED step. Peak areas of CFS line intensitieswere evaluated.

For the solid sample measurements of the stan-dard reference material BCR CRM 189Wholemeal flour 100 mg for the determination ofMn, respectively, 600 mg for the determination ofCu and Fe, was placed in the graphite boat and

Žweighed with a M5SA balance Mettler-Toledo.GmbH, Germany and covered with 20 ml 5%

HNO . The boat was inserted into the ETV and3

the sample vaporized after an appropriate pyroly-sis step. Bracketing calibration against aqueoussolutions was used with multi-element standardsolutions containing the elements of interest inconcentrations slightly lower and higher than thesolid sample.

In order to prevent water condensation insidethe ETV furnace outlet, tubing and glass capillaryduring the drying step, the furnace was kept openon the side of the manual shutter and a secondarygas flow from the opposite side of the furnaceused at a flow-rate of 1500 mlrmin to sweep outthe vapor. The same was done during the pyroly-sis step with BCR CRM 189 Wholemeal flour.

Table 1

( )a Furnace program for ETV1. Drying step 40 s at 1208C2. Pyrolysis step 60 s at 4008C3. Intermediate step 20 s for closing the ETV, positioning of the capillary in the graphite furnace by the

autosampler of the CFS spectrometer and allowing the Ar gas stream and the dischargeto stabilize

4. Vaporization step 8 s at 25008C5. Intermediate step 15 s for opening the ETV, positioning of the capillary in the park position by the

autosampler of the CFS spectrometer6. Cleaning step 6 s at 25008C

( )b Furnace program for HGA-600Step Temperature Ramp time Hold time Internal gas flow-rate

Ž . Ž . Ž . Ž .8C s s mlrmin

Cool down 20 1 9 300Atomize 2500 0 6 0Cool down 20 1 9 300Clean 2500 0 6 300Cool down 20 1 9 300


3. Results and discussion

3.1. Optimization of ETV operation parameters

3.1.1. Carrier gas flow-rateThe carrier gas flow-rate was varied from 50 to

400 mlrmin. The variation of the carrier gasflow-rate in this range had no influence on thedischarge. Fig. 3a shows ETA-CFS signals forlead for different Ar flow-rates. Maximum peak

area is achieved by using a gas flow-rate of 150mlrmin. A decrease of the signal was observedfor lower as well as higher gas flow-rates. Themeasurements show small pre-peaks at gas flow-rates between 200 and 400 mlrmin which arepresented with higher magnification as shown inFig. 3b. These pre-peaks are likely caused bydepositing analyte partly on the tube wall withincomplete deposition on the platform at higherflow rates. In the case of lower gas flow-rates thelosses are likely caused by diffusion of the parti-

Ž . Ž .Fig. 3. Time-resolved CFS signals obtained with 4 ng Pb in the ETV for different carrier gas flow-rates: a full time interval; bexpanded time-window of magnified pre-peaks at higher flow rates.


cles to the surfaces of various components of thetransport system.

3.1.2. Secondary gas flow-rateIn addition to the carrier gas flow, a secondary

or coolant gas flow with cold Ar was introducedbehind the evaporation tube in an effort to accel-

w xerate rapid formation of a stable aerosol 11 . Fig.4 shows the response for lead for different combi-nations of carrier and cooling gas flow-rates at aconstant total flow of 150 mlrmin obtained with 4ng Pb primary sample in the ETV. The maximumsignal is achieved without a cooling gas flow. Thedecrease in signal is likely caused by the fact thatthe entrance of the cooling gas flow is too farbehind the graphite furnace tube. Consequently,aerosol formation is not enhanced and turbu-lences caused by the coolant gas flow result inlosses in the adapter tube.

3.2. Transport efficiencies of the ETV-ED processwith liquid primary samples

Assuming almost quantitative electrostatic de-

position of the produced aerosol, reaching thegraphite furnace of the CFS-spectrometer, allowsus to determine the transport efficiencies by com-parison of signals achieved by direct measure-ments of liquid samples with signals obtained byemploying electrothermal vaporization and elec-trostatic deposition.

3.2.1. Dependence on the gas flow-rateThe total efficiency comprises the efficiencies

of the electrothermal vaporization process, thetransport of the aerosol and its electrostatic pre-cipitation. Fig. 5 shows transport efficiencies fordifferent carrier gas flow-rates with a primarysample of a nitric acid solution containing 4 ng Pbin the ETV. A maximum transport efficiency of15.2"0.2% is obtained by employing a transportgas flow-rate of 150 mlrmin. Each point repre-sents an average of five measurements. The stan-dard deviations are in the range of 1.6]8.5%.

3.2.2. Dependence on the sample massThe transport efficiency as a function of the

primary sample mass is shown in Fig. 6 for a nitric

Fig. 4. CFS signals from deposited mass employing 4 ng Pb dosed as nitric acid aqueous solution into the ETV for differentcombinations of carrier and cooling gas flow-rate.


acid solution of Pb. The transport efficiency in-creases from approximately 10% with a primaryanalyte mass of 1 ng Pb to approximately 20% fora mass of 20 ng. Each point represents an averageof five measurements. The standard deviationsare in the range of 3]5%. For lower samplemasses a non-linear behavior is observed. Thesame effect is reported from ETV-ICP-MS forsmall sample masses in the absence of additional

w xcarrier 12,13,16 . Higher primary sample massesare not shown because the observed Pb CS-CFSline intensity reaches saturation.

3.2.3. Dependence of the analyte transport efficiency( )on the mass of added Pd NO modifier3 2

Fig. 7 shows the transport efficiency of a con-stant primary analyte mass of 1 ng of Pb as afunction of added Pd-modifier mass. The transportefficiencies increase from approximately 10%without modifier to more than 21% with additionof 200 ng Pd modifier on the boat in the ETV.The standard deviation is 1.5]6.2%.

3.2.4. Transport efficiencies with a multi-elementsolution

The transport efficiency as a function of thecontent of a multi-element solution used as pri-

mary sample is shown in Fig. 8. The total metalmass introduced into the ETV was varied from148 to 594 ng with constant element ratio. For thevolatile elements that present by far the highestmass fraction clearly increased transport effi-ciencies up to 29% for Pb, 28% for Zn and 24%for Mn are found. For Fe only a slight, and for Nialmost no increase of the transfer efficiency isobserved. This is likely caused by the fact that thelower volatile elements are released later thanthe volatile physical carrier generators. Thetransport efficiencies were reproducible withinstandard deviations of 0.5]2%.

3.3. Analytical results with solid sampling of BCRCRM 189 Wholemeal flour

Table 2 shows results of the determination ofMn, Cu and Fe in the standard reference materialBCR CRM 189 Wholemeal flour. The measuredconcentration is in good agreement with the certi-

Ž .fied value. The relative standard deviation R.S.D.is 7.6% for Mn, 9.1% for Cu and 5.2% for Fe.With the used sample masses the contents of Znand Pb are in the range of the limits of detectionand could not be determined simultaneously.

Fig. 5. Transport efficiencies with 4 ng Pb dosed as nitric acid aqueous solution into the ETV for different carrier gas flow-rates.


Fig. 6. Determined deposited Pb mass transferred with ETV-ED into the CFS spectrometer as a function of the primary mass of PbB dosed as nitric acid aqueous solution into the ETV. A non-linear behavior is observed at smaller analyte masses. The solid linesrepresent calculated transport efficiencies of 10, 15 and 20%.

Fig. 7. Transport efficiency with constant primary analyte mass of 1 ng Pb dosed into the ETV for different masses of added Pdmodifier.


Fig. 8. Transport efficiencies of Pb, Zn, Mn, Ni and Fe with constant mass ratio as a function of the total mass of the metals: B, Pb21.5 mgrg; `, Zn 672.8 mgrg; I, Mn 2.2 mgrg; l, Fe 21.5 mgrg; v, Ni 215.3 mgrg. The signals were determined simultaneously.

3.4. Distribution and size of particulates precipitatedwith ETV-ED

Fig. 9 shows a SEM micrograph of particulatesdeposited on the graphite platform with ETV-EDfrom a primary sample dosed as an nitric acidaqueous solution of 2 mg Pb into the ETV. Thedeposited particles form chains and webs of muchsmaller particles with diameters of approximately

Ž200]500 nm. The particles are unresolved -30.nm . Previous transmission electron microscopy

Ž .TEM micrographs of particulates obtained with

LA which were electrostatically deposited withthe same arrangement ranged down to 5]10 nmw x36 .

4. Conclusions

Electrothermal vaporization and subsequentelectrostatic deposition of the generated aerosolin combination with electrothermal atomizationanalysis is suited for liquid and, especially, forsolid primary samples as well. The possibility oftransferring a sample from a primary to a sec-

Table 2Ž .Accuracy and precision with BCR CRM 189 Wholemeal four ns5

Element Wavelength R.S.D. LODConcentrationŽ . Ž . Ž .nm % ngCertified Measured S.D.

Ž . Ž . Ž .mgrg mgrg mgrg

Mn 279.8 63.3"1.6 63.8 4.9 7.6 0.17Cu 324.8 6.4"0.2 6.1 0.5 9.1 0.33Fe 298.4 68.3"1.9 63.2 3.3 5.2 2.95

Primary sample mass 100 mg for Mn and 600 mg for Cu and Fe.


Fig. 9. SEM micrograph of particles deposited with ETV-EDon the platform with 2 mg Pb dosed as nitric acid aqueoussolution into the ETV showing chain and ‘web-like’ agglomer-ation of smaller particles in the border range of the depositionzone.

ondary platform of the same type, that can beanalyzed with the same analytical instrument, al-lows optimal direct determination of transportefficiencies and optimization of the transferprocess. A higher primary sample mass, achieved

Ž .by adding Pd NO modifier as well as the addi-3 2tional elements contained in multi-element solu-tions enhance physical carrier generation and in-creases the transport efficiency from 10% up toapproximately 30% for Pb dosed as primary sam-ple in aqueous solution. Increased transport ef-ficiencies are also observed for Zn and Mn withmulti-element solutions. The results obtained bysolid sampling of BCR CRM 189 for Mn, Cu andFe are in a good agreement with the certifiedvalues.

Use of other modifiers, generating physical car-riers, and improvements with the ETV are to betested with different samples, elements, and sam-ple masses. The size distribution and the depen-dence on the sample matrix for vaporized andelectrostatically deposited particles must be fur-ther investigated with SEM of higher resolutionor with TEM.

Acknowledgements

The authors gratefully acknowledge the sup-

port for the SM30 graphite furnace and graphiteparts by Grun Analytische Mess-Systeme GmbH,¨Ehringshausen, Germany. The authors are alsoindebted to Schunk Kohlenstoffwerke GmbH,Heuchelheim, Germany for support with graphiteparts and Dr Koyro of the ‘Zentrale Biotechnis-che Betriebtseinheit, Justus-Liebig-Universitat¨Giessen’ for performing SEM.

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Solid sampling by electrothermal vaporization in combination with electrostatic particle deposition...

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