Direct Determination of VolatileElements in Nickel Alloys byElectrothermal Vaporization InductivelyCoupled Plasma Mass Spectrometry

MICHAEL W. HINDS *a, D. CONRAD GREGOIRE b AND ELISA A. OZAKI c

aRoyal Canadian Mint, 320 Sussex Drive, Ottawa, Ontario, Canada K1A 0G8bGeological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K1A 0E8cV illares Metals SA, Rod. Anhangvera Km 113, Nova Veneza, Sumare, CEP 13177–900, SP, Brazil

A method is described for the direct determination of Bi, Pb owing to the co-vaporization of analyte with matrix compo-nents. This concept was studied in greater detail by Vanhaeckeand Te in solid Ni alloys by ETV-ICP-MS. Samples are

introduced into the graphite tube as small filings or chips et al.9 who used the argon dimer as both a tool for detectingmatrix interferences and for use as an internal standard forweighing up to 3 mg. Using diluted sea water as a physical

carrier, both Bi and Pb could be determined in solid Ni using direct solids analysis by ETV-ICP-MS. Fonesca and Miller-Ihli10 reported on the use of Pd as a physical carrier andexternal calibration with aqueous samples although results for

Pb were biased low. Better results in terms of accuracy and chemical modifier and oxygen ashing for the analysis ofbiological RMs by slurry sampling ETV-ICP-MS. Vanhaeckeprecision were obtained when solid RMs (Ni) were used for

calibration. LODs of 14 and 44 ng g−1 were obtained for Bi et al.11 used solid sampling ETV-ICP-MS for the direct deter-mination of As in RMs of plant origin. A detection limit forand Pb, respectively, using a reduced sensitivity mode

(OmniRange). Based on signals obtained for solution standards As of 1 ng g−1 was reported. Ren et al.12 reported on theanalysis for Cd in solid samples compressed into graphitemeasured at the highest sensitivity, LODs of 0.002 and

0.004 ng g−1 are possible for Bi and Pb, respectively. The pellets. The graphite pellet was mounted in a custom-madeETV device and heated electrothermally to produce a signal.determination of Te by this technique was not successful using

either solution or solids calibration. Tellurium did not show a The requirements for successful direct solids analysis wereoutlined by Moens et al.13 along with a review of the currentlinear instrument response with concentration, which was

probably due to an interaction between the Te and one or more literature on the subject. A second review on solid samplingusing electrothermal devices interfaced with ICP atomic emis-matrix components in the solid phase that alters the release

mechanism(s) for Te from those observed for Pb and Bi. sion and mass spectrometers was recently published by Darkeand Tyson.14

Keywords: Electrothermal vaporization; inductively coupled The objective of the present study was to extend workplasma mass spectrometry ; solid sample; nickel alloys; volatile completed on the direct analysis of solid Ni using ETAAS toelements ETV-ICP-MS and to demonstrate the applicability of the

technique for the analysis of metals for volatile trace elements.Trace levels (mg g−1) of Bi, Pb, Te and other volatile elementsare known to alter the mechanical properties of steels and EXPERIMENTALnickel alloys.1 Rapid and accurate determinations of these

Instrumentationelements during the melting process are required to maintainboth material quality and the mechanical properties of the A Perkin-Elmer SCIEX ELAN 5000 ICP mass spectrometeralloy. Sample dissolution is slow due to the corrosion resistant equipped with an HGA-600MS electrothermal vaporizer wasproperties of these alloys. The determination of trace elements used. The electrothermal vaporizer system was equipped within Ni alloys by direct weighing solid sample ETAAS has been a Model AS-60 autosampler. Pyrolytic graphite coated graphiteshown to be effective using calibration with solid RMs2,3 and tubes were used throughout. The experimental conditions forwith aqueous standards.4,5 More recently, Te and Sb were the ELAN 5000 and the HGA-600MS are given in Table 1.determined in Ni alloys by solid sample LEAFS in a graphite Optimization of plasma and mass spectrometer conditions wasfurnace.6 It was considered that ETV-ICP-MS would also be accomplished using solution nebulization sample introductionapplicable to the solid sampling problem. and aqueous standards (High Purity Standards, Charleston,Sample introduction using ETV is well suited to the directanalysis of solid samples. Along with providing femtogramlevels of detection, ETV-ICP-MS allows for in-tube sample Table 1 Instrumental operating conditions and data acquisitionpre-treatment with possible elimination of interfering species parametersand the use of chemical modifiers to vaporize either analyte

ICP mass spectrometer—or matrix components selectively. In this way, both spectralRf power/W 1000and non-spectral interferences can be avoided.Outer argon flow rate/l min−1 15.0Solid sampling for analysis using ETV-ICP-MS has been Intermediate argon flow rate/ml min−1 850reported by Voellkopf et al.7who used a ‘cup-in-tube’ technique Carrier argon flow rate/ml min−1 900

and slurry sampling for the analysis of coal. Gregoire et al.8 Sampler/skimmer Nickelused slurry sampling for the direct analysis of biological

Data acquisition—materials and coal. LODs ranged from 0.07 ng g−1 for Co toDwell time (ETV) 20 ms3.2 ng g−1 for Cr using 2 mg samples. Gregoire et al.8 used theScan mode Peak hoppingargon dimer at m/z=80 as a tool to monitor the analyte signal Points per spectral peak 1for possible interference effects such as signal suppression

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NC, USA). The HGA-600MS was interfaced to the argon valve switching. No pyrolysis step was required. The optimizedatomization temperature was chosen to volatilize the analytesplasma via an 80 cm length of 6 mm (id) PTFE tube. The

operation of the HGA-600MS was completely computer con- but also to vaporize a minimum amount of the metal matrix,which avoided saturating the detector and causing analytetrolled. During the dry and pyrolysis stages of the temperature

program, opposing flows of argon gas (300 ml min−1) originat- signal suppression. This temperature (1300 °C) is close to themelting-point (1453 °C) of Ni metal.ing from both ends of the graphite tube removed water and

other vapours through the dosing hole of the graphite tube.Prior to and during the high temperature or vaporization step,

Reference Materials and Solutionsa graphite probe was pneumatically activated to seal the dosinghole. Once the graphite tube was sealed, a valve located at Samples were Ni-based high-temperature alloy RMs:one end of the HGA-600MS workhead directed the carrier Tracealloys A and B, SRMs 897 and 898 (NIST, Gaithersburg,argon flow originating from the far end of the graphite tube MD, USA) and BCS CRMs 345 and 346 (Bureau of Analyseddirectly to the argon plasma at a flow rate of 900 ml min−1. Samples, Middlesbrough, Cleveland, UK). Metal RMs came

in the form of fine turnings which did not require any furthertreatment. Samples (0.3–1.5 mg) were weighed on a Model

Electrothermal Vaporizer Temperature Program UM3 balance (Mettler Instruments, Greifensee, Switzerland).Once each sample had been weighed, it was wrapped in aThe optimized temperature program used for the electrother-

mal vaporizing unit (HGA-600MS) is shown in Table 2. A small piece of pre-cut white paper and labelled with the RMnumber and the mass in milligrams. The calibration of theclean-out step begins the program to remove any residual

analytes present in the graphite furnace. For conventional balance was verified with a 1 mg weight traceable to theCanadian prototype of the kilogram, K74 (Institute of Nationaltemperature programs used in ETAAS, a clean-out step often

immediately follows the atomization step. The current con- Measurement Standards, National Research Council ofCanada, Ottawa, Ontario, Canada).figuration of the HGA-600MS ETV-ICP-MS system does not

allow the sealing probe to be raised and for vapours to be Calibration solutions were prepared by serial dilution from1000 mg ml−1 stock solutions (High Purity Standards) withvented through the dosing hole, once the high-temperature

vaporization step is engaged. A cleaning step placed at the end 1% HNO3 (Seastar Chemical, Sidney, British Columbia,Canada). Distilled, de-ionized water (Millipore, Bedford, MA,of the heating cycle would unnecessarily contaminate the valve

system, transport tube, plasma torch and interface with large USA) was used throughout.amounts of Ni.

The second temperature program step allows adequate timeRESULTS AND DISCUSSIONfor manually placing the small solid sample into the graphite

tube. For the analysis of liquid samples, an autosampler Vaporization of Analyteinjection volume of 20 ml plus 10 ml of NASS-3 (diluted A typical transient signal is shown for all three analytes and500-fold) Open Ocean Seawater (National Research Council the Ni matrix in Fig. 1. The peak shapes for each analyteof Canada, Ottawa, Ontario, Canada) was used. The addition returned to the baseline well before the end of the heatingof diluted sea water provides for a physical carrier used cycle. This observation is consistent with complete analyteprimarily to equalize the transport of analyte be it from vaporization from the solid sample, although very little of theaqueous standards or solid samples.15 For solid metal samples, matrix was vaporized. It was also observed that for Pb and Bia disposable glass pasteur pipette (which was cut to an outside the determined concentrations were in agreement with RMdiameter of 2 mm, 5 mm from the tip) was placed into the concentrations. This could not have occurred unless completedosing hole of the graphite furnace to act as a funnel. A pre- analyte vaporization was achieved. Higher vaporization tem-weighed sample was picked up with curved nosed forceps and peratures were tried but too much Ni matrix was vaporizeddropped into the funnel. The funnel was then removed. This and could potentially alter the sampling orifice to the masswas followed by the addition of a 10 ml aliquot of NASS-3 sea spectrometer. Solid Ni samples remained in the graphite tubewater (diluted 500-fold) as for the solution samples. The time virtually unchanged after several high-temperature heating cycles.required to complete this step may be reduced from that No analyte signal was observed when the remaining Ni samplesuggested (Table 1) as one becomes more skilled in handling was re-heated to 1300 °C following the initial (measurement)the funnel and solid samples. The argon gas flow was stoppedduring this step to prevent the metal chips from being blownback out of the vaporizer.

Two drying steps were used: one to dry the samples and asecond to prevent pressure build-up and to allow the argonplasma to stabilize prior to the high-temperature measurementstep. During the second drying step, the dosing hole of thegraphite tube is sealed and the switching valve is activated.This occurs 5 s prior to the measurement step. The additional5 s at zero internal flow prevents a build-up of pressure during

Table 2 ETV temperature program

Temperature/ Gas flow/Step °C Ramp/s Hold/s ml min−1 Flow

Clean out 2700 1 3 300 VentSample in* 20 1 90 0 VentDry I 120 120 45 300 VentDry II 120 1 10 900 ICPVaporize 1300 0 8 900 ICP/readCool 20 1 15 900 ICP

Fig. 1 ETV-ICP-MS peak profiles for Ni, Bi, Pb and Te from a solidsample of SRM 898 Nickel Alloy.* Sample+10 ml NASS-3 (diluted 500-fold).

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Fig. 3 Ni ETV-ICP-MS background signals for: A, maximum powerheating to 1300 °C; B, 1 s ramp heating to 1300 °C. Line C is the 13Csignal recorded during maximum power heating to 1300 °C, whichcorrelated with the surface temperature of the graphite (ETV).

Fig. 2 ETV-ICP-MS peak profiles for Bi, Pb and Te from a solidsample of Nickel Alloy BCS 346.

heating cycle. It was visually observed that the metal samplechanged from an irregular shape to a smoother and flattershape indicative of melting (without the clean-out step at2700 °C). This observation is consistent with those reported inthe literature,16 where it was found that Si was completelyremoved from a solid Au sample without vaporization of themetal sample. A follow-up study by Hinds et al.17 showed thatthe movement of the analyte from the bulk to the surfacecould not occur owing to diffusion but rather through convec-tive currents within the sample bulk moving to the surface,where the analyte is vaporized. These currents in the molten

Fig. 4 Ni ETV-ICP-MS signals obtained during A, the first samplesample are believed to be induced by surface tension inhomo- vaporization (new vaporizer) and B, subsequent sample vaporization.geneities which occur as a result of a continuous increase inthe temperature during the early stages of analyte vaporization. re-condensed metal from within the graphite tubes or contactMultiple analyte peaks, as shown in Fig. 2, were commonly cones rather than from a fresh sample. The Ni signal obtainedobserved and could be indicative of the presence of different using a new graphite tube containing a fresh Ni sample isphysical forms of analyte species, as has been suggested from shown in Fig. 4 (curve A). No large transient signal wasETAAS solid sampling studies. Multiple peaks did not occur observed. However, a second temperature cycle using the samefor every sample, however, double peaks were observed for sample (no other sample added) showed a Ni signal (curve B)each analyte studied. Impurity elements (such as Pb, Bi, and similar to that obtained in Fig. 3 (curve A). The Ni contributingTe) in Ni and Ni alloys could be located on the outside surface to this signal originated from metal which had vaporized,of the sample particle, in the grain boundaries within a metal re-distributed and condensed throughout the graphite tubesample and within the bulk of the metal. There is, however, and contact cones during the clean-out step beginning theinsufficient information concerning the forms of analyte species second cycle. The first cycle also began with a clean-out cyclethat are present in the complex alloys used in this investigation but without solid Ni sample present.with which to correlate the observed multiple peaks. During the high-temperature vaporization step, the amount

of Ni vaporized was insufficient to result in analyte signalsuppression. No change in the argon dimer signal at m/z=80Vaporization of Matrixwas observed during this study indicating that analyte signalIt is of interest to note that the Ni signal, along with those of was unaffected by the vaporization of Ni.8,9the trace analytes, also returned to the baseline during the

vaporization step. The Ni signal generated using two heatingramp rates is shown in Fig. 3. The 1 s (curve B) ramp rate Analytical Results(time to a pre-set maximum temperature) produced little signal

Aqueous solution calibrationwhereas a substantial Ni peak (curve A) resulted from maxi-mum power heating (‘0’ s ramp). It is probable that the A comparison of analyte concentrations determined in solid

samples by ETV-ICP-MS with calibration by aqueous solu-transient nature of the Ni signal is due to the temperatureover-ramp (heating above the maximum pre-set temperature) tions is given in Table 3. The determination of Pb by this

calibration scheme gives concentration values that are close towhich occurs in maximum power heating. An isotope ofcarbon, 13C, was recorded during a maximum power ramp the reference concentration values for BCS 346 and for NIST

SRM 897, but are systematically biased low. Samples with lowand is also shown in Fig. 3 (curve C). A correlation betweenthe 13C signal and the graphite surface temperature has been concentrations of Pb such as BCS 345 and NIST SRM 898

gave Pb concentrations that are about half of the reportedshown by Gilmutdinov et al.18 The 13C signal followed asimilar transient pattern as observed for Ni, which confirms values with aqueous calibration. This could occur because of

differences in the mechanism of vaporization of the Pb in thethat analyte and matrix vaporization occurs as a result of thetemperature over-shoot. Ni sample and the Pb in the aqueous sample when vaporized

at 1300 °C. Complete vaporization–atomization has beenThe observed Ni signal probably originates from

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Table 3 Determined concentration of analytes (mg g−1 ) in Ni andsteel RMs by different calibration schemes

Calibration method

ReferenceSample Aqueous Solid sample* value

Pb—BCS 346 16.1±1.9 19.3±3.7 21±2BCS 345 0.11±0.02 0.14±0.03 0.2NIST SRM 897 9.29±1.19 11.7±1.5 11.7±0.8NIST SRM 898 1.02±0.22 2.2±0.4 2.5±0.6

Bi—BCS 346 9.47±1.07 9.87±1.03 10±1BCS 345 0.0083±0.0028 0.0075±0.0022 <0.2NIST SRM 897 0.53±0.08 0.56±0.08 <0.5NIST SRM 898 1.02±0.22 1.07±0.23 1

T e—BCS 346 0.50±0.23 38±10 12±1NIST SRM 897 0.20±0.03 8.6±1.3 1.05±0.07NIST SRM 898 0.09±0.01 4.6±1.4 0.54±0.02

* Signals from the four highest mass BCS 346 samples were used assolid sample calibration standards.

observed from the same RMs by ETAAS5 at higher vaporiz-ation–atomization temperatures. Unfortunately, heating at

Fig. 5 Analyte mass versus peak area response from solid Ni alloythese temperatures would also vaporize large amounts of the samples by ETV-ICP-MS (a) x Pb and & Bi; (b) 2 Te.Ni alloy, which would have ultimately caused analyte signalsuppression during ICP-MS measurements. Despite the slightly

possible that there is an interaction between the Te and onelow bias of the results, solution calibration is convenient andor more matrix components in the solid phase, which alterscould best be used as a rapid screening method where onethe release mechanism(s) for Te from those observed for Pbseeks to identify samples that have analyte levels above orand Bi. This variability in the release mechanism for Te frombelow certain concentration levels. More accurate determi-different alloys makes it impossible for the method of standardnations, if required, can be completed on only those samplesadditions to be used because there is a considerable differenceflagged by rapid screening.in vaporization for Te from aqueous standards as well as fromResults for Bi showed that the reference values are includeddifferent alloys. It may be possible to use an internal standardwithin the data spread from the determinations of each RMbut this must have the same release mechanisms and also beexamined. This verifies that calibration with solution standardspresent in known concentrations which would be extremelyis an accurate method for determining Bi from solid Ni anddifficult to find in a solid sample.steel alloy pieces.

As shown in Table 3 the Pb and Bi concentration determinedIt should be noted that accurate results using aqueousfor each RM overlapped with the corresponding referencestandards were only obtained after the graphite tube had beenvalue. In general, this was as expected. However, as notedtreated with a solid Ni sample. It is probable that the residualpreviously, the calibration was carried out with solid samplesamount of Ni remaining in the graphite tube acts as a physicalof one RM (BCS 346 ), which has a different composition fromcarrier along with added diluted sea water.the other NIST SRMs used in this study (Table 4). ThisTellurium in solid Ni alloys (discussed below) did not showsuggests that for all the metal alloys examined in the presenta linear instrument response to analyte mass, which indicatedstudy the release of Pb and Bi is virtually complete after onethat Te was a poor candidate element for this analyticalvaporization cycle. This is supported by the observation thatmethod. The results displayed in Table 3 confirm thesethe analytical signal for each analyte returns to the baselineobservations. Neither aqueous calibration nor solid sampleduring vaporization (Fig. 2) and no significant analyte signalcalibration resulted in Te concentration values that matchedwas observed during a second vaporization cycle.the RM values.

Table 4 Elemental composition (%) of the RMs used in the studySolid sample calibration

Element BCS 345/346 NIST SRM 897/898Plots of the integrated signal versus the analyte mass in a solidsample (sample mass multiplied by the certified concentration) C 0.15 0.12

Si — —are shown for Bi and Pb [Fig. 5(a)] and Te [Fig 5 (b)]. DifferentMn — —RMs were used to cover a wide analyte mass range. To a firstCr 10 12approximation, a direct correlation is shown for both Bi andMo 3 —Pb, which indicates that the analyte mass can be directly Al 5.5 2

related to instrument response over two orders of magnitude. Co 15 8.5This may also be an indication that the processes involved in Ti 5 2

V 1 —analyte release from the solid and in analyte vaporization areW — 1.7similar for the different metal alloy RMs used in this experi-Nb — 0.9ment. However, Te did not show this correlation [Fig. 5 (b)].Ta — 1.7It is thus unlikely that Te can be determined in this case by Hf — 1.2

direct solid sampling because a linear correlation is not Ni 60.35 69.88observed for the analyte response and analyte mass. It is

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Table 5 Analytical figures of merit (n=10, k=3) to the determination of volatile elements in other samplesheated, as for Ni, almost to the melting-point of the sample.

Reduced High Alternatively, changing the physical form of the sample to aAnalyte LOD sensitivity* sensitivity more highly divided state may make the low-temperatureBismuth Absolute 13 pg 0.002 pg removal of a volatile analyte possible. Research in these areas

Relative† 13 ng g−1 0.002 ng g−1 of study are currently underway in these laboratories.Lead Absolute 44 pg 0.004 pg

The authors are grateful to S. Gilbert for technical assistanceRelative† 44 ng g−1 0.004 ng g−1during the course of this work. GSC publication No. 1996134.

* OmniRange settings were used to attenuate analyte and samplesignal to avoid detector saturation for most samples determined.

REFERENCES† Based on a 1 mg solid sample.

1 ASM International Committee, Metals Handbook, ASMInternational, Materials Park, OH, 10th edn., 1990, vol. 1, p. 950.

L imits of detection 2 Marks, J. Y., Welcher, G. G., and Spellman, R. J., Appl. Spectrosc.,1977, 31, 9.The LODs are summarized in Table 5 for Pb and Bi. These 3 Backman, S., and Karlsson, R. W., Analyst, 1979, 104, 1017.values were estimated from ten replicate determinations of a 4 Headridge, J. B., and Riddington, I. M., Mikrochim. Acta, 1982,

single Ni alloy sample (after an initial ETV firing that removed 11, 457.analytes from the sample). Two sensitivity settings were used 5 Irwin, R. L., Mikkelsen, A., Michel, R. G., Dougherty, J. P., and

Preli, F. R., Spectrochim. Acta, Part B, 1990, 45, 903.for these estimates. Under normal operation with the Ni alloys6 Liang, Z., Lonardo, R. F., and Michel, R. G., Spectrochim. Acta,used in this study, the sensitivity was attenuated using the

Part B, 1993, 48, 7.OmniRange facility on the mass spectrometer system in order 7 Voellkopf, U., Paul, M., and Denoyer, E. R., Fresenius’ J. Anal.to bring the signals on scale. This is achieved automatically Chem., 1992, 342, 917.by changing an applied voltage on the mass spectrometer 8 Gregoire, D. C., Miller-Ihli, N. J., and Sturgeon, R. E., J. Anal.reducing ion throughput at any selected m/z. Relatively low At. Spectrom., 1994, 9, 605.

9 Vanhaecke, F., Galbacs, G., Boonen, S., Moens, L., and Dams, R.,LODs were observed even with the detector attenuation. AtJ. Anal. At. Spectrom., 1995, 10, 1047.maximum sensitivity, the LOD drops by a factor of 6000. This

10 Fonseca, R. W., and Miller-Ihli, N. J., Appl. Spectrosc., 1995,indicates that very low levels of Pb and Bi can be determined49, 1403.in individual samples. However, the homogeneity of these trace 11 Vanhaecke, F., Boonen, S., Moens, L. and Dams, R., J. Anal. At.

elements in the bulk material at very low concentrations is not Spectrom., 1995, 10, 81.known but can perhaps be estimated through solid sampling 12 Ren, J. M., Rattray, R., Salin, E. D., and Gregoire, D. C., J. Anal.

At. Spectrom., 1995, 10, 1027.ETV-ICP-MS.13 Moens, L., Verrept, P., Boonen, S., Vanhaecke, F., and Dams, R.,

Spectrochim. Acta, Part B, 1995, 50, 463.14 Darke, S. A., and Tyson, J., Microchem. J., 1994, 50, 310.CONCLUSIONS15 Hughes, D. M., Chakrabarti, C. L., Goltz, D. M., Gregoire, D. C.,

ETV-ICP-MS is applicable to direct solid sample determi- Sturgeon, R. E., and Byrne, J. P., Spectrochim. Acta, Part B, 1995,nation of Pb and Bi in Ni alloys. Calibration with RMs of 50, 425.

16 Hinds, M. W., and Kogan, V. V., J. Anal. At. Spectrom., 1994,similar alloy composition gives the most accurate results for9, 451.Pb. Either aqueous solution standards or solid RMs result in

17 Hinds, M. W., Brown, G. N., and Styris, D. L., J. Anal. At.accurate determinations for Bi in the Ni alloys studied.Spectrom., 1994, 9, 1411.Tellurium was not successfully determined probably because 18 Gilmutdinov, A. Kh., Staroverov, A. E., Gregoire, D. C.,

of variable interactions between the analyte and the matrix, Sturgeon, R. E., and Chakrabarti, C. L., Spectrochim. Acta,which prevent complete analyte vaporization. The very low Part B, 1994, 49, 1007.LODs observed indicate that this technique can be used forthe determination of volatile elements in solid samples either Paper 6/03313Jrepresentative of the bulk material or in micro-samples. Received May 13, 1996

It is possible that the proposed technique can be extended Accepted August 27, 1996

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