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Trace Element Analyses of Rocks and Minerals by ICP-MS

Knaack, C., Cornelius, S.B., and Hooper, P.R.,
GeoAnalytical Lab, Washington State University, December, 1994

INTRODUCTION
The ICP-MS (inductively coupled plasma source mass spectrometer) consists of a quadrupole mass spectrometer with an inductively coupled argon plasma as an ion source. Liquids introduced into the plasma (7000°C) are ionized and then passed to the mass spectrometer through a two-stage ion extraction interface. The ICP-MS is capable of quantitatively determining trace elements in liquids in the range of fractions of a part per billion. For routine REE analysis of rocks and minerals, the detection limit is at or below chondrite levels. Its capability for rapid multi-element analysis at low cost, high sensitivity, and relative freedom from interferences make the ICP-MS an excellent instrument for the determination of many trace elements in rocks and minerals.

In the routine procedure practiced in the GeoAnalytical Laboratory for trace elements in rocks and minerals, the following 26 elements are analyzed: all 14 naturally occurring rare earth elements (La through Lu) together with Ba, Rb, Y, Nb, Cs, Hf, Ta, Pb, Th, U, Sr and Zr. Zr is measured only as a check for complete dissolution of the sample.

SAMPLE PREPARATION
Rock and mineral samples must be completely dissolved prior to analysis. The digestion technique used for routine analysis is adapted from Crock and Lichte (1982) and our own experimentation. Samples are first ground in an iron bowl in a shatterbox swing mill. Two grams of this rock or mineral powder is then mixed with an equal amount of lithium tetraborate (Li2B4O7) flux, placed into a carbon crucible and fused in a 1000° muffle furnace for 30 minutes. The resulting fusion bead is briefly ground again in the shatterbox and 250 mg of this powder is dissolved on a hotplate at 110°C, using 6 ml HF, 2 ml HNO3, and 2 ml HClO4 in an open teflon vial. The sample is evaporated to dryness, followed by an additional evaporation with 2 ml HClO4 at 165°C to convert insoluble fluorides to soluble perchlorates. 3 ml HNO3, 8 drops H2O2, 5 drops of HF and an internal standard of In, Re, and Ru are added to the sample which is then is diluted up to 60 ml final volume (1:240 final dilution). This combined fusion/dissolution procedure ensures the complete dissolution of zircons and other refractory phases such as garnets, while removing silica and boron as matrix elements by volitilizing them as gaseous fluorides.

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INSTRUMENT OPERATION
The instrumentation consists of a Sciex Elan model 250 ICP-MS equipped with a Babington nebulizer, water cooled spray chamber, and Brooks mass flow controllers. Samples are introduced into the argon plasma at 1.0 ml/min using a peristaltic pump and an automatic sampler. Plasma power is 1500 watts. Under these conditions MO+/M+ (the proportion of metal ions forming oxides) is minimized. The instrument is run in "multi-element" mode averaging 10 repeats of 0.5 sec/element for a total integrated count time of 5 sec/element.

Most elements have more than one isotope. For these elements the selection of the isotope for measurement is based on relative abundance and freedom from oxide and isobaric interferences. Table 1 lists the isotopes measured and their potential interfering oxide.

CALIBRATION
Unknown samples are run in sets of 17. One acid blank and two samples each of the 3 in-house rock standards BCR-P, GMP-01, and MON-01 (table 2) are run with each batch, totaling 24 standard and unknown samples per batch.

The three in-house standards have been calibrated against 17 international standards (figure 1). Their elemental concentrations so derived are listed in table 2. Independent values on these three in-house standards by Walsh (Kings College, London, U.K., by ICP-OES), Meier (U.S.G.S., Denver, by ICP-MS), Bailey and Conrey (WSU, Pullman, WA, by INAA) are also listed.

CALCULATION OF CONCENTRATIONS
Intensities for standards and unknowns are downloaded to a personal computer and reduced using a conventional spreadsheet program. Raw intensities are corrected for oxide and isobaric interferences as listed in table 1 (modified from Lichte et. al., 1987) and corrected for drift using the In, Re, and Ru internal standards (after Doherty, 1989). Calibration curves for each element are then constructed from the six (2x3) standard samples and single acid blank by plotting given values (table 2) against the corrected intensities. Concentrations for the unknown samples are then computed from this curve. These calculations assume that the isotopic proportions of the unknowns and standards do not vary significantly from the average of the earth's crust.

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PRECISION
The reproducibility (total of sample preparation plus instrumental precision) is illustrated in table 3, which includes 24 separate preparations of BCR-P analyzed in 12 separate runs between September 1994 and December 1994.

ACCURACY
As always, no absolute values for the standard samples are available to determine the calibration curves; only a consensus of the most likely values. It follows that exact measurements of accuracy cannot be made. Perhaps the best way of estimating analytical accuracy in secondary techniques, as employed here, is to calculate the scatter of individual standards from a calibration line which represents the best fit for all the standards.

This is demonstrated visually for each element in the attached series of 24 plots (Figure 1). Values for each of these standards, treated as unknowns, have been calculated from these calibration curves and are listed in table 2. This table also reproduces the recommended values (Govindaraju, 1994) used in constructing the calibration curves.

REFERENCES CITED
Crock, J.G., Lichte, F.E., 1982, Determination of rare earth elements in geologic materials by inductively couple argon plasma/atomic emission spectrometry. Analytical Chemistry, 54, 1329-1332.

Doherty, W., 1989, An internal standardization procedure for the determination of yttrium and the rare earth elements in geological materials by inductively coupled plasma mass spectrometry. Spectrochemica Acta, 44B, 263-280.

Lichte, F.E., Meier, A.L., Crock, J.G., 1987, Determination of rare earth elements in geological materials by inductively coupled mass spectrometry. Analytical Chemistry, 59, 1150-1157.

REFERENCES FOR TABLE 4
Govindaraju, K., 1994 compilation of working values and sample descriptions for 383 geostandards. Geostandards Newsletter, Special Issue, July 1994, 18, 15-53.