The ICP magnetic sector multiple collector mass spectrometer (MC-ICPMS) Plasma 54 is a new double focusing instrument that combines the advantages of high efficiency plasma ionization with the superior peak shapes and precision of multiple collector magnetic sector mass spectrometry. This enables high precision isotopic composition and isotope dilution (ID) concentration measurements of elements that are otherwise difficult to analyze using thermal ionization mass spectrometry. Reliable concentration measurements for the platinum-group elements (PGE) are important to reach a better understanding of the Earth's accretion and internal differentiation. At the present time, however, only a limited amount of high-quality data are available for these elements. This is particularly true for terrestrial samples where PGE concentrations are typically in the ppb to ppt concentration range.
We are currently developing new analytical methods for the determination of PGE abundances in geological samples based upon isotope-dilution techniques in conjunction with high-precision MC-ICPMS measurements. Following spiking and sample digestion the elements of interest must first be separated from the rock matrix to increase analytical sensitivity and eliminate any isobaric interferences.
Our first efforts have focused on the elements Pd, Ir and Pt. We have developed a simple anion-exchange separation scheme that permits the isolation of these elements in sufficient purity for mass spectrometric analysis at yields >75%. The high affinity of Pd, Ir and Pt to anion-exchange resins permits the use of small (~1.25 ml) columns for the large samples (~1-2g) that are required to ensure that the analyses are not biased by the heterogeneous distribution of the PGE in rocks ("nugget effect").
Following loading of the sample solution onto the column and matrix elution, (Pd) and (Ir, Pt) are stripped from the resin with 8M HNO3 and 14M HNO3, respectively. The fractions are then evaporated, redissolved in dilute mineral acid and analyzed by MC-ICPMS using static multicollection. Fractionation correction during ID analyses is achieved either by internal normalization or by monitoring the mass discrimination of an element of similar mass that is admixed with the sample solutions (e.g., Ag for Pd; Os for Ir and Pt).
We have measured Pd, Ir and Pt concentrations in two standard reference materials (PTC-1a and SU-1a) and the basalt BTHO, an olivine tholeiite from Iceland, to evaluate the reliability of our analytical procedure. Two digestion techniques were utilized during these studies: (1) conventional acid-dissolution with HF and aqua regia in closed Savillex® beakers and (2) a similar procedure in which the aqua regia step, however, was carried out at high temperature (195C) and pressure inside a Parr® digestion bomb.
Our results, particularly for Pt and Ir, indicate that conventional acid digestion techniques are insufficient to achieve complete dissolution of resistant PGE containing phases. The Parr® bomb digestion results for Pd and Pt on SU-1a (Pd: 0.346±0.027 ppm; Pt: 0.283±0.004 ppm) and PTC-1a (Pd: 4.27±0.06 ppm; Pt: 3.06±0.01 ppm) display excellent reproducibility and are generally in good agreement with previously published values. Our Ir data (SU-1a: 14.3±4.5 ppb; PTC-1a: 47.5±9.5 ppb), however, display more scatter and are low compared to reference values. This may be the result of incomplete sample digestion and/or significant sample heterogeneity.
The results of two analyses (based upon separate
dissolutions) for basalt BTHO (Pd: 9.64±0.21 ppb;
Pt: 5.36±0.06 ppb) agree to within ¾3% for both elements, indicating that the technique is well suited for the analyses of typical geological samples with low PGE abundances.
We are presently modifying our ion-exchange chemistry to separate further elements, such as Re, Ru, Ag and Zn, from the same sample splits for MC-ICPMS analysis. We are additionally planning to use a Carius-tube technique for sample digestion in order to ensure complete dissolution of resistant PGE containing phases and obtain full spike-sample equilibration.