The need for a rapid, inexpensive, and sensitive method for determination of the platinum-group elements (PGEs) in geochemical exploration samples initiated the development of a method incorporating microwave acid digestion followed by quantification of the PGEs, gold, silver, and related elements with a Micromass Platform ICP-HEX-MS. The technique is fast, inexpensive, accurate, precise and applicable to soils, sediments, and vegetation.
Microwave digestion with aqua regia for 45 minutes at 220°C effectively digests the relatively insoluble platinum-group minerals. The main bomb digestion step is preceded by treatment with hot hydrochloric and hydrofluoric acids, either in sealed bombs or in open vessels, to promote the dissolution of chromite and magnetite when sample media are rocks or soils. Quantification of the PGEs with ICP-MS proceeds directly, without additional separations or extractions. Certain interferences exist, and these are quantified and subtracted unless their concentrations are abnormally high. The hexapole collision cell inherent in the Micromass Platform design suppresses polyatomic interferences which would otherwise produce serious analytical errors. Background variations are corrected by monitoring fluctuations in 129Xe rather than by adding internal standards.
Relatively small sample sizes are ideal for maintaining low analytical costs related to ultrapure water and high-purity acids, but can produce relatively large sampling imprecisions due to geological sample heterogeneity. This precision improves in most exploration samples from approximately 25% RSD at 100 mg sample size to a more acceptable 5% RSD at 500 mg sample size with dilution to 100 ml or 500 ml, respectively. Replicates should be digested and analyzed to monitor precision of the analyses.
At the current state of development of this method, lower limits of detection are approximately 1 to 10 ng/g, limited by ability to monitor and correct interferences and background variations rather than by instrument sensitivity. The method avoids use of fire assay pre-concentration and matrix removal which is a necessary step in many other analytical approaches.