Isotope dilution mass spectrometry as a calibration method for the analysis of trace elements in powder samples by LA-ICP-MS

The applicability of the isotope dilution (ID) technique for trace element determinations in powdered solid samples by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is demonstrated. LA-ICP-IDMS combines the advantage of the isotope dilution technique as a definitive method with the powerful capability of LA-ICP-MS for a fast and direct multi-element analysis of solid samples. LA-ICP-IDMS therefore allows matrix-matched quantification of element concentrations without any external standard, which is not possible by other calibration strategies used in LA-ICP-MS, up to now. The sample preparation effort is kept minimal and consists of an addition of the corresponding isotope-enriched spike solutions to the powder sample, subsequent drying and pressing of the isotope diluted sample to a pellet. For quantification, only one isotope ratio per analyte must be measured in the isotope diluted sample. It is shown that this internal standardisation corrects for all signal variations during analysis, e. g. instrumental drift or varying mass ablation rates. The LA-ICP-IDMS method is validated by the analysis of Cr, Fe, Cu, Zn, Sr, Cd, and Pb in seven certified reference materials (BCR 60, BCR 150, BCR 151, SRM 1567a, SRM 1577b, CRM 320, and SRM 1646) of different (organic and inorganic) matrix composition. 28 of a total number of 32 trace element concentrations determined by LA-ICP-IDMS are in agreement within the uncertainties of the certified values. The precision obtained is better than 10% for elements with concentrations >0.03 mug g(-1). Only for two samples the Cr and Fe results differ significantly from the certified values.