レーザーアブレーション-誘導結合プラズマ質量分析法による地球化学試料の微量元素分析

Abstract

This paper describes advances in elemental and isotopic ratio measurements that have been made with inductively coupled plasma mass spectrometry (ICP-MS) coupled with a laser-ablation sample introduction technique. Laser ablation utilizing UV light (frequency quadrupled to 266 nm or quintupled to 213 nm by a Nd-YAG laser, or an ArF Excimer 193 nm laser) is now the most widely used system because of its higher analytical precision and analytical capability to most of solid samples. However, even with UV lasers, serious elemental fractionation may occur during laser ablation, mainly due to differences in the element volatility. Therefore, the reduc- tion of elemental fractionation during ablation is necessary for further precise and accurate ele- mental and isotopic analyses of trace elements. This is particularly the case for high quality-iso- tope data for age determination. In the field of geochemistry, great efforts have been made to obtain accurate Pb/U ratios, because 206 Pb/ 238 U ratios provide chronological information. The basic problem is that Pb/U results using conventional LA-ICPMS techniques show serious frac- tionations of the ratio over a prolonged measurement. The observed Pb/U ratio increases dur- ing the measurement because of the greater volatility of Pb. The Pb/U elemental fractionation is successfully reduced by an active focus technique, chemically assisted-laser ablation technique, or by soft-ablation technique. Moreover, recently developed techniques including signal sta- biliser, a correction for the slow response of a Faraday amplifier and removal of isobaric interfer- ence by cool plasma conditions, will also expand the application fields and analytical accuracies of the LA-ICPMS technique. Outlines of these techniques are introduced.