[S-CG51] ハイブリッド年代学−ジルコン年代学の次へ−

[SCG51-P01] Optimization of quantitative analysis of trace elements in zircon using laser ablation ICP-MS/MS with collision/reaction cell technique

*坂田 周平¹、大野 剛² (1.東京大学地震研究所、2.学習院大学)

キーワード：ジルコン、微量元素、LA-ICP-MS/MS、アナログカウンティング、入出力直線性

Concentration of trace elements in zircon (ZrSiO₄) provides useful information for geochemical process in its parent melt. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been widely used in recent years as a relatively low-cost and quick method for measuring trace element composition in zircon. However, in probe analysis such as LA-ICP-MS, the amount of sample is limited, and in order to determine the accurate elemental composition, it is necessary to improve sensitivity, reduce background signal intensity, and improve the input/output linearity of the detector to enable simultaneous measurement of signals with large differences in concentration. In particular, in the conventional LA-ICP-MS method, it is an urgent issue to reduce spectral interferences caused by polyatomic and polyvalent ions originating from Si, Zr, and ion sources, which are the main components of zircon. An ICP mass spectrometer with a collision/reaction cell sandwiched between two quadrupole mass separation sections (ICP-MS/MS) has attracted attention as a promising solution to these problems. In the conventional method, it is difficult to measure trace amounts of elements such as P, Sc, Ti, and Nb in zircon, but the LA-ICP-MS/MS method can greatly reduce spectral interference and obtain a high signal-to-noise ratio by mass shift method using oxygen gas. However, the mass shift method alone is not sufficient to determine more accurate concentrations, and it is necessary to correct for interfering ions that have passed through the filters of the two mass separation sections. Moreover, the internal standard elements needed to calculate the concentration also need to be considered. In the case of zircon, the candidate internal standard elements are Si and Zr, of which Zr is easy to measure because of its low background signal intensity. However, when Zr is used as an internal standard, the difference in Zr concentration between the primary standard material for elemental ratio calibration and zircon is about 3-4 orders of magnitude different, so the input-output linearity of the detector must be evaluated and corrected if necessary. Secondary electron multipliers normally used in quadrupole ICP mass spectrometers switch between pulse counting mode and analog counting mode depending on the strength of the signal. However, the analog counting mode may be inferior to Faraday detector with a negative feedback DC current circuit in terms of input/output linearity, and it is important to rigorously evaluate the performance of the analog counting mode for quantitative analysis using Zr internal standards. In this presentation we would like to report on the problems encountered in the measurement of trace elements in zircon using LA-ICP-MS/MS combined with a collision/reaction cell, based on experimental data on the mass shift method, correction calculations, and detector evaluation.