应用 Argus 多接收稀有气体质谱仪准确测量空气的 Ar 同位素组成

摘要

稀有气体质谱仪准确测量氩同位素组成是 Ar －Ar 法高精度定年的前提，目前测量氩同位素主要应用单接收或多接收质谱仪，其中多接收稀有气体质谱仪在数据准确性和重现性等方面具备优势。本文研究了Argus 多接收稀有气体质谱仪应用于测量 Ar 同位素过程中一些主要因素对测量结果准确度和重现性的影响情况。结果表明，整套系统在静态模式下不同时间段的本底值极低，不影响测定；仪器电子倍增器的接收效率优于99.67％，可显著提高 Ar 低含量样品测量精度，当40 Ar信号强度低于0.5 V 时，用电子倍增器测量40 Ar／36 Ar组成的标准偏差仅为0.11％，而用法拉第杯测量40 Ar／36 Ar组成的标准偏差为0.53％；仪器的质量歧视效应可通过多次循环测量并采用指数定律获得稳定的质量歧视校正因子（此值相对标准偏差为0.0434％），实现对 Ar 同位素组成的准确校正。本文以测量空气中的氩同位素组成为例，证明了 Argus 多接收稀有气体质谱仪的测试效率比单个接收器跳峰方式的测试效率高，测试结果更精确，因此适合年轻样品或含钾量极低的样品的 Ar －Ar 高精度定年工作。%Accurate measurement of Argon isotopes is the premise of Ar-Ar dating with high precision.Two main kinds of instruments for Ar isotope measurement are Single-collector and Multi-collector Mass Spectrometers, among which the Multi-collector Noble Gas Mass Spectrometer can produce data with better accuracy and reproducibility.Some main factors affecting the accuracy and reproducibility of the results during the measurement of the Ar isotopes by the Argus Multi-collector Noble Gas Mass Spectrometer are discussed in this paper.Results show that the background of the whole system in static mode is very low and has little effect on the measurement of Ar isotope composition.The efficiency of the multiplier of the instrument is better than 99.67%,which can improve the measurement precision of low Ar samples.When the signal of 40 Ar is less than 0.5 V,the standard error is only 0.11% for the 40 Ar/36 Ar ratios measured by the CDD, while the standard error is 0.53% when the ratios are measured using a Faraday cup.The mass discrimination effect of the instrument could be calibrated with the logarithmic law through the repeated measurement of argon isotopes in air and the relative standard error of the mass discrimination is only 0.0434%.Using the example of the determination of argon isotopes in air,it can be demonstrated that the efficiency of the measurement by Argus Multi-collector Mass Spectrometer is higher than that of the Single-collector Mass Spectrometer with higher precision and accuracy.Therefore,the Multi-collector Mass Spectrometer is highly suitable for the high-precision Ar-Ar dating of young and potasium-poor samples.