低质量数元素同位素在线连续流同位素比值质谱分析的质量控制和数据标准化

同位素比值质谱分析方法是准确测量各种同位素相对丰度的标准方法。连续流同位素质谱的出现不仅提高运行效率,也降低了样品用量并提高灵敏度。但是,要使这种方法获得更好准确度和精度的同位素数据,并做到所获得数据可与其他实验室结果进行类比,从而得到可靠的同位素数据,这就需要好的分析策略和运行方案,还需要对仪器日常性能和数据质量进行严密的监视管控,而且还取决于原始数据如何进一步标准化到国际同位素尺度上。因此,同位素比值质谱结合元素分析仪(或热转换元素分析仪)连续流方法要实现可靠的稳定同位素分析需要:①设备安装和环境控制、测试准备、样品制备和称量、标准物质选择及序列等规范化质量控制措施;②严格校准仪器系统(包括调节灵敏度和线性,背景值监测,稳定性检测,H+3系数校正等);③可靠的数据处理。目前不同的实验室,采用标准物质来标定系统、对测量的同位素数据进行标准化,以及利用控制曲线来监测系统稳定性并对不确定度的计算,这些策略往往都不同。因此,统一的数据处理方案是被高度期待的。目前最好的执行方案是基于线性回归的两点或多点标准化方法。如果每一批样品中测量两个不同的标准物质四次,或者测量四个标准物质两次,那么不确定度会降低50%。当前同位素比值质谱能够测定同位素比值的不确定度一般要好于0.02‰。但是,标准物质的使用既要考虑样品的性质,同时要涵盖它们未知同位素组成的范围,尤其氢同位素在现阶段缺乏标准物质和测量的仪器精度较差(比碳、氮、氧等要低一个数量级)的情况下,这显然是稳定同位素分析者的一个重大挑战。本文概括了同位素比值质谱结合元素分析仪(或热转换元素分析仪)的基本操作原理和分析实践,将数据处理运用到同位素比值分析之中,获得连续流同位素比值质谱分析结果的合理准确度和精度。

Data Normalization and Quality Control of Light Element Stable Isotope Analyses by Means of Continuous Flow Isotope Ratio Mass Spectrometry

The technique of Isotope Ratio Mass Spectrometry(IRMS)is the gold standard of accurate and precise analyses for all sorts of isotopic relative abundance amongst analytical techniques. In many fields, the extensive application of continuous flow IRMS not only improves effective runs, but also reduces sample size as well as enhancing sensitivity. However, for the purpose of obtaining more accurate results, which can be compared between different laboratories, it is imperative to have good strategies and protocols in the run, monitoring of routine analytical performance and quality. In addition, detailed calibration for the raw data is highly desirable. In order to obtain reliable data by using IRMS coupled with EA (TC/EA), requirements need to be met as follows: (1) instrument set-up and environmental control, measurement preparation, sample preparation and weight, reference materials utilization and selection, sequence etc; (2) strictly calibrated systems including tuning sensitivity and linearity, monitoring the background, stability detection, H₃₊ correction; (3) reliable data reduction. The strategies of calibrating, normalization and control linear by using reference materials are discriminated in different labs. So it is expected to have a unified data-processing programme. Two-point normalization (or multi-point normalization) is the best executive program at present. If two different reference materials are used four times for normalization, or four different reference materials are used two times for each batch of samples, this may reduce the normalization uncertainty by 50%. The modern IRMS is capable of measuring natural isotope ratio variations with an uncertainty better than 0.02. Nevertheless, it is a challenge to the use of reference materials with the cover range of unknown isotopes, especially for H isotope absence of the standard materials and poor precision (an order of magnitude lower compared with carbon, nitrogen, oxygen isotope). This paper aims to highlight general principles of IRMS coupled to an EA or a TC/EA and the practices of analytical application of stable isotope ratio measurement. The knowledge on the protocols of continuous flow IRMS analyses is necessary for the acquisition of reasonable accuracy and precision for stable isotope composition.