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.