HT-IRMS测定有机物氧同位素过程的影响因素及方法改进

IMPROVEMENT OF ANALYTICAL METHOD AND THE INFLUENCE FACTORS OF OXYGEN ISOTOPIC COMPOSITION IN ORGANIC MATTER BY HT-IRMS

高温裂解元素分析仪与气体同位素质谱仪联用系统(HT-IRMS)是测定有机物氧同位素的重要方法,优化HT-IRMS的参数、提高其可靠性和稳定性一直是各实验室努力的方向。本研究基于国际标准物质IAEA-601和IAEA-C3的多次测量,对HT-IRMS测定结果的稳定性进行了检验。结果显示,手动进样模式下,测定值比较稳定(标准偏差分别为0.16‰和0.25‰),而自动进样模式的测定结果波动变化显著且呈现偏负漂移趋势,IAEA-601和IAEA-C3最大波动程度分别达到3.00‰和4.26‰。通过对比分析测定结果,本文推断自动进样模式下氧同位素的漂移可能源于空气中的水汽的污染。通过在自动进样盘位置加装隔离罩装置,将自动进样盘与实验室空气进行隔离之后,自动进样模式下再次重复测定结果较为稳定(标准偏差分别为0.35‰和0.25‰),有机物氧同位素负漂移趋势基本消除,表明使用隔离罩装置可以有效减少实验室空气中水汽对样品的干扰,降低实验室环境参数变化对同位素测定结果的影响,提高HT-IRMS系统测定有机物氧同位素的稳定性。

The combination system of high-temperature pyrolysis element analyzer and gas isotope ratio mass spectrometer(HT-IRMS)is an important method for the measurement of oxygen isotopes in organic matter.The improvements such as the trying to optimize the parameters,efforts to improve the reliability of HT-IRMS technique made this method more user friendly.In this paper,we tested the stability of HT-IRMS system in long-term and discussed the major sources of uncertainty.Based on repeated analysis of the international standard materials(IAEA-601:benzoic acid,C7 H6 O2,δ18 OV-SMOW=23.14±0.19‰,IAEA-C3:cellulose,δ18 OV-SMOW=32.20±0.13‰)using HT-IRMS system at the stable isotope center of Fujian Normal University,the results showed that the measurements from the manual injection mode were relatively stable(standard deviations were 0.16‰,0.25‰respectively),while the results from automatic injection mode showed significant fluctuations.The measurement data of the two international standard materials from automatic injection mode both showed a trend of negative drift with the time,and the maximum data difference of IAEA-601 and IAEA-C3 among repeated analysis reached 3.00‰and 4.26‰respectively.On the comparative analysis of the results,it was found that the water vapor in the air could pollute the samples stored on the automatic injector,when the standard materials were waiting for testing in turn using the automatic injection mode.It was concluded that the negative trend could be derived from the contamination of air vapor.We proposed that the use of an isolation cover on the automatic injector of HT-IRMS system to isolate the instrument from air and help to reduce the pollution from air in the laboratory.The use of isolation cover reduced the trend of negative drift with time and improved the stability of the measurement data,when the measurement was done using automatic injection mode.Based on the comparation of the standard deviations of repeated analysis,the standard deviations(0.35‰and 0.25‰respectively)with the using of isolation cover from automatic injection mode were very close to manual injection mode within the error range.The monitoring data also showed that the internal humidity of the isolation cover decreased to 10%~20%,indicating that the using of the isolation cover could help to reduce the interference of water vapor in the laboratory room effectively,and to improve the stability of the HT-IRMS system for the measurements of oxygen isotopes in organic matter as well.The isolation cover device used in this study was made with polymethyl methacrylate and had a low cost,which made this device easy to be applied in other laboratories and to improve the data stability and comparability over long-term.