Merritt D A, Hayes J M
Biogeochemical Laboratories, Department of Chemistry, Indiana University, Bloomington 47405-1403, USA.
Anal Chem. 1994 Jul 15;66(14):2336-47. doi: 10.1021/ac00086a020.
The performance of systems in which picomole quantities of sample are mixed with a carrier gas and passed through an isotope-ratio mass spectrometer system was examined experimentally and theoretically. Two different mass spectrometers were used, both having electron-impact ion sources and Faraday cup collector systems. One had an accelerating potential of 10kV and accepted 0.2 mL of He/min, producing, under those conditions, a maximum efficiency of 1 CO2 molecular ion collected per 700 molecules introduced. Comparable figures for the second instrument were 3 kV, 0.5 mL of He/min, and 14000 molecules/ion. Signal pathways were adjusted so that response times were <200 ms. Sample-related ion currents appeared as peaks with widths of 3-30 s. Isotope ratios were determined by comparison to signals produced by standard gases. In spite of rapid variations in signals, observed levels of performance were within a factor of 2 of shot-noise limits. For the 10-kV instrument, sample requirements for standard deviations of 0.1 and 0.5% were 45 and 1.7 pmol, respectively. Comparable requirements for the 3-kV instrument were 900 and 36 pmol. Drifts in instrumental characteristics were adequately neutralized when standards were observed at 20-min intervals. For the 10-kV instrument, computed isotopic compositions were independent of sample size and signal strength over the ranges examined. Nonlinearities of <0.04%/V were observed for the 3-kV system. Procedures for observation and subtraction of background ion currents were examined experimentally and theoretically. For sample/ background ratios varying from >10 to 0.3, precision is expected and observed to decrease approximately 2-fold and to depend only weakly on the precision with which background ion currents have been measured.
对皮摩尔量的样品与载气混合并通过同位素比质谱仪系统的系统性能进行了实验和理论研究。使用了两种不同的质谱仪，均具有电子轰击离子源和法拉第杯收集器系统。一种加速电压为10kV，每分钟接受0.2mL氦气，在这些条件下，每引入700个分子可收集到1个二氧化碳分子离子，最大效率。第二种仪器的可比数据为3kV、每分钟0.5mL氦气和14000个分子/离子。调整信号通路以使响应时间<200毫秒。与样品相关的离子电流呈现为宽度为3 - 30秒的峰。通过与标准气体产生的信号进行比较来确定同位素比。尽管信号快速变化，但观察到的性能水平在散粒噪声极限的2倍以内。对于10kV的仪器，标准偏差为0.1%和0.5%时的样品需求量分别为45和1.7皮摩尔。3kV仪器的可比需求量为900和36皮摩尔。当每隔20分钟观察一次标准时，仪器特性的漂移得到了充分抵消。对于10kV的仪器，在所研究的范围内，计算出的同位素组成与样品大小和信号强度无关。3kV系统观察到的非线性<0.04%/V。对背景离子电流的观察和扣除程序进行了实验和理论研究。对于样品/背景比从>10到0.3的变化，预计并观察到精度大约降低2倍，并且仅微弱地依赖于测量背景离子电流的精度。
