Zha Xiang-Ping, Gong Bing, Zheng Yong-Fei, Chen Yi-Xiang
CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China.
Rapid Commun Mass Spectrom. 2018 Jan 15;32(1):48-56. doi: 10.1002/rcm.8008.
Continuous-flow isotope ratio mass spectrometry (CF-IRMS) is a specialized technique used to quickly analyze very small amounts of sample. We have used CF-IRMS to assess the influences of sample weight and relative carbon content on the accuracy and precision of the δ C values of micro amounts of carbonate and non-carbonate in silicate rocks.
The analytical work was performed on a Gasbench II (GB) sample preparation device and on an Elemental Analyzer (EA), which were both interfaced to CF-IRMS instruments. Potential silicate matrix effects on the carbon isotopic analyses were investigated by measuring mixtures of calibrated carbon reference materials and quartz powder. The calibration lines, established by the measured raw values and the known values of three reference materials mixed with quartz powder, were used to calibrate the δ C values of basalt samples from eastern China.
The δ C values measured by GB-CF-IRMS of one national carbonate reference material, GBW04416, deviate slightly from the known value for approximately 20-70 μg of carbonate contained in 4.5-mL vials; the smaller the sample size, the lower the measured δ C values. External precision better than 0.1‰ (1σ, n = 26) is achieved at a signal intensity for mass 44 of between 868 and 1614 mV, corresponding to a sample weight of 30.8-50.2 μg, whereas it is reduced to 0.27‰ (1σ, n = 34) at a signal intensity between 519 and 1614 mV, corresponding to a sample weight of 21.1-50.2 μg. In the EA-CF-IRMS experiments for non-carbonate carbon, at high carbon concentration (greater than 800 ppm) and at optimum sample weights, the accuracy and precision are both better than 0.2‰. For carbon concentrations less than 500 ppm, the measured δ C values deviate from the average by up to -1.2‰ and the precision is 0.74‰.
The measured δ C values decrease substantially at lower carbon concentration and higher sample weights, and poorer precision is obtained. Suggestions are made to measure repeatedly the same carbon concentration of sample and reference materials in order to obtain not only reproducible, but also accurate carbon isotope ratios.
连续流同位素比率质谱法（CF - IRMS）是一种用于快速分析极少量样品的专业技术。我们已使用CF - IRMS来评估样品重量和相对碳含量对硅酸盐岩石中微量碳酸盐和非碳酸盐δC值的准确度和精密度的影响。
分析工作在与CF - IRMS仪器相连的Gasbench II（GB）样品制备装置和元素分析仪（EA）上进行。通过测量校准过的碳参考物质与石英粉的混合物，研究了潜在的硅酸盐基体对碳同位素分析的影响。由测量的原始值和与石英粉混合的三种参考物质的已知值建立的校准曲线，用于校准来自中国东部的玄武岩样品的δC值。
对于4.5 mL小瓶中约20 - 70 μg的碳酸盐，用GB - CF - IRMS测量的一种国家碳酸盐参考物质GBW04416的δC值与已知值略有偏差；样品量越小，测量的δC值越低。在质量数44的信号强度为868至1614 mV时，外部精密度优于0.1‰（1σ，n = 26），对应样品重量为30.8 - 50.2 μg；而在信号强度为519至1614 mV时，精密度降至0.27‰（1σ，n = 34），对应样品重量为21.1 - 50.2 μg。在EA - CF - IRMS对非碳酸盐碳的实验中，在高碳浓度（大于800 ppm）和最佳样品重量下，准确度和精密度均优于0.2‰。对于碳浓度小于500 ppm的情况，测量的δC值与平均值的偏差高达 - 1.2‰，精密度为0.74‰。
在较低碳浓度和较高样品重量下，测量的δC值大幅下降，且精密度较差。建议对样品和参考物质的相同碳浓度进行重复测量，以便不仅获得可重现的，而且准确的碳同位素比率。
