Slight pressure imbalances can affect accuracy and precision of dual inlet-based clumped isotope analysis
Slight pressure imbalances can affect accuracy and precision of dual inlet-based clumped isotope analysis
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DOI：
10.1080/10256016.2015.1010531
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发表时间：
2016-03-03
影响因子：
1.3
通讯作者：
Wacker, Ulrike
中科院分区：
文献类型：
--
作者：
Fiebig, Jens;Hofmann, Sven;Wacker, Ulrike
众所周知，在CO2的聚集同位素分析过程中可能会出现微妙的非线性，如果不加以解决，就会限制准确性。非线性是由m/z 47离子法拉第杯上的负背景引起的，其大小与m/z 44离子束的强度相关。负背景的来源仍然不清楚，但可能是由于二次电子。通常，具有不同整体同位素组成的CO2气体在1000摄氏度下平衡，并与样品一起沿着测量，以便能够校正这种效应。或者，在监测m/z 47上的负背景如何随m/z 44离子束的强度演变之后，可以针对二次电子的贡献来校正测量的m/z 47束强度。如果m/z 44杯比m/z 47杯具有更宽的狭缝宽度，则后一种校正程序似乎工作良好。在这里，我们表明，负m/z 47背景影响精度的双入口为基础的聚集同位素测量CO2，除非原始m/z 47强度直接校正的二次电子的贡献。此外，即使使用加热气体方法来校正所观察到的非线性，也可能获得不准确的结果。负背景对准确度和精密度的影响来自参考和样品CO2测量之间m/z 44离子束强度的微小不平衡。二次电子对m/z 47信号的相对贡献越大，并且将进入离子源的CO2通量率设置得越高，其变得越显著。这些问题可以通过在将这些贡献缩放到相应的m/z 49离子束的强度之后，针对源自二次电子的贡献校正样品和参考气体的测量的m/z 47离子束强度来克服。三个内部碳酸盐标准的聚集同位素分析证明了这种校正的准确度和精密度。如果m/z 49法拉第杯的狭缝宽度大于m/z 47杯的狭缝宽度，则所提出的校正方案可以容易地应用。
It is well known that a subtle nonlinearity can occur during clumped isotope analysis of CO2 that - if remaining unaddressed - limits accuracy. The nonlinearity is induced by a negative background on the m/z 47 ion Faraday cup, whose magnitude is correlated with the intensity of the m/z 44 ion beam. The origin of the negative background remains unclear, but is possibly due to secondary electrons. Usually, CO2 gases of distinct bulk isotopic compositions are equilibrated at 1000 degrees C and measured along with the samples in order to be able to correct for this effect. Alternatively, measured m/z 47 beam intensities can be corrected for the contribution of secondary electrons after monitoring how the negative background on m/z 47 evolves with the intensity of the m/z 44 ion beam. The latter correction procedure seems to work well if the m/z 44 cup exhibits a wider slit width than the m/z 47 cup. Here we show that the negative m/z 47 background affects precision of dual inlet-based clumped isotope measurements of CO2 unless raw m/z 47 intensities are directly corrected for the contribution of secondary electrons. Moreover, inaccurate results can be obtained even if the heated gas approach is used to correct for the observed nonlinearity. The impact of the negative background on accuracy and precision arises from small imbalances in m/z 44 ion beam intensities between reference and sample CO2 measurements. It becomes the more significant the larger the relative contribution of secondary electrons to the m/z 47 signal is and the higher the flux rate of CO2 into the ion source is set. These problems can be overcome by correcting the measured m/z 47 ion beam intensities of sample and reference gas for the contributions deriving from secondary electrons after scaling these contributions to the intensities of the corresponding m/z 49 ion beams. Accuracy and precision of this correction are demonstrated by clumped isotope analysis of three internal carbonate standards. The proposed correction scheme can be easily applied if the slit width of the m/z 49 Faraday cup is bigger than that of the m/z 47 cup.