A Reference Gas Inlet System for Compound Specific Hydrogen Isotopic Composition Mass Spectrometry



Compound specific isotope measurements represent a powerful analytical tool to identify sources and sinks of carbon species in natural environments.  One particular example in hydrothermal systems is microbial methane in subsurface environments, which can be distinguished from thermogenic methane based on its carbon and hydrogen isotopic composition.  The key limitation with hydrogen isotopic measurements is in reducing the analytical uncertainty to a level that allows for more robust and subtle interpretations.

Current methods for standardization utilize pulses of H2 gas injected directly into the ion source of a mass spectrometer and bypass a conversion step.  Inherent in this conversion step are certain biases that discriminate in favor of one isotope of hydrogen.  These biases, induced by the required conversion, are only expressed in the unknown samples and not in the reference standards.  Briefly, our approach will inject discrete peaks of propane into the stream of a gas chromatograph.  These peaks of propane will be separated and converted, along with unknown compounds of interest, into H2 gas for analysis.  With the proposed injection of propane, the biases become equal between sample and standard, and therefore generate a much higher quality data.

This method of standardization will allow for more precise data to be collected and become the standard method of calibration for all compound-specific hydrogen isotope data in the future.  We request funds to synthesize and interpret results from this development in preparation for publication.  This work will allow WHOI to obtain the rightful credit for the development of this new technique.