Memory effects in compound-specific D/H analysis by gas chromatography/pyrolysis/isotope-ratio mass spectrometry

Compound-specific analyses of lipid D/H ratios often encounter ranges of 300 per thousand or more, and experiments using D-enriched water to study fractionations often extend the range up to 1000 per thousand. Here we show that for such large dynamic ranges in D/H ratio, isotopic "memory" between adjacent peaks can be significant. Memory effects have not been previously reported for GC/P/IRMS systems but can have a significant impact on many measurements, even those exploring only natural-abundance variations in D/H. To quantitatively evaluate these effects, we synthesized two series of organic standards with deltaD values varying from -230 to +800 per thousand. We then analyzed chromatograms in which analyte deltaD values, retention times, or relative abundances were independently varied. For two sequential GC peaks, isotopic memory is measured to be typically 2-4% of the difference in deltaD values between the two. Roughly half of this effect can be attributed to unknown processes within the GC itself, and the other half to surface adsorption processes in the pyrolytic conversion of analytes to H2. Isotopic memory increases with decreasing time separation between peaks, with decreasing analyte abundance, and with increasing age of pyrolysis reactors. A simple numerical model that simulates dynamic adsorption of H2 on pyrolytic carbon can reproduce many aspects of the experimental data, suggesting that this is likely to be an important mechanism in isotopic memory. Several steps to mitigate memory effects in routine analyses are suggested.