Analytical challenges in the quantitative determination of 2H/1H ratios of methyl iodide

Abstract

A published method to determine the hydrogen isotope ((2)H/(1)H) ratios of methoxyl groups, cleaved from lignin by hydroiodic acid (HI), calls for the analysis of methyl iodide (CH(3)I). However, analysis of halogenated compounds by gas chromatography/pyrolysis/isotope ratio mass spectrometry (GC/P/IRMS) yields incomplete conversion into H(2) and unwanted HX products post-pyrolysis. We tested analytical capabilities when measuring a pure CH(3) I standard using a GC Isolink pyrolysis furnace at 1420 °C and a DELTA V(Plus) ™ isotope ratio mass spectrometer, both by Thermo Scientific. We added and tested the efficacy of either a secondary reduction furnace containing heated metals or a cold trap to eliminate unwanted HI prior to IRMS analysis for (2)H/(1) H determinations. While 94% of the hydrogen in CH(3) I is converted into H(2) in GC/P/IRMS, 6% yields HI. The use of either a secondary reduction furnace or cold trap can eliminate HI. However, secondary reduction is untenable given changes in reduction/transmission efficiency and (2)H/(1) H drift. A cold trap provides a pragmatic solution: the δ(2)H values of -97 ‰ were measured from injections of 23-57 µg of CH(3)I to a precision of 4.5 ‰ (n = 26), within the error of the offline measurements. We have refined an earlier method to measure the δ(2)H values of CH(3)I by GC/P/IRMS by adding a cold trap to remove HI. This approach should facilitate future experimentation with the measurement of CH(3)I liberated from natural samples for potentially diverse applications including forensic sourcing of wood and tree-wood-based reconstructions of source water.