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

RATIONALE 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. METHODS 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. RESULTS 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. CONCLUSIONS 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. Copyright © 2012 John Wiley & Sons, Ltd.