N2: a potential pitfall for bulk 2H isotope analysis of explosives and other nitrogen-rich compounds by continuous-flow isotope-ratio mass spectrometry

Observations made during the C-13 isotope analysis of gaseous CO2 in the simultaneous presence of argon in the ion source of the isotope ratio mass spectrometer prompted us to investigate what influence the simultaneous presence of nitrogen would have on both accuracy and precision of bulk H-2 isotope analysis of nitrogen-rich organic compounds. Initially an international reference material, IAEA-CH7, was mixed with silver nitrate in various ratios to assess the impact that N-2 evolved from the pyrolysis of nitrogen-rich organic compounds would have on measured delta H-2-values of IAEA-CH7. In a subsequent experiment, benzoic acid was mixed with silver nitrate to mimic the N:H ratio of organic-rich nitrogen compounds such as cellulose nitrate and RDX. The results of both experiments showed a significant deterioration of both accuracy and precision for the expected delta H-2 values for IAEA-CH7 and benzoic acid when model mixtures were converted into hydrogen and nitrogen, and subsequently separated by gas chromatography using standard experimental conditions, namely a 60 cm packed column with molecular sieve 5 angstrom as stationary phase held at a temperature of 85 degrees C. It was found that bulk H-2 stable isotope analysis of nitrogen-rich organic compounds employing published standard conditions can result in a loss of accuracy and precision yielding delta H-2 values that are 5 to 25 parts per thousand too negative, thus suggesting, for example, that tree-ring H-2 isotope data based on cellulose nitrate may have to be revised. Copyright (C) 2009 John Wiley & Sons, Ltd.