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Improved accuracy in high‐temperature conversion elemental analyzer δ 18 O measurements of nitrogen‐rich organics
Author(s) -
Hunsinger Glendon B.,
Stern Libby A.
Publication year - 2012
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.6132
Subject(s) - chemistry , pentaerythritol tetranitrate , nitrogen , analytical chemistry (journal) , dilution , mass spectrometry , benzoic acid , ion chromatography , chromatography , organic chemistry , physics , thermodynamics , explosive material
RATIONALE The use of high‐temperature conversion (HTC) reduction systems interfaced with isotope ratio mass spectrometers for δ 18 O measurements of nitrogen‐containing organic materials is complicated by isobaric interference from 14 N 16 O + . This ion is produced in the ion source when N 2 reacts with trace oxygen shifting the m/z 30 baseline prior to elution of CO. METHODS We compared adaptations to a typical HTC system (TC/EA) to determine the best method to measure the δ 18 O values of nitrogen‐rich organic substrates including: (1) 0.6 and 1.5 m 5 Å molecular sieve GC columns; (2) reduction of N 2 peak via He dilution; and (3) diversion of N 2 to waste via an automated four‐port valve. These methods were applied to caffeine (IAEA‐600), glycine, 4‐nitroacetanilide, pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX), as well as pure and sodium azide‐doped benzoic acid (IAEA‐601) and sucrose (IAEA‐CH6). RESULTS The efficiency of N 2 production in the HTC interface was highly variable among these compounds. Both the longer column and the dilutor improved, but did not eliminate, the adverse effects of nitrogen. CONCLUSIONS The diversion of N 2 adequately addressed the nitrogen‐induced problems as indicated by: (1) consistent m/z 30 background offset between reference and sample CO for both N‐free and N‐rich materials; (2) production of the highest δ 18 O values; and (3) high correlation between the increase in the δ 18 O values relative to the GC‐only measurements and the N 2 peak area. Additional validation would require N‐rich oxygen isotope standards for inter‐laboratory comparisons. Further, more stringent methodology may improve the poor inter‐laboratory δ 18 O reproducibility of IAEA‐600. Published in 2012 by John Wiley & Sons, Ltd.