Improved accuracy in high-temperature conversion elemental analyzer δ18O measurements of nitrogen-rich organics
This article is a U.S. Government work and is in the public domain in the U.S.A.
L. A. Stern, Counterterrorism and Forensic Science Research Unit, FBI Laboratory, Quantico, VA 22135, USA.
The use of high-temperature conversion (HTC) reduction systems interfaced with isotope ratio mass spectrometers for δ18O measurements of nitrogen-containing organic materials is complicated by isobaric interference from 14N16O+. This ion is produced in the ion source when N2 reacts with trace oxygen shifting the m/z 30 baseline prior to elution of CO.
We compared adaptations to a typical HTC system (TC/EA) to determine the best method to measure the δ18O values of nitrogen-rich organic substrates including: (1) 0.6 and 1.5 m 5 Å molecular sieve GC columns; (2) reduction of N2 peak via He dilution; and (3) diversion of N2 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).
The efficiency of N2 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.
The diversion of N2 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 δ18O values; and (3) high correlation between the increase in the δ18O values relative to the GC-only measurements and the N2 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 δ18O reproducibility of IAEA-600. Published in 2012 by John Wiley & Sons, Ltd.