Improved accuracy in high‐temperature conversion elemental analyzer δ¹⁸O measurements of nitrogen‐rich organics

Abstract

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.

Isotopic disproportionation during hydrogen isotopic analysis of nitrogen‐bearing organic compounds

“…Interference by molecular nitrogen (N 2 ) was investigated by Meier‐Augenstein et al ., who suspected insufficient chromatographic separation between H 2 and N 2 as a major cause for the observed deviations in δ 2 H results. The conversion of nitrogen‐bearing organic compounds was investigated more recently by Sieper et al ., and in greater detail by Hunsinger et al . Sieper et al .…”

Elimination of nitrogen interference during online oxygen isotope analysis of nitrogen‐doped organics using the “NiCat” nickel reduction system

“…Several of the factors that control the magnitude of the interfering N 2 peak vary between instrumental configurations,and within the same instrument over time, rendering it difficult to effectively eliminate the interference across a wide range of substrates and analytical conditions. These factors include the oxidation state of nitrogen and the types of functional groups comprising the sample material, reactor conditions influencing nitrogen intermediates, IRMS source conditions such as oxygen availability, and the condition of the GC column . In contrast, the NiCat system circumvents each of these issues by converting the CO into CO 2 and measuring δ 18 O values using m/z 44 and 46, thus eliminating the fundamental problem of using CO as the analyte.…”

“…The NiCat system may also be preferable for nitrogen‐bearing compounds that have proven exceptionally difficult to analyze via conventional HTC. For example, published values for the international reference material IAEA‐600 caffeine (C 8 H 10 N 4 O 2 ) vary widely, from −0.58 to ‐–4.63‰ . The difficulty in analyzing caffeine might be the result of slow‐elution of N 2 formed from the breakdown of precursor molecules such as paracyanogen ((NC‐CN) x ) during the HTC reaction, giving rise to a variable m/z 30 background that is difficult to predict and correct .…”

“…Researchers employ two strategies to mitigate the interference of N 2 with the results of δ 18 O analyses. The first involves optimization of N 2 and CO peak separation to prevent N 2 from reaching the source . The second strategy relies upon modified baseline corrections to the raw data post‐analysis .…”

Gas Source Isotope Ratio Mass Spectrometry (IRMS)

“…Compared to plain oxidation with O 2 , the high-temperature reduction technique is chemically less well constrained. [100][101][102] The reactions of sample material containing nitrogen are not well understood and postconversion reactions like recombination between CO and H 2 may occur. Numerous gaseous byproducts (e.g.…”