Offline oxygen isotope analysis of organic compounds with high N:O

Abstract

Although the advantages of online δ(18)O analysis of organic compounds make its broad application desirable, researchers have encountered NO(+) isobaric interference with CO(+) at m/z 30 (e.g. (14)N(16)O(+), (12)C(18)O(+)) when analyzing nitrogenous substrates. If the δ(18)O value of inter-laboratory standards for substrates with high N:O value could be confirmed offline, these materials could be analyzed periodically and used to evaluate δ(18)O data produced online for nitrogenous unknowns. To this end, we present an offline method based on modifications of the methods of Schimmelmann and Deniro (Anal. Chem. 1985; 57: 2644) and Sauer and Sternberg (Anal. Chem. 1994; 66: 2409), whereby all the N(2) from the gas products of a chlorinated pyrolysis was eliminated, resulting in purified CO(2) for analysis via a dual-inlet isotope ratio mass spectrometry system. We evaluated our method by comparing observed δ(18)O values with previously published or inter-laboratory calibrated δ(18)O values for five nitrogen-free working reference materials; finding isotopic agreement to within ±0.2‰ for SIGMA® cellulose, IAEA-CH3 cellulose (C(6)H(10)O(5)) and IAEA-CH6 sucrose (C(12)H(22)O(11)), and within ±1.8‰ for IAEA-601 and IAEA-602 benzoic acids (C(7)H(6)O(2)). We also compared the δ(18)O values of IAEA-CH3 cellulose and IAEA-CH6 sucrose that was nitrogen-'doped' with adenine (C(5)H(5)N(5)), imidazole (C(3)H(4)N(2)) and 2-aminopyrimidine (C(4)H(5)N(3)) with the undoped δ(18)O values for the same substrates; yielding isotopic agreement to within ±0.7‰. Finally, we provide an independent analysis of the δ(18)O value of IAEA-600 caffeine (C(8)H(10)N(4)O(2)), previously characterized using online systems exclusively, and discuss the reasons for an average 1.4‰ enrichment in δ(18)O observed offline relative to the consensus online δ(18)O value.

Environmental and taxonomic controls of carbon and oxygen stable isotope composition in <i>Sphagnum</i> across broad climatic and geographic ranges

“…Carbon isotope data were corrected using sucrose (IAEA-CH-6, -10.449‰), acetanilide (in house, -37.07‰), and caffeine (IAEA-600, -27.771‰). Oxygen isotope data were corrected using sucrose (IAEA-CH-6, 36.4‰), cellulose (IAEA-C3, 31.9‰) and caffeine (IAEA-600, -3.5‰) with values from Hunsinger et al (2010). Oxygen isotope standardization was further checked with the whole wood standards USGS54 and USGS56.…”

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

“…Lastly, one of the most noteworthy general observations from these experiments was the lack of agreement between the IAEA‐600 caffeine δ 18 O values, averaging ‐0.58 ‰ (Table ) across all treatments in this study, whereas the δ 18 O values from the inter‐laboratory comparison study averaged ‐3.48 ± 0.53 ‰ . Higher δ 18 O values were also observed during offline and modified HTC analyses of IAEA‐600. This material received a great deal of discussion during the inter‐laboratory comparison, due to the large variations in results among the participating laboratories, and this may indicate that specific methodological recommendations are needed when using IAEA‐600 as an oxygen isotope reference material.…”

“…alkaloids, amino acids, pharmaceuticals, keratin, and explosives >15% N), which can complicate oxygen and hydrogen isotope ratio measurements of these materials. Traditional approaches to oxygen isotope analysis require labor‐intensive offline separation techniques to eliminate the N 2 component and isolate oxygen as CO 2 for oxygen isotope analysis on a dual‐inlet isotope ratio mass spectrometer . The standard practice for the determination of δ 18 O values in organic materials ( δ 18 O is defined in Eqn.…”

“…Traditional approaches to oxygen isotope analysis require labor-intensive offline separation techniques to eliminate the N 2 component and isolate oxygen as CO 2 for oxygen isotope analysis on a dual-inlet isotope ratio mass spectrometer. [1][2][3] The standard practice for the determination of d 18 O values in organic materials (d 18 O is defined in Eqn. (1)) is high-temperature conversion (HTC) into reduced gases (i.e.…”

“…[4][5][6][7][8] This rapid, automated, and reproducible technique has shown great promise for the application of d 18 O values (and d 2 H values) in diverse fields of study including geochemistry, [9] food authenticity, [10] biomedical, [11] animal migration, [12] and forensics. [13] Despite increasing HTC usage, several recent studies have indicated that the presence of nitrogen in analytes compromises both precision and accuracy of d 18 O [3,[6][7][8][14][15][16][17][18][19][20][21] and d 2 H measurements. [20] For instance, a recent inter-laboratory comparison for d 18 O reference materials showed high external reproducibility for caffeine (IAEA-600) with mean d 18 O values from various laboratories ranging from À4.43 to À3.19 %, with an outlier laboratory obtaining a value of À12.63 %.…”