Improved online δ18O measurements of nitrogen‐ and sulfur‐bearing organic materials and a proposed analytical protocol

It is well known that N₂ in the ion source of a mass spectrometer interferes with the CO background during the δ¹⁸O measurement of carbon monoxide. A similar problem arises with the high‐temperature conversion (HTC) analysis of nitrogenous O‐bearing samples (e.g. nitrates and keratins) to CO for δ¹⁸O measurement, where the sample introduces a significant N₂ peak before the CO peak, making determination of accurate oxygen isotope ratios difficult. Although using a gas chromatography (GC) column longer than that commonly provided by manufacturers (0.6 m) can improve the efficiency of separation of CO and N₂ and using a valve to divert nitrogen and prevent it from entering the ion source of a mass spectrometer improved measurement results, biased δ¹⁸O values could still be obtained. A careful evaluation of the performance of the GC separation column was carried out. With optimal GC columns, the δ¹⁸O reproducibility of human hair keratins and other keratin materials was better than ±0.15 ‰ (n = 5; for the internal analytical reproducibility), and better than ±0.10 ‰ (n = 4; for the external analytical reproducibility).