Comparison of δ18O measurements in nitrate by different combustion techniques

Abstract

Three different KNO 3 salts with δ 18 O values ranging from about −31 to +54‰ relative to VSMOW were used to compare three off-line, sealed glass tube combustion methods (widely used for isotope studies) with a more recently developed on-line carbon combustion technique. All methods yielded roughly similar isotope ratios for KNO 3 samples with δ 18 O values in the midpoint of the δ 18 O scale near that of the nitrate reference material IAEA-NO-3 (around +21 to +25‰). This reference material has been used previously for one-point interlaboratory and intertechnique calibrations. However, the isotope ratio scale factors by all of the off-line combustion techniques are compressed such that they are between 0.3 and 0.7 times that of the on-line combustion technique. The contraction of the δ 18 O scale in the off-line preparations apparently is caused by O isotope exchange between the sample and the glass combustion tubes. These results reinforce the need for nitrate reference materials with δ 18 O values far from that of atmospheric O 2 , to improve interlaboratory comparability.