Comparison of δ18O measurements in nitrate by different combustion techniques

Three different KNO₃ salts with δ¹⁸O 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 KNO₃ samples with δ¹⁸O values in the midpoint of the δ¹⁸O 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 δ¹⁸O 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 δ¹⁸O values far from that of atmospheric O₂, to improve interlaboratory comparability.