Comparison of δ18O measurements in nitrate by different combustion techniques

Three different KNO3 salts with delta(18)O values ranging from about -31 to +54parts per thousand 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 delta(18)O values in the midpoint of the delta(18)O scale near that of the nitrate reference material IAEA-NO-3 (around +21 to +25parts per thousand). 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-fine 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 delta(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 delta(18)O values far from that of atmospheric O-2, to improve interlaboratory comparability.