John Karl Bohlke Kinga M. Revesz 2002 <div id="" class="NLM_sec NLM_sec_level_1"><div class="NLM_p last"><span>Three different KNO</span>₃<span>&nbsp;salts with δ</span>¹⁸<span>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</span>₃<span>&nbsp;samples with δ</span>¹⁸<span>O values in the midpoint of the δ</span>¹⁸<span>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</span>¹⁸<span>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</span>¹⁸<span>O values far from that of atmospheric O</span>₂<span>, to improve interlaboratory comparability.</span></div></div> application/pdf 10.1021/ac025854b en American Chemical Society Comparison of δ18O measurements in nitrate by different combustion techniques article