A rapid on-line technique for determination of oxygen isotope composition of nitrogen-containing organic matter and water

Abstract

A rapid, inexpensive on-line technique for the analysis of stable isotopes of oxygen in organic matter and water is described. The oxygen in the sample is pyrolysed over nickelized carbon using a modified Unterzaucher technique and converted to carbon monoxide. Separation of the CO from N 2 by gas chromatography before mass spectrometric measurement of the 18 O/ 16 O ratio allows application of the technique to nitrogen containing samples. The precision of measurements is generally as good as, or better than, that obtained with other methods (σ = 0.2‰ for n = 10). A major advantage of the method is its speed, each sample taking less than 7 minutes to analyse. The technique improves on those used previously, in being applicable to a wide range of samples including N-containing organic matter and water and in being rapid, simple and inexpensive while retaining precision. © 1997 by John Wiley & Sons, Ltd. Received 21 May 1997; Accepted 22 July 1997 Rapid. Commun. Mass Spectrom. 11, 1554-1560(1997 Effective and reliable analysis of stable isotopes of oxygen in organic matter and in water has become an attractive goal in recent years. Oxygen isotope analysis has many applications related to ecology (sources of water, seasonality of growth), agriculture (screen for stomatal conductance), physiology/biochemistry (water pathways, kinetic limitations), climate reconstruction (tree rings), forensic science (origins of drugs, paper, cotton, wine, etc.) and global climate change (sinks and sources of CO 2 ). Whilst rapid, accurate techniques are well established for measurement of stable isotopes of other biologically important elements (carbon, hydrogen and nitrogen), studies employing 18 O have been hampered by the lack of a rapid, generally applicable analytical technique. This paper describes a rapid, inexpensive on-line method for measuring the oxygen isotope ratio of organic and water samples.Traditionally, analysis of oxygen isotopes has involved decomposition of the sample by pyrolysis and conversion of the oxygen-containing products to carbon dioxide. The focus has been on CO 2 as the target gas because it is readily formed by chemical action and is easy to trap and purify. The Schutze-Unterzaucher procedure is well established as a technique for oxygen elemental analysis [1][2][3][4][5] and, more recently, for isotope analysis. 6 The sample is pyrolysed over nickelized carbon at 1060 °C to convert all oxygen products to carbon monoxide, which is then oxidized to CO 2 with iodine pentoxide. As well as correcting for any contamination of the oxygen pool in CO 2 by an oxygen blank (associated with the cups used for containing the sample for pyrolysis) and memory (oxygen from previous samples), it is necessary to correct for the oxygen contributed by iodine pentoxide to the CO 2 .Other methods used include those of Rittenberg and Ponticorvo 7 and Thompson and Gray. 8 The first, involving pyrolysing the sample in the presence of mercuric chloride at around 500 °C, has been improved more recently by Schimmelmann and De...

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