Common to all these methods is that they require relatively high amounts of sample material, the conversion takes place in vacuo, and the reactions do not always lead to a single oxygen-bearing product, necessitating a second reaction step to convert other oxygen-containing products to CO 2 .A prerequisite for an on-line technique for d 18 O measurement is a fast and quantitative conversion of the sample oxygen to a single gaseous product. To produce CO or CO 2 as a measuring gas for the isotope ratio mass spectrometer (IRMS) during a carbon reduction reaction, a reagent providing carbon and reduction equivalents is needed. Although the ratio between CO and CO 2 in a static system is according to the Boudouard equilibrium 99:1 in the presence of carbon at temperatures above 1000°C, even higher temperatures are necessary in dynamic systems. 29In a previously described on-line system for d18 O and d 15 N measurement of nitrate, 30,31 samples were converted at 1200°C with graphite, but the main product was still CO 2 , accompanied by small amounts of CO, probably due to the construction of the heating device.

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A method for the automated sample conversion and on-line oxygen isotope ratio (delta(18)O) determination for organic and inorganic substances is presented. The samples are pyrolytically decomposed at 1400 degrees C in the presence of nickelized graphite. With the system presented organic as well as inorganic samples such as nitrates, sulphates and phosphates of 50-100 &mgr;g O can be analyzed for their delta(18)O values with a standard deviation usually better than 0.5 per thousand. Additionally, carbon isotope ratios of organic substances and nitrogen isotope ratios of inorganic nitrogenous compounds are available in the same sample run. Data for international and some inter-laboratory reference materials are presented to show the accuracy and reliability of the method. The effect of some additives on the CO yield was checked for substances which do not pyrolyze completely. Copyright 1999 John Wiley & Sons, Ltd.

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