18 O-analysi
8Pages

analysis of nitrogen containing organic compounds using low temperature pyrolysis The quantitative conversion of organically bound oxygen into CO for the 18O-analysis of organic compounds is traditionally performed by high temperature pyrolysis at ~1430°C. This procedure demands a complicated and expensive equipment. Theoretically, the conversion should be possible already at ~1200°C and by using carbon black instead of glassy carbon as catalyst. This technique is in use for oxygen elemental analysis for many decades. When using the ordinary GC-separation for separating the pyrolysis product gases, Ncontaining compounds yield incorrect results, although the bound oxygen is quantitatively converted into CO. This is due to partly overlapping NO and CO peaks at m/z 30 and overlapping N2 and CO peaks at m/z 28 when GC columns are used. NO is formed in the ion source by the reaction of N2 (a pyrolysis product) and traces of oxygen (from water or small leaks) on the hot surface of the filament. The 18O measurements for N-containing samples is therefore biased (see Accoe et al. [2008]). By using the purge and trap chromatography technique, which is standard in all Elementar Elemental analyzers, this overlap can be eliminated by heating the CO column first when the nitrogen peak is completely finished. For this technique, the time between the nitrogen and CO peaks can be freely selected to ensure full peak separation, in contrast to the GC instruments which have a fixed short time between the two peaks. During pyrolysis, the bulk of the bound nitrogen is not converted directly to N2, but into Cand N-containing non-volatile polymers, which decompose slowly to N2 and other compounds and interfere with the isotope ratio measurement of CO. To prevent these secondary pyrolysis gases to enter the mass spectrometer they are vented before the CO trap by the newly developed back-flush system. In this report 18O analyses are shown using low temperature pyrolysis (LTP) at 1170°C with the Elementar elemental analyzer (EA) vario ISOTOPE cube, equipped as standard with the purge and trap chromatography technique and the back-flush system, coupled with an Isoprime isotope ratio mass spectrometer (IRMS). These analyses show the excellent performance of the EA-IRMS system for low temperature pyrolysis of nitrogen containing organic samples.

Application report s§>isoprime General description of the system The vario ISOTOPE cube is run in oxygen mode using the oxygen upgrade kit (order nr. 51.^0-0001). The quartz pyrolysis tube is filled with carbon black and is heated to 1170°C. Since carbon black has a larger surface as glassy carbon, the velocity of the CO/CO2 + C equilibration is raised, which ensures the complete conversion of all 0 to CO. The operating mode of the vario ISOTOPE cube is shown by means of the functional diagram Isoprime House Earl Road, Cheadle Hulme Email: info@ isoprime. co.uk

Application report s§>isoprime 1. The sample drops from the sample tray through the ball valve into the pyrolysis 2. The sample is pyrolysed and the product gases stream with the carrier gas helium through several absorption tubes to eliminate water, basic and acid pyrolysis 3. The gas stream, containing He, N2 and CO, pass through the CO adsorption column where all CO is adsorbed and only He and N2 remain. k. The He-N2 mixture passes through the thermal conductivity detector (TCD). The TCD signal gives, however, no indication for the total amount of nitrogen in the sample, since not all...

Application report The next Figure shows the excellent peak separation between the N2 and CO peaks using the purge and trap technique and the stable base line of the TCD during the CO peak using the backflush system. Indicated is also the TCD signal without backflush, showing the slowly decreasing N2 background signal of the secondary pyrolysis gases. without hackflu^h isoprime House Earl Road, Cheadle Hulme Email: info@i sop rime, co.uk

analysis of nitrogen containing organic compounds using low temperature pyrolysis Ten different standard materials have been analyzed using the vario ISOTOPE cube in oxygen mode connected to an ISOPRIME isotope ratio mass spectrometer. The determined 18O ratio has been compared with the nominal values. The samples have been weighed into silver boats, with a sample weight corresponding to 350 µg O absolute. All samples have been measured six times. * absolute standard deviation < 0.2

analysis of nitrogen containing organic compounds using low temperature pyrolysis For comparison, a study of the 18O value of different reference materials, measured with different high temperature conversion IRMS instruments at different institutes, shows that the measured 18O values range over up to 3 ‰ [Brand et al., 2009]. This is an indication for the accuracy of commonly used IRMS systems for oxygen isotope ratio measurements. This study also shows that the precision of the 18O measurements, given by the standard deviation of eight subsequent samples measured in one laboratory on...

Application report s§>isoprime The major advantages of the vario ISOTOPE-ISOPRIME system presented here are: Due to the lower operation temperature less expensive pyrolysis tubes (quartz glass instead of glassy carbon/ceramics) and ovens can be used, which additionally have a longer lifetime as the high temperature ones. The vario ISOTOPE cube comes with a standard e/ementarfurnace with 10 years warranty. Due to the purge and trap technique a full separation between the N2 and CO peaks Memory effects causing drifts in the 180 analyses are minimized by the back flush The additional...

Application report s§>isoprime F. Accoe, F., M. Berglund, B. Geypens and P. Taylor, Methods to reduce interference effects in thermal conversion elemental analyzer/continuous flow isotope ratio mass spectrometry 180 measurements of nitrogen-containing compounds, Rapid Communications in Mass Spectrometry, 2280-2286,2008. Brand, W. A., T. B. Coplen, A. T. Aerts-Bijma, J. K. Bohlke, M. Gehre, H. Geilmann, M. Grbning, H. G. Jansen, H. A. J. Meijer, S. J. Mroczkowksi, H. Qi, K. Soergel, H. Stuart- Williams, S. M. Weise und R. A. Werner, Comprehensive inter-laboratory calibration of reference...