Determination of Average Isotope Abundances in Organic Compounds

Methods for the Determination of Average Isotope Abundances in Organic Compounds 

1 Classic Elemental Analysis Procedures and Isotope Ratio Mass Spectrometry 

The common instruments use electron-impact ionisation, an ion separation by a (permanent or an electro-) magnet, high voltage ion acceleration and a double or triple Faraday cup set for ion current measurement. The ion current for the compound with the lower abundance isotopologue is amplified 100 to 1000 times in correspondence to that for the main isotopologue. For more details about the technical side of mass spectrometers and the measuring procedures, see e.g. references [134, 135], 

The reference principle for on-line methods has to be different as compared to classic dual-inlet isotope ratio analysis, as it is not possible to introduce sample and reference gas in exactly the same maimer into the mass spectrometer. Reference gas pulses have to be set at different points during one mn, but also laboratory standards with knovm 8-values have to be analysed periodically and exactly in the same manner and under the same conditions as the samples under investigation (identical treatment principle ]13]). A general problem of on-line HPLC and GC methods in isotope ratio analysis is the lack of international reference materials suitable to fulfil the above mentioned requirements. This implies the necessity to establish suitable laboratory standards by EA (elemental analysis) or classic measurements. Meanwhile, also efforts in supplying suitable international organic standards have been overtaken by the IAEA (e.g. caffeine, glutamic acid ]176, 177]). This programme (benzoic acid reference materials with different 8 0-values) will also support the on-line EA and GC 8 0-meas-urement by reductive pyrolysis (carbon reduction) methods (standardisation problems are compiled in ]178]). 

2 On-line Coupling of Gas Chromatography to Stable Isotope Ratio Mass Spectrometry 

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