THE MEASUREMENT OF STABLE ISOTOPE NATURAL ABUNDANCE VARIATIONS

Precise stable isotope natural abundance analysis of the elements of organic matter, yields a wealth of information for the biologist. Robust sample preparation methodology and analytical instrumentation is necessary to achieve precise results. Basic principles of isotope ratio mass spectrometry (IRMS) are detailed, with particular regard to sample size, gas production and transfer into the IRMS ion source. Gas preparation methods developed to give quantitative yields of pure simple gases from organic and inorganic materials include vacuum line combustion, ampoule combustion and automated elemental analysers used off and on-line. The new technique of GC-C-IRMS, where individual volatile organic compounds are separated by GC, combusted and analysed on-line by IRMS, is also described. The conventional dual batch inlet developed by geochemists for the most precise analysis of stable isotopes, is contrasted with continuous flow-IRMS analysis. The needs of the biological scientist for rapid throughput of small samples are discussed in this context. It is argued that the development of new instrumental approaches will permit many new applications of stable isotope methodology in the biological sciences.