Gas Chromatography Combustion Isotope Ratio Mass Spectrometry (GC/C/IRMS)
Gas chromatography combustion isotope ratio mass spectrometry (GC/C/IRMS) is a highly specialised instrumental technique used to ascertain the realative ratio of light stable isotopes of carbon (13C/12C), hydrogen (2H/1H), nitrogen (15N/14N) or oxygen (18O/160) in individual compounds separated from often complex mixtures of components. The ratio of these isotopes in natural materials varies slightly as a result of isotopic fractionation during physical, chemical and biological processes resulting, in some cases, with the relative isotopic ratio of specific compounds being highly diagnostic of key environmental processes. Furthermore, use of growth substrates incorporating compounds artificially enriched in the heavier isotope can aid in the deconvolution of often highly complex and obscure biogeochemical pathways. The primary prerequisite for GC/C/IRMS is that the compounds constituting the sample mixture are amenable to GC, i.e. they are suitably volatile and thermally stable. Polar compounds may require further chemical modification (derivatization) and in such cases the relative stable isotope ratio of the derivatization agent must also be determined.
Figure 1 depicts a schematic of a typical GC/C/IRMS instrument (based on a ThermoElectron Delta XP). Just as for GC/MS the sample solution is injected into the GC inlet where it is vaporized and swept onto a chromatographic column by the carrier gas (usually helium). The sample flows through the column and the compounds comprising the mixture of interest are separated by virtue of their relative interaction with the coating of the column (stationary phase) and the carrier gas (mobile phase). For carbon and nitrogen compounds eluting from the chromatographic column then pass through a combustion reactor (an alumina tube containing Cu, Ni and Pt wires maintained at 940 ºC) where they are oxidatively combusted. This is followed by a reduction reactor (an alumina tube containing three Cu wires maintained at 600 ºC) to reduce any nitrogen oxides to nitrogen. For hydrogen and oxygen a high temperature thermal conversion reactor is required (not shown). Water is then removed in a water separator by passing the gas stream through a tube constructed from a water permeable nafion membrane. The sample is then introduced into the ion source of the MS by an open split interface.
Ionisation of the analyte gases (CO2, H2, N2 or CO) is achieved using electron ionisation (EI) which has already been described under the section relating to GC/MS. The ionised gases are separated in a single magnetic sector analyser by virtue of their momentum and are detected by an array of Faraday cups the output from which is used to calculate the final stable isotope ratio. This is calculated relative to a standard of known isotopic composition and expressed using the dimensionless 'per mil' notation. A more extensive overview of GC/C/IRMS and the various methodologies associated with it may be downloaded here.
Figure 1 A schematic of a typical GC/C/IRMS instrument