Performance and Optimization of a Combustion Interface for Isotope Ratio Monitoring Gas Chromatography/Mass Spectrometry

Abstract

Conditions and systems for on-line combustion of effluents from capillary gas chromatographic columns and for removal of water vapor from product streams were tested. Organic carbon in gas chromatographic peaks 15 s wide and containing up to 30 nanomoles of carbon was quantitatively converted to CO2 by tubular combustion reactors, 200 x 0.5 mm, packed with CuO or NiO. No auxiliary source of O2 was required because oxygen was supplied by metal oxides. Spontaneous degradation of CuO limited the life of CuO reactors at T > 850 degrees C. Since NiO does not spontaneously degrade, its use might be favored, but Ni-bound carbon phases form and lead to inaccurate isotopic results at T < 1050 degrees C if gas-phase O2 is not added. For all compounds tested except CH4, equivalent isotopic results are provided by CuO at 850 degrees C, NiO + O2 (gas-phase mole fraction, 10(-3)) at 1050 degrees C and NiO at 1150 degrees C. The combustion interface did not contribute additional analytical uncertainty, thus observed standard deviations of 13C/12C ratios were within a factor of 2 of shot-noise limits. For combustion and isotopic analyses of CH4, in which quantitative combustion required T approximately 950 degrees C, NiO-based systems are preferred, and precision is approximately 2 times lower than that observed for other analytes. Water must be removed from the gas stream transmitted to the mass spectrometer or else protonation of CO2 will lead to inaccuracy in isotopic analyses. Although thresholds for this effect vary between mass spectrometers, differential permeation of H2O through Nafion tubing was effective in both cases tested, but the required length of the Nafion membrane was 4 times greater for the more sensitive mass spectrometer.

Isotope‐ratio detection for gas chromatography

“…The optimal reactor conditions for achieving both complete combustion and long lifetime are the subject of some debate, and vary with analyte chemistry. Merritt et al [25] provide a comprehensive discussion of the subject. The two primary considerations are temperature and source of oxidizing power.…”

“…CuO is the better reagent at temperatures below l8008C, whereas NiO becomes favorable at higher temperatures. The variation arises from the temperature dependence of O 2 partial pressure over those reagents [25]. A widely applicable solution developed by ThermoElectron is to include both CuO and NiO in the reactor, together with Pt as a catalyst, operated at 900-9508C [55].…”

“…This can be done periodically -typically each night -by flushing the reactor with high concentrations of O 2 while diverting flow from the IRMS. Alternatively, the regeneration can be continuous by adding a small amount of O 2 to the carrier gas at a point between the analytical column and combustion reactor [25]. The size of this "O 2 trickle" must be carefully adjusted so as not to damage the IRMS filament, and a common practice is to set its flow to produce a l20 nA ion current at m/z 32 in the IRMS.…”

“…The first utilizes a narrow-bore glass or ceramic tube, typically 0.5 mm inner diameter. Reagent metal wires (Cu, Ni, and/or Pt), l0.1 mm diameter, are twisted together and threaded into the tube [25,55]. The assembled reactor tube is inserted into a resistively heated tube furnace, connected to fused-silica capillaries at each end (Fig.…”

“…5). By simultaneously opening both valves, helium is added such that flow through the reactor is reversed and GC column effluent is vented to the atmosphere [25,73]. Such a system is commonly called "backflushing," and is operated under computer control.…”