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 15s wide and containing up to 30 nmol of carbon was quantitatively converted to CO2by tubular combustion reactors, 200 x 0.5 mm, packed with CuO or NiO. No auxiliary source of O2was required because oxygen was supplied by metal oxides. Spontaneous degradation of CuO limited the life of CuO reactors at T > 850 °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 °C if gas-phase O2is not added. For all compounds tested except CH4, equivalent isotopic results are provided by CuO at 850 °C, NiO + O2(gas-phase mole fraction, 10-3) at 1050 °C, and NiO at 1150 °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 ≈a 950 °C, NiO-based systems are preferred, and precision is ~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 CO2will 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.
All Science Journal Classification (ASJC) codes
- Analytical Chemistry