PERFORMANCE AND OPTIMIZATION OF A COMBUSTION INTERFACE FOR ISOTOPE RATIO MONITORING GAS-CHROMATOGRAPHY MASS-SPECTROMETRY

Conditions and systems for on-line combustion of effluents from capill ary gas chromatographic columns and for removal of water vapor from pr oduct streams were tested. Organic carbon in gas chromatographic peaks 15 s wide and containing up to 30 nmol of carbon was quantitatively c onverted to CO2 by tubular combustion reactors, 200 x 0.5 mm, packed w ith CuO or NiO. No auxiliary source of O-2 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 spon taneously degrade, its use might be favored, but Ni-bound carbon phase s form and lead to inaccurate isotopic results at T < 1050 degrees C i f gas-phase O-2 is not added. For all compounds tested except CH4, equ ivalent isotopic results are provided by CuO at 850 degrees C, NiO + O -2 (gas-phase mole fraction, 10(-3)) at 1050 degrees C, and NiO at 115 0 degrees C. The combustion interface did not contribute additional an alytical uncertainty, thus observed standard deviations of C-13/C-12 r atios were within a factor of 2 of shot-noise limits. For combustion a nd isotopic analyses of CH4, in which quantitative combustion required T approximate to 950 degrees C, NiO-based systems are preferred, and precision is similar to 2 times lower than that observed for other ana lytes. Water must be removed from the gas stream transmitted to the ma ss spectrometer or else protonation of CO2 will lead to inaccuracy in isotopic analyses. Although thresholds for this effect vary between ma ss spectrometers, differential permeation of H2O through Nafion tubing was effective in both cases tested, but the required length of the Na fion membrane was 4 times greater for the more sensitive mass spectrom eter.