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Low Temperature Catalytic Combustion Reactors for High Precision Carbon Isotope Measurements in Gas Chromatography Combustion Isotope Ratio Mass Spectrometry

Tobias, Herbert J., Jones, Andrew, Spanjers, Charlie, Bowers, Larry, Brenna, J. Thomas
Analytical chemistry 2019 v.91 no.4 pp. 2901-2907
alkanes, carbon, carbon dioxide, catalysts, ceramics, combustion, gas chromatography, mass spectrometry, oxygen, silica, stable isotopes, standard deviation, steroids, temperature
Metal oxide-filled reactors constructed with ceramic tubes or fused silica capillary are widely used for combustion in gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). However, they tend to be easily cracked or broken and prone to leaks at operating temperatures of ∼950 °C. Here we introduce a modified commercially available catalytic combustion/reduction methanizer to quantitatively convert organics to CO₂ for δ¹³C analysis while retaining chromatographic resolution. These modified “ARC” reactors operate with a transition-metal catalyst that requires a flowing O₂ gas to enable complete conversion to CO₂ at lower temperature (620 °C) with acceptable reactor life, reduced complexity, and improved robustness. Performance of two versions of the ARC reactors with different combustion volumes was characterized by analysis of steroid and alkane isotopic standard materials. Linearity of steroid isotopic standards ranged from 0.02 to 0.60 ‰/V in the range of 25 to 200 ng of each steroid injected. Precisions and accuracies of measurements for steroids and alkanes had average standard deviations of SD(δ¹³C) less than ±0.18 ‰ and average accuracy of better than 0.19 ‰ δ¹³CVPDB. Peak width expansion within both devices were similar to that in traditionally used metal oxide reactors. These data demonstrate for the first time that novel combustion schemes enable operation at lower temperatures as an alternative approach comparable to high temperature techniques to yield high precision δ¹³C data with GCC-IRMS.