Evaluation of spurious results in the infrared measurement of CO2 isotope ratios due to spectral effects: a computer simulation study

The application of infrared spectroscopy to the measurement of carbon isoto pe ratio breath tests is a promising alternative to conventional techniques , offering relative simplicity and lower costs. However, when designing suc h an instrument one should be conscious of several spectral effects that ma y be misinterpreted as changes in the isotope concentration and which there fore lead to spurious results. Through a series of computer simulations whi ch model the behaviour of the CO2 absorption spectrum, the risk these effec ts pose to reliable measurement of (CO2)-C-13/(CO2)-C-12 ratios and the mea sures required to eliminate them are evaluated. The computer model provides a flexible high-resolution spectrum of the four main isotopomer fundamenta l transitions and fifteen of their most significant hotband transitions. It is demonstrated that the infrared source, infrared windows and breath samp le itself all exhibit strong temperature-induced errors but pressure effect s do not produce significant errors. We conclude that for reliable measurem ent of (CO2)-C-13/(CO2)-C-12 ratios using infrared spectroscopy no pressure controls are required, window effects are eliminated using windows wedged at a minimum angle of 0.8-2.2 mrad, depending on the material, and the temp erature sensitivity of source and gas cells necessitates stabilization to a n accuracy of at least 0.2 K.