Simultaneous Sensitive Determination of δ13C, δ18O, and δ17O in Human Breath CO2 Based on ICL Direct Absorption Spectroscopy

Abstract

Previous research revealed that isotopes ¹³C and ¹⁸O of exhaled CO₂ have the potential link with <i>Helicobacter pylori</i>; however, the ¹⁷O isotope has received very little attention. We developed a sensitive spectroscopic sensor for simultaneous <i>δ</i>¹³C, <i>δ</i>¹⁸O, and <i>δ</i>¹⁷O analysis of human breath CO₂ based on mid-infrared laser direct absorption spectroscopy with an interband cascade laser (ICL) at 4.33 μm. There was a gas cell with a small volume of less than 5 mL, and the pressure in the gas cell was precisely controlled with a standard deviation of 0.0035 Torr. Moreover, real-time breath sampling and batch operation were achieved in gas inlets. The theoretical drifts for <i>δ</i>¹³C, <i>δ</i>¹⁸O, and <i>δ</i>¹⁷O measurement caused by temperature were minimized to 0.017‱, 0.024‱, and 0.021‱, respectively, thanks to the precise temperature control with a standard deviation of 0.0013 °C. After absolute temperature correction, the error between the system responded <i>δ</i>-value and the reference is less than 0.3‱. According to Allan variance analysis, the system precisions for <i>δ</i>¹³C, <i>δ</i>¹⁸O, and <i>δ</i>¹⁷O were 0.12‱, 0.18‱, and 0.47‱, respectively, at 1 s integration time, which were close to the real-time measurement errors of six repeated exhalations.