| Summary: | Previous research revealed that isotopes <sup>13</sup>C and <sup>18</sup>O of exhaled CO<sub>2</sub> have the potential link with <i>Helicobacter pylori</i>; however, the <sup>17</sup>O isotope has received very little attention. We developed a sensitive spectroscopic sensor for simultaneous <i>δ</i><sup>13</sup>C, <i>δ</i><sup>18</sup>O, and <i>δ</i><sup>17</sup>O analysis of human breath CO<sub>2</sub> 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><sup>13</sup>C, <i>δ</i><sup>18</sup>O, and <i>δ</i><sup>17</sup>O 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><sup>13</sup>C, <i>δ</i><sup>18</sup>O, and <i>δ</i><sup>17</sup>O 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.
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