Fig. 1. Selection of mid-infrared region spectral lines. (a) Absorption distribution of NO in infrared spectral region; (b) absorption peak distribution of NO, CO₂ and H₂O in wavenumber range of 1886-1904 cm^-1; (c) comparison of spectral line distribution and intensity of NO, CO₂ and H₂O with low pressure and low concentration in region with less H₂O absorption

Fig. 2. Structural diagram of system

Fig. 3. Peak value of NO demodulation signal varying with modulation amplitude

Fig. 4. Result of standard gas calibration experiment. (a) NO demodulation signal; (b) linearity description

Fig. 5. Peak measurement for 2f/R_1f signal of NO with volume fraction of 25×10^-9. (a) Variation of peak value in one hour; (b) result of Allan variance analysis

Fig. 6. Measurement signal of breath sample and results of multivariate linear fitting. （a） Measured spectral demodulation signals and multivariate linear fitting results of 6 samples； （b） comparison between peak values of fitted signal and measured signal； （c） linear relationship between peak values of fitted signal and measured signal

Fig. 7. Results of measured concentrations of volunteers' breath samples