E.V. Stepanov, P.V. Zyrianov, V.A. Miliaev, S.G. Kasoev, A.A. Panov

Human exhalation contains traces of several hundreds of organic and nonorganic molecular com-pounds that are endogenously producing in living organism. The concentration of such molecules in exhaled air is considerably low, 5-8 orders of magnitude, than the concentration of main me-tabolites, CO2 and H2O, in expired air. Development of new instrumental approachs to the analysis of such molecular gases in exhalation obtaining necessary sensitivity, selectivity and accuracy of the detection could give new possibilities for the investigation of the molecules in human organism, their production, transportation, utilization and illumination, and then for using to biomedical diagnostics. Methane (CH4) and ethylene (C2H4) are the molecules that could be considered as possible biomarkers. CH4 production could be mainly related with bacterial activity in intestine and its variation in exhaled air could be useful in diagnosing problems in the digestion processes. Production of C2H4 in living organism could be related with free radical oxygenation and oxidative stress and thus ethylene could be used for assessment of these pathological processes. In frames of the present study a selection of spectral regions best for the detection of methane and ethylene in exhaled air was provided. For this purpose rotational-vibration spectra of CH4, C2H4, NH3, CO2 and H2O in the middle and near IR spectral regions were calculated and analyzed. The best spectral regions for the detection of CH4 were found to be located at 3060 and 6053 cm-1. The best spectral regions for the detection of C2H4 is located at 997 cm-1. An adaptation of optical sche-matic of gas analyzer based on tunable diode lasers was provided. Electrical, thermal and spectral properties of tunable diode laser operated in this spectral region and used for the methane and ethylene detection were carefully studied. To increase the detection sensitivity of the spectral analysis two channel optical scheme and multipass gas cell with optical path of 15 m were used. Special algorithms of spectral data filtering and processing as well as noise suppression were applied to minimize systematic and random errors of CH4 and C2H4 concentration analysis in breath and to provide this measurements in real time regime. The applied optical schematics and electronic solutions have provided the analytical parameters of the system necessary for quantitative analysis of CH4 and C2H4 content in exhaled air. Particularly, the concentration sensitivity was at the level of 0.1 ppb, accuracy - 3-5%, selectivity to CO2 and H2O - close to 100%, speed of the analysis ? 3-10 s, volume of gas sample - 0,5-1,0 l.