An experimental setup and a method for analyzing multicomponent gas mixtures, including human exhaled air, are presented. The installation consists of a quantum cascade laser, tunable in the wavelength range of 5.3-12.8 µm, with a peak power of 150 mW, and a multi-pass Herriot gas cell, which allows obtaining an optical path of up to 76 m. The registration time of a single spectrum is about 50 ms. For acetone and ethanol, which are potential biomarkers of some human diseases, the sensitivity threshold at the sub-ppm level was experimentally determined. A system of sample preparation and pre-drying is proposed, which allows analyzing both multicomponent gas mixtures and the air exhaled by a person. The variants of application of the described installation in biomedical applications are proposed.