## 2 Materials and Methods
A frequency-swept source signal is converted to the frequency range of the radiation source with a wideband phase-locked loop. The frequency bands are controlled by the PC. The radiation (as in the previous spectrometer presented above) transfers through the gas cell with the sample of gas under study and interacts with the gas molecules. The detector receives this signal. This signal is recorded and then sent to the amplifier. After amplifying, it is digitized by an analogue-to-digital converter (ADC), and afterwards, the digital signal passes to the accumulator and is then recorded and processed in the PC. It takes the spectrometer 30 s to sweep the 118-178 GHz range. The sensitivity of the absorption coefficient with a cell length of 1 m is \(10^{-7}\div 1.5\times 10^{-8}\) cm\({}^{-1}\). The sensitivities of concentration are 1.6 ppm \(\div\) 250 ppb for H\({}_{2}\)S (f = 168762.7624 MHz, lggl = \(-\)2.8376) [11] in this case. To study the composition of vapors and of the thermal decomposition products of tissue samples aimed at detecting metabolites and potential markers of pathologies, the following tissue sections were used: a relatively healthy mucosa of the nasal cavity (n = 10) obtained as a result of standard access during a sphenetomy (fragments of the mucosa from the posterior ends of the inferior turbinates) and samples of pathological material obtained from the sphenoid sinus (n = 10), as well as from the maxillary sinus (n = 10). Part of the material (3-5 mg) was placed in a container with distilled water for analysis using the spectrometers described above. In the laboratory, the sample tissue under study was put in a flask connected to a measuring cell pre-evacuated to 0.0005 mbar. The sample was dehydrated under low vacuum pressure, and then the flask was pre-evacuated to 0.005 mbar. After that, the flask was heated up to 200-250 \({}^{\circ}\)C, and the vapors and products of thermal decomposition of the sample were let into the measuring cell, increasing the pressure up to about 0.05 mbar (working pressure). The dynamics of substance concentration were monitored in the phase switching mode, and fast sweeping made it possible to track all absorption lines in a given range and detect the presence of various substances. With the use of two variants of devices,  
Figure 2: The scheme of a fast frequency sweep spectrometer, where PC is a personal computer and ADC is an analogue-to-digital converter. we obtained an optimal picture of the thermal decomposition of biological samples, taking into account the chemical composition of the gas mixture in a more complete version, including the dynamics of the formation of the compounds. Substances were identified by the absorption lines using the electron databases [11; 12]. The frequency range corresponding to the operating ranges of both spectrometers contains strictly fixed absorption lines for specific substances, which are unambiguous signs of the presence of this substance in the studied multicomponent gas mixture. The absorption coefficient of each specific line is proportional to the concentration of a specific substance that interacts with the radiation contained in the mixture under study [15; 16]. Therefore, the number of lines recorded in measurements of the spectra of a multicomponent gas mixture of products at thermal decomposition for the compared samples when measurements are taken under the same conditions (maintaining working pressure in the cell at a certain level and the same stage of heating the sample)may be an informative sign for a qualitative analysis of changes in the composition (an increase or decrease in the relative concentration of substances from sample to sample). It is important to note that in our study we used identical (for the phase switching spectrometer) and working (for the fast frequency sweep spectrometer) frequency ranges, which made it possible to qualitatively compare the contents of substances by estimating the number of recorded absorption lines in the spectrum of the sample.