## 2 Materials and Methods  
The method of THz spectroscopy based on the effect of freely decaying polarization works as follows. If a polarization is created in a gas sample under resonant conditions, then after the oscillator is pulled out of resonance, the polarization wave will emit the same field mode that excited it. This is done by periodically switching the phase or frequency of the probe radiation resonantly interacting with the medium, which leads to non-stationary radiation and absorption, i.e., to the periodic appearance and decay of induced macroscopic polarization. For this we have developed a spectrometer with phase switching of radiation and a fast frequency sweep spectrometer [10]. The phase switching mode [13] allows measurements to be performed at a particular frequency and is needed to monitor the dynamics of the selected substance concentration. The principle of operation of the spectrometer with phase switching of the radiation acting on the gas (the block diagram is shown in Figure 1) is as follows: the frequency of the radiation source (RS) is modulated using a phase modulator (PM) and is set by the frequency of the frequency synthesizer (FS) preset by a computer (PC). Passing through the cell with the gas under study and interacting with the gas absorption line, the radiation enters the radiation detector (RD), where it is detected, and then amplified by an amplifier (AMP). After conversion from analog to digital with using an analog-to-digital converter (ADC), the signal containing information about the gas absorption line is accumulated in the storage device and then detected and processed in the computer. There are two possible ways to detect the absorption line:  
* To detect the absorption line amplitude and the change in this amplitude over time by selecting one frequency (the absorption line center);
* To tune the frequency of the radiation source in a needed range and detect the absorption lines that lie within this range. In the first case the sensitivity of the spectrometer with phase switching of the radiation acting on the gas amounts to \(5\times 10^{-10}\) cm\({}^{-1}\) and in the second case is \(3\times 10^{-9}\) cm\({}^{-1}\). Another approach is fast frequency passage (sweep) through the absorption line [14]. Fast sweep makes it possible to detect all absorption lines in the entire frequency range of the spectrometer and determine the presence of several substances at once. A block diagram of a fast frequency sweep spectrometer is shown in Figure 2.  
The signal of the fast frequency sweep source is transferred to the frequency range of the radiation source (RS) using a broadband phase lock-loop (PLL) system. The frequency ranges are PC controlled. The radiation passes through a cell with a sample of the gas under study, interacts with it and enters the radiation detector (RD). The signal from the RD is amplified by the amplifier (AMP) and converted to digital form by an analog-to-digital converter (ADC). This signal contains information about all the absorption lines of substances in the studied gas mixture, which lie in the sweep frequency range. Further,  
Figure 1: Block diagram of a spectrometer with phase switching of the radiation source signal. the digital signal is accumulated in the digital storage accumulator (DSA) to reduce the receiver noise and thereby increase the spectrometer sensitivity. From the DSA the signal is forwarded to the PC, where it is processed and further accumulated. It takes the spectrometer 30 s to cover the frequency range of 60 GHz. The sensitivity of the recorded absorption coefficient for a cell length of 1 m is from \(10^{-7}\) cm\({}^{-1}\) to \(5\times 10^{-8}\) cm\({}^{-1}\). Using the method of THz non-stationary spectroscopy, we studied the composition of thermal decomposition products for samples of tissues of the nasal cavity and paranasal sinuses in norm and pathology, taken from patients during surgery performed according to strict medical indication, based on the functional endoscopic sinus surgery (FESS) principles.