## 2 Materials and Methods  
The THz spectroscopy approach based on nonstationary effects is as follows: the radiation interacts with the molecules of gas under resonant conditions, and the macroscopic polarization is observed in a gas sample in this case, then the radiation is taken out of resonance (by switching off or changing the frequency or phase) with the time shorter than gas polarization relaxation times, the polarization signal will emit the same mode of electromagnetic field mode as the mode inducing the polarization. This approach is realized by periodic phase or frequency switching. The corresponding devices are implemented in two forms: a phase switching spectrometer and a fast frequency sweeping spectrometer [7]. The phase switching mode enables the detection at a certain frequency of the rotational transition of a certain substance and is used for monitoring the dynamics of its concentration. The spectrometer with phase switching of radiation influencing gas [6] (the scheme is presented in Figure 1) has the radiation source generating the frequency of the signal for probing, a phase modulator modulating the frequency of the radiation source, a frequency synthesizer setting this frequency, and a personal computer (PC) controlling the frequency synthesizer. The detector receives the signal after transferring through the gas cell with the sample of gas under study and after interacting with the gas molecules. This signal is recorded and then sent to the amplifier. After amplifying, it is digitized by an analogue-to-digital converter (ADC), and after that, the digital signal passes to the accumulator and is then recorded and processed in the PC. The information about the rotational absorption line is in the signal that arrived at the detector and will not be lost at the next manipulation. There are two possible ways to record absorption lines:  
* Choose the frequency of the absorption line center of the substances under study and record the dynamics of the line amplitude during the measurement time;
* Tune the radiation source frequency over the frequency band and record the rotational lines falling within this band. In the first mode, the sensitivity (minimal absorption coefficient) of the phase switching spectrometer reaches 5 \(\times\) 10\({}^{-10}\) cm\({}^{-1}\), and in the second case, 3 \(\times\) 10\({}^{-9}\) cm\({}^{-1}\). If the absorption lines have integrated intensity of 0.001 nm\({}^{2}\)\(\times\) MHz (the lgI = \(-\)3 presented in database [11; 12]) as forHzS (f = 168762.7624 MHz, lgI = \(-\)2.8376) [11], the concentration sensitivity (for absorption coefficient sensitivities of spectrometers presented in our work) is approximately 8.3 ppb (for minimal absorption coefficient of 5 \(\times\) 10\({}^{-10}\) cm\({}^{-1}\)) and 50 ppb (for a minimal absorption coefficient of 3 \(\times\) 10\({}^{-9}\) cm\({}^{-1}\)). Most of the absorption lines in the range of 118-178 GHz detected in the gas mixture under study have the lgI at a level of \(-\)4 or \(-\)5. For those lines, the minimal concentration would be at the level of 100 ppb \(-\)1 \(\div\) 10 ppm. The levels of concentration sensitivities are better (for H\({}_{2}\)S) or comparable (for some other substances) with the sensitivity (relative to concentration) in [10; 13]. Figure 1: The scheme of a phase switching spectrometer, where PC is a personal computer and ADC is an analogue-to-digital converter. The approach based on a fast frequency sweep through the absorption line was used in the spectrometer with a radiation source on a backward wave oscillator operating in the range from 118 to 178 GHz [14]. Fast sweeping allows for the recording of all the absorption lines throughout the spectrometer's frequency range and detecting several substances simultaneously. The scheme of the fast frequency sweep spectrometer is shown in Figure 2.  
The radiation source has a built-in module consisting of a commercial reference frequency synthesizer, the LMX2594 (Texas Instruments, Dallas, TX, USA), used as the reference oscillator.