## 3 Results and Discussion
### Evaluation of HS Sampling and Identification of Pollutants in HS Gas  
Organic solvents, which are used during paint production and are the typical representation of VOCs, are a group of relevant environmental pollutants [31]. One of the ideal methods for sampling VOCs is the headspace technique. The major advantages of this simple method are the cost (it is a cheap technique), the instrumentation (without using complicated instruments), and its solventless character. Headspace sampling requires only the thermodynamic equilibrium between the aquatic and gaseous phases in a closed-tempered vial. After reaching equilibrium, the volatile compounds are represented in gas phase which is injected into the GC and analyzed without the influence of the matrix or extraction solvent. On the other hand, the disadvantage of headspace sampling is the restriction of the sensitivity of the method by the concentration of the VOCs in the headspace phase. The concentration of VOCs in the headspace phase depends on several parameters, such as the initial concentration in the water, the phase ratio between the liquid phase and gas phase, and the water/air distribution constant [31]. The paint-industrial water effluent underwent analysis by GC-MS in full scan mode, and some of the VOCs were identified in the HS, namely toluene, ethylbenzene, chlorobenzene, and the isomers of xylene. The headspace sampling parameters were studied by comparing the chromatographic peak areas of the identified compound with those recorded at different sampling parameters. Different parameters affect the effective transfer of analytes to a headspace gas. The effects of sampling temperature, sampling time, stirring rate, and the volume of the injected gas phase on the efficiency of the VOC transfer to the gas phase were investigated and optimized independently. At first, the optimal volume of the injected HS phase into the GC-MS system was selected. A 30 mL sample of industrial water was placed in a vial, which was heated in a water bath to a temperature of 60 \({}^{\circ}\)C. After the HS sampling time, a volume of 10, 20, 30, 40, and 50 \(\upmu\)L was injected into the gas chromatograph, and the chromatographic peaks of the analytes were subsequently compared. A gradual increase in the peak areas of the analytes was observed with an increase in the injected volume of the gas phase. The largest peak areas were obtained in the case of injecting 40 \(\upmu\)L of the headspace phase. When the volume was further increased, the chromatographic peak areas of the identified analytesbegan to decrease, as a result of which, 40 \(\upmu\)L was selected as the optimal injecting volume of the HS phase. As the next parameter, the sampling temperature was examined. The enhanced temperature could increase the distribution of VOCs between a sample and the headspace because higher temperatures cause higher vapor pressure of the analyte, and consequently, its concentration in the headspace increases [28]. According to the literature, the usually applied HS sampling temperature for the BETX sampling from water samples is between 70 and 80 \({}^{\circ}\)C [31; 32; 33]. Therefore, for the selection of the sampling temperature, the sample was exposed for 20 min to temperatures ranging from 60 to 80 \({}^{\circ}\)C. HS sampling at the laboratory temperature was also tested, but the extracted analyte concentration was below the limit of detection. It was observed that the quantity of the extracted compounds increases with the temperature (Figure 1). Headspace sampling at 80 \({}^{\circ}\)C also caused lower peak resolution than at 70 \({}^{\circ}\)C, which complicated the identification of the analytes. At higher temperatures, a higher amount of water may be injected into the GC system, which could cause an increase in the background level [32]. Therefore, to reduce water vapor, 70 \({}^{\circ}\)C was preferred as the optimum sampling temperature.