## 3 Results and Discussion
### GC-Qtof-Ms
#### 3.1.2 Target Analysis  
The quantitative curve used for target analysis was established using the Agilent MassHunter (QQQ) software. The identification of target analytes in standard substances was carried out by obtaining up to two micro-window extracted ion chromatograms (quantitative and qualitative ions) at selected _m/z_ ions for every compound. Thirty-two VOCs were selected as target compounds in this study, including halogenated hydrocarbons and aromatic hydrocarbons. The accurate _m/z_ of the quantitative and qualitative ions utilized  
Figure 1: Identification of nontarget DMDS (dimethyl disulfide) by GC-QTOF-MS (gas chromatography quadrupole-time-of-flight mass spectrometry): (**A**) Extracted-ion chromatograms for five _m/z_ ions; (**B**) Deconvoluted accurate mass spectrum in the sample; (**C**) Commercial library mass spectrum of DMDS at nominal mass. Figure 2: Identification of nontarget DMTS (dimethyl trisulfide) by GC-QTOF-MS: (**A**) Extracted-ion chromatograms for five _m/z_ ions; (**B**) Deconvoluted accurate mass spectrum in the sample; (**C**) Commercial library mass spectrum of DMTS at nominal mass. in the method was determined by analyzing the standard substances. The most abundant fragment ion for each compound was selected as the quantitative ion, and typically the second most abundant ion was chosen as the qualitative ion. All the target analytes had unique quantitative and qualitative ions, and all of them were separated during chromatography except for trans- and cis-1,3-dichlorpropylene, which co-eluted at 4.024 min, o-, m- and p-xylene, which co-eluted at 8.35 min, and 1,3,5- and 1,2,4-trimethylbenzene, which co-eluted at 12.946 min. These co-eluted compounds were, respectively, constitutional isomers and analyzed together in this method. The information of the target analytes was summarized in Table S3. The results of precision, accuracy, linearity, selectivity, limits of detection (LODs) and limits of quantification (LOQs) are shown in Table S4. Precision  
Intra-day repeatability of the method was assessed using mixing standard at two different levels (\(n=5\), each level) on the same day. Precision was calculated using the mean, standard deviation, and relative standard deviation (RSD, %). The results, expressed as relative standard deviations (RSD, %), were \(<\)15%, while RSD values were below 10% for all target VOCs at 50 ng/L level. The method displayed RSDs between 0.81% and 11.11%, confirming its excellent precision. Accuracy  
The accuracy was the percentage recovery of a known amount of target analyte added to the sample, which was presented as the mean recovery of the analyte from the spiked matrix. Both spiked and non-spiked samples were further subjected to the sample preparation procedure described above, and spiked sample was spiked with two different concentrations (50 ng/L and 500 ng/L). The peak areas of the analyte from the spiked and non-spiked samples were used to calculate the concentration of each analyte. Influent wastewater was used in recovery test, in which spiked and non-spiked samples were measured five times in parallel to calculate the recovery rates and RSDs. The results of recovery test were shown in Table S4. The results (recovery rates of 70.11-115.05% and RSDs of 1.08-17.71%) showed accuracy was found to be satisfactory with quantitative requirements. In this study, selectivity was assessed by injecting five blank samples into GC-QTOF-MS by the above SPME procedure. No compound was detectable that could interfere in the identification and quantitation of the target analytes. Due to excellent mass identification power (narrow mass windows) in QTOF-MS, it was possible to identify in case of co-elution and operate trace-level target analysis. Calibration standards (standard concentrations of 50, 100, 200, 500 and 1000 ng/L in triplicate) were prepared to set up calibration equation and evaluate the linearity of the present method. The present method displayed good linearity (\(t^{2}>0.99\)) over the concentration range of 50-1000 ng/L for each analyte.