## 2 Materials and Methods
### Sample Pretreatment for Analyses  
The pretreatment in MSM for qualitative analysis [28]: the solid phase microextraction method (SPME) was performed mainly to analyze metabolites of UDMH by a manual holder (Supelco, Bellefonte, PA, USA). Four milliliters of aqueous sample were transferred into a 10 mL headspace vial with a pipette. One gram of NaCl was added to the vial, which was stirred to dissolve. The vial was penetrated by the SPME manual holder with 75 \(\upmu\)m Carboxen/Polydimethylsiloxane (CAR/PDMS) \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Values** \\ \hline Sand (50–2000 \(\upmu\)m) (\%) & 48.83 \\ Silk (2–50 \(\upmu\)m) (\%) & 35.47 \\ Clay (<2 \(\upmu\)m) (\%) & 15.70 \\ Density (g\(\cdot\) cm\({}^{-3}\)) & 1.55 \\ pH (in water) & 7.63 \\ Water-holding capacity WHC (\%) & 23.04 \\ Organic carbon (\%) & 1.26 \\ Total nitrogen content (\%) & 0.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical and chemical properties of soil used in this experiment. SPME fiber. We adjusted the manual holder to expose the fiber to the headspace of the water. The sample was extracted for 10 min with magnetic stirring. The SPME fibers were conditioned in the Gas Chromatography (GC) (Agilent 6890A, Santa Clara, CA, USA) inlet at 250 \({}^{\circ}\)C for 30 min before first use. SPME-based sample desorption was carried out at a GC inlet temperature of 250 \({}^{\circ}\)C for 15 min. For the pretreatment in the MSM for determining \(N\)-nitrosodimethylamine (NDMA) [29], the steam distillation method was used in this study. One hundred milliliters of the aqueous sample were transferred into the distilling flask of the steam distillation apparatus. Thirty grams of NaCl were added to the flask, which was shaken to dissolve. An Erlenmeyer flask containing 40 mL dichloromethane was placed in an ice bath to collect the distillate. When the volume of the distillate was 200 mL, 40 g NaCl and 2 mL sulfuric acid (1 + 3) were added. The Erlenmeyer flask was shaken to make the NaCl dissolve thoroughly, and then the solution was transferred into a 500 mL separatory funnel. After shaking for 5 min, the solution was kept still for 10 min to allow stratification. The dichloromethane layer was transferred into another Erlenmeyer flask. The water layer was extracted by 80 mL dichloromethane twice. All the extractant was merged together and dried by anhydrous sodium sulfate. Finally, the extractant was condensed by the nitrogen blowing instrument to a dry residue. The dry residue was dissolved in 1 mL dichloromethane for subsequent GC/MS determination of NDMA. The pretreatment in the soil for qualitative analysis [1]: the solid phase microextraction method (SPME) was performed, mainly to analyze the metabolites of UDMH by a manual holder (Supelco). A 5 g soil sample was transferred into a 10 mL headspace vial. One gram of NaCl was added to the vial. The vial was penetrated by the SPME manual holder with 75 \(\upmu\)m Carboxen (CAR)/PDMS SPME fiber. We adjusted the manual holder to expose the fiber to the headspace of the soil. The sample was extracted for 18 h at room temperature. SPME-based sample desorption was carried out at a GC inlet temperature of 250 \({}^{\circ}\)C for 15 min. The pretreatment for determining UDMH in the soil: 5 g of soil, 2 g of Na\({}_{2}\)S \(\cdot\)9H\({}_{2}\)O, and 50 mL of a 40% solution of NaOH were placed in a 250 mL distillation flask; the flask was attached to a condenser, and the liquid was distilled to dryness into a receiver containing 10 mL of 0.1 mol\(\cdot\) L\({}^{-1}\) H\({}_{2}\)SO\({}_{4}\). The distillate was transferred to a 100 mL flask and diluted to the mark with deionized water [26]. Ten milliliters of liquid from the 100 mL flask and 0.2 mL salicaldehyde were added into a 25 mL colorimetric tube with a stopper.