Table 1. Preconcentrator settings

Stage	Trapping temperature	Desorption condition
First trap	−40 °C	30 °C
Second trap	−80 °C	230 °C
Third trap	−185 °C	30 s desorption time

Table 2. GC-MS settings and gases

Parameter	Setting
GC column	DB-624
Oven program	35 °C for 5 min; 5 °C/min to 150 °C, hold 7 min; 10 °C/min to 200 °C, hold 4 min
Ionization	Electron ionization (EI)
Ion source temperature	300 °C
Acquisition mode	Full scan
Gases	Nitrogen and helium, each ≥99.999% purity

The oven program was used for a standard mixture containing the 11 target compounds and additional VOCs. Any shorter program for a narrower target list should be verified separately.

Chromatographic analysis

Figure 1 shows the mixed-standard chromatogram. The numbered target peaks are identified in Table 4.

Figure 1. Chromatogram of the 11 target compounds in a VOC standard mixture.

Calibration design

Table 3. Equivalent calibration concentrations in nmol/mol

I and II identify the two working standard mixtures. H2S = hydrogen sulfide; thiols = methanethiol and ethanethiol; other targets = the remaining eight compounds in Table 4. The listed volumes are actual standard-mixture loading volumes.

Linearity and method detection limits

The 11 compounds gave calibration correlation coefficients r of 0.9994–1.0000. Table 4 lists the compound identities and reported method detection limits. These limits apply to the study configuration.

Table 4. Target compounds and analytical performance

Methanethiol and ethanethiol are also known as methyl mercaptan and ethyl mercaptan. Ethyl methyl sulfide is also called methyl ethyl sulfide.

Detection limit assessment

Precision and matrix spike recovery

Nitrogen samples were tested at low, medium, and high concentrations. Ambient-air samples were tested at low and high concentrations. Each compound and test condition was measured six times. Table 5 summarizes the ranges across the 11 compounds; all 55 sets of results are provided in the appendix.

Table 5. Precision and spike recovery by test condition

Appendix Detailed precision and recovery results

Each row represents a condition tested with six replicate measurements. N2 denotes high-purity nitrogen; Air denotes the ambient-air matrix. Added and measured concentrations are in nmol/mol. RSD and recovery are percentages. Measured concentrations are rounded, so ratios calculated from the displayed values may differ slightly from the reported recovery percentages.

Table 6. Compound-specific precision and spike recovery data