## 2 Experimental Section  
### Instrumentation  
The true positive rate (TPR) studies were conducted with two different techniques: PSPME-IMS (bench-top instrument and portable instrument) and SPME-GC-MS. The bench top IMS system used was an IONSCAN 400B (Smiths Detection, Mississauga, ON, Canada) which was used in both negative and positive polarity with nicotinamide and hexachloroethane dopants, as recommended by the manufacturer. A Morpho Detection Hardened MobileTrace was used as the portable IMS system and operated in the Explosives Particle Mode with dichloromethane (VICI Metronics, Inc., Poulsbo, WA, USA) and ammonia (Real Sensors, Inc., Hayward, CA, USA) dopants. For both instruments, the instrumental parameters were kept at the manufacturer's default parameters. The parameters for the benchtop IMS used the drift tube temperatures of 115 \(\,\mathrm{\SIUnitSymbolCelsius}\) and 235 \(\,\mathrm{\SIUnitSymbolCelsius}\) in the negative and positive polarity, respectively. The portable IMS system allowed for detection of analytes in both polarities, using the explosives particle mode setting with a drift tube temperature of 162 \(\,\mathrm{\SIUnitSymbolCelsius}\). Alarms for compounds not present in the library were added and the parameters used were similar to the alarms in the library. The alarm thresholds for the analytes of interest were adjusted to the minimum alarm threshold for true positive and false positive rate studies but a full listing of the alarm thresholds for each analyte in both IMS systems is presented in Table 2.  
The GC-MS studies were performed using a Varian (Palo Alto, CA, USA) CP 3800 gas chromatograph coupled to a Saturn 2000 ion trap mass spectrometer and equipped with an CP 8400 autosampler (Varian Inc., Walnut Creek, CA, USA). The sample was introduced to the GC with an inlet temperature of 180 \(\,\mathrm{\SIUnitSymbolCelsius}\) (split ratio 5:1) and analyzed using a 30 m \(\times\) 0.25 mm ID \(\times\) 0.25 \(\,\mathrm{\SIUnitSymbolMicro m}\) DB-5 MS UI (Agilent Technologies, Inc., Santa Clara, CA, USA) with a constant flow rate of Helium at 2.0 mL*min\({}^{-1}\). The method length was 29.3 min, in which the GC oven started at a temperature of 40 \(\,\mathrm{\SIUnitSymbolCelsius}\) and held for 1 min, followed a ramp to 200 \(\,\mathrm{\SIUnitSymbolCelsius}\) at 15 \(\,\mathrm{\SIUnitSymbolCelsius}\) min\({}^{-1}\), then held for 1 min, another ramp to 240 \(\,\mathrm{\SIUnitSymbolCelsius}\) at 15 \(\,\mathrm{\SIUnitSymbolCelsius}\) min\({}^{-1}\), held for 6.5 min, a third ramp to 270 \(\,\mathrm{\SIUnitSymbolCelsius}\) at 25 \(\,\mathrm{\SIUnitSymbolCelsius}\) min\({}^{-1}\), then a final ramp to 280 \(\,\mathrm{\SIUnitSymbolCelsius}\) at 5 \(\,\mathrm{\SIUnitSymbolCelsius}\) min\({}^{-1}\), held for 4 min. The transfer line to the ion trap was set to 280 \(\,\mathrm{\SIUnitSymbolCelsius}\) and the ion trap  
\begin{table}
\begin{tabular}{|c|c|c|c|} \hline
**Name** & **Chemical Structure** & **Vapor Pressure, Torr (25 \(\,\mathrm{\SIUnitSymbolCelsius}\))** & **Reduced Mobility,** \\ \hline Cyclotrimethyl- & & & \\ enterinitramine (RDX) & 1.7 \(\times 10^{-8}\) & 1.3129 \\ \hline
2,4,6- & & & \\ Trinitrotoluene (TNT) & 8.8 \(\times 10^{-6}\) & 1.4488 \\ \hline \end{tabular}
\end{table}
Table 1: _Cont._was maintained at 180 \(\,\)C. Each compound of interest was identified by the retention of their pure standards and identifying the resulting peak using the NIST mass spectral library.