## 3 Results and Discussion
### True Positives Rate Studies of Smokeless Powders  
#### 3.2.1 PSPME Coupled with IMS  
PSPME coupled with both portable and bench-top IMS systems resulted in excellent detection performance for both 2,4-DNT and NG. The TPR values were calculated based on the fraction of  
\begin{table}
\begin{tabular}{|c|c|c|c|c|c|c|c|} \hline
**Container** & \begin{tabular}{c} **Equilibrium** \\ **Time (h)** \\ \end{tabular} & **Extraction** & \begin{tabular}{c} **Sample** \\ **Size (mg)** \\ \end{tabular} & \begin{tabular}{c} **Sampling** \\ **Time** \\ **(min)** \\ \end{tabular} & \begin{tabular}{c} **PSPME-IMS** \\ **(Portable)** \\ \end{tabular} & \begin{tabular}{c} **PSPME-IMS** \\ **(Bench Top)** \\ \end{tabular} & \begin{tabular}{c} **SPME-GC-MS** \\ **(Bench Top)** \\ \end{tabular} \\ \hline \multirow{4}{*}{\begin{tabular}{c} Quart can \\ (0.94 L) \\ \end{tabular} } & \multirow{4}{*}{24} & \multirow{4}{*}{static} & 10 & 10 & \begin{tabular}{c} NG, \\ 2,4-DNT \\ \end{tabular} & \begin{tabular}{c} NG, DPA, \\ 2,4-DNT \\ \end{tabular} & \begin{tabular}{c} NG, \\ 2,4-DNT \\ \end{tabular} \\ \cline{3-8}  & & & \multirow{2}{*}{50} & \multirow{2}{*}{10} & NG, DPA, \\  & & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline \multirow{4}{*}{\begin{tabular}{c} Gallon can \\ (3.8 L) \\ \end{tabular} } & \multirow{4}{*}{24} & \multirow{4}{*}{static} & 10 & 10 & \begin{tabular}{c} NG, DPA, \\ 2,4-DNT \\ \end{tabular} & \begin{tabular}{c} NG, \\ 2,4-DNT \\ \end{tabular} & \
\begin{tabular}{c} NG, DPA, \\ 2,4-DNT \\ \end{tabular} \\ \cline{3-8}  & & & \multirow{2}{*}{50} & \multirow{2}{*}{10} & NG, DPA, \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\  & & & & & & \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Plastic \\ container (45L) \\ \end{tabular} } & \multirow{4}{*}{24–48} & \multirow{4}{*}{dynamic} & 10 & 1 & NG, DPA, \\  & & & & & & \\  & & & & & & \\ \cline{3-8}  & & & & & & \\ \cline{3-8}  & & & & & & & \\ \cline{3-8}  & & & & & & \\ \hline \end{tabular}
\end{table}
Table 3: Detection of analytes of interest (NG and DPA from All Unique smokeless powder; 2,4-DNT from IMR 4198 smokeless powder) for different sampling parameters.  
containers containing smokeless powders that resulted with a maximum signal (in height, mV) greater than the predetermined alarm threshold value. The TPR curves with respect to the alarm threshold set on the instrument for NG for both IMS systems are shown in Figure 1. The TPR decreases with increased alarm threshold with a TPR of 1.0 observed with a minimum detection equivalent to 8 ng and 2 ng of NG in the portable and bench-top IMS, respectively. A complete list of the TPRs results for the different scenarios for the three analytes of interest with the minimum alarm threshold are shown in Table 4. Static extractions for both IMS instruments showed a greater TPR values in comparison to dynamic extractions, nevertheless, the TPRs for 2,4-DNT and NG in the two systems for all the different set conditions were greater than 0.80. Detection of DPA was not very successful, resulting with the highest TPR of 0.82 for static extractions and 0.53 for dynamic extractions in the bench top IMS system. The highest TPRs for the DPA detection were 0.58 and 0.47 from static and dynamic extractions, respectively. Since DPA is a stabilizer [23, 31] in the smokeless powders, the presence of other chemicals such as the explosive NG is required for a positive alarm of low explosives. The presence of DPA can use used as a confirmation for the detection of smokeless powders.  
Figure 1: True positive rates for the (**a**) portable and (**b**) benchtop IMS systems. Comparison of true positive rates for the two extraction methods are shown with varying alarm threshold.