## 4 Conclusions  
Within this study, we successfully showed the performance of an improved APLD setup. Various parameters, such as temperature of sampling, desorption area, or desorption head material were modified, while retaining easy handling and flexibility of the system. The novel system showed improved LODs, which are significantly below the original APLD approach and mostly comparable to other atmospheric pressure ionization methods or spectroscopic techniques. The LOD, at laboratory measurements, was 0.5 ng for trinitrotoluene, 17 ng for cocaine hydrochloride, and 6 ng for ketamine, to mention only some of the tested materials. In particular, the investigation of real-world scenarios exhibited the capabilities of the flexible laser desorption sampling, the soft chemical ionization, and fast ion-trap mass spectrometric detection. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Compared Analytical Technique** & **Measured Substance** & \
\begin{tabular}{c} **LOD** \\ **Ref. Technique/ng** \\ \end{tabular} &
\begin{tabular}{c} **LOD** \\ **APLD/ng** \\ \end{tabular} \\ \hline \multirow{2}{*}{Raman spectroscopy} & 24-DNT & 5 & 1.5 \\ \cline{2-4}  & Cocaine HCl & 1000 & 17 \\ \hline \multirow{2}{*}{Ion mobility spectrometry} & TNT & 0.35 & 0.5 \\ \cline{2-4}  & Cocaine HCl & 5 & 5 \\ \hline \multirow{2}{*}{Thermal desorption MS} & TNT & 5 & 0.5 \\ \cline{2-4}  & Cocaine HCl & 388 & 17 \\ \cline{2-4}  & MDMA & 2 & 10 \\ \hline \multirow{2}{*}{Desorption spray ionization} & TNT & 10 fg & 0.5 \\ \cline{2-4}  & Amphetamine & 14 & 20 \\ \hline Low-temperature plasma probe & Tetryl & 0.25 & 2 \\ \hline \multirow{2}{*}{DART-TD-MS} & Tetryl & 2 & 2 \\ \cline{2-4}  & MDMA & 2 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of LOD of different analytical techniques to APLD for explosives and drugs. DART-TD-MS = Direct analyses in real time-thermal desorption-mass spectrometry. This allows detection of TNT on bomb shell residues and residues of a new psychoactive substance on the inside of its package. Furthermore, the system was extended by a unit for the automatized investigation of surfaces by mapping individual pixels. Consequently, the manual laser desorption sampling is no longer required for such surfaces, which improves systematic mapping capabilities. In summary, APLD with an improved sampling does not require pretreatment of the surface and rapidly exploits a chemical profile. Therefore, it gives indices for conclusions of first responders or at onsite screening operations. By choosing an appropriate transferline length, better LODs or more flexibility can be achieved, depending on the scenario. For further investigations, cross-sensitivities as well as high throughput behavior would be interesting. In addition, an attempt could be made to use the mapping capability at ambient conditions to scan thin layer chromatography plates, for example, to separate isomeric compounds that are difficult for exclusively MS-based approaches. The use of a more mobile and flexible MS could be beneficial for highly mobile application scenarios if the inherent restrictions in system performance are acceptable. Therefore, APLD in combination with a portable ion trap [46] is promising. The following are available online at [[http://www.mdpi.com/2076-3417/8/6/933/s1](http://www.mdpi.com/2076-3417/8/6/933/s1)]([http://www.mdpi.com/2076-3417/8/6/933/s1](http://www.mdpi.com/2076-3417/8/6/933/s1)). LabVIEW script for generated heat map, used in Figure 5.  
[PERSON] and [PERSON] conceived and designed the experiments; [PERSON] and [PERSON] performed the experiments; [PERSON] analyzed the data; [PERSON] and [PERSON] contributed reagents, samples, measurement tools, and expert knowledge for those; [PERSON] wrote the paper; [PERSON] and [PERSON] contributed significantly to the discussion and revision. This investigation was performed within the SEMFreS project funded by the Federal Ministry of the Interior (BMI) and the Federal Office of Civil Protection and Disaster Assistance, FP406 \"SEMFreS\".