## 3 Results and Discussion
### Additional Capabilities of the APLD Approach  
The LODs and the measurement speed of the APLD system allows not only for the measurement of samples on surfaces at trace levels, but it also enables an automated measurement of surfaces without sample pretreatment. For this purpose, a computer controlled X-Y table was mounted in  
Figure 4: APLD EI-mass spectra of the herbal blend material. Direct laser desorption exhibits intense signals at higher \(m/z\), tentatively assigned to 5F-Cumyl-PINACA from an herbal blend sold as “Rollin’ High”. This new psychoactive substance could be detected directly from herbal blend as well as residues sticking on the bag. The upper part shows a mass spectrum achieved by desorbing the inner side of the bag and the lower one a direct measurement of the herbal blend. A similar spectrum compared to the inside of the empty bag is revealed. front of the APLD head. In consequence, the automated mapping of flat surfaces can be performed, which reveals APLD spectra with sub-millimeter lateral resolution. Analysis of non-flat surfaces could be difficult due to delocalization of target compounds by touching the PTFE ring. The actual system performance is illustrated in Figure 5. This figure shows the possibility to detect the spatial distribution of an artificially prepared substance trace on a surface. An imaginable scenario in context of a preliminary investigation that benefits from APLD capabilities could be the analysis of fingerprint residues from plastic explosives. In this case, the identification can be challenging due to the strong presence of matrix signals in the plastic explosive. Figure 5: APLD mapping capabilities shown at a TNT example generated with CI negative detection mode. Two typical mass spectra can be seen above. If the position of the X-Y table and the signal intensity is combined at any point of the surface the image on the top right can be achieved. This image shows a prepared stainless steel surface that was labeled with the word TNT. This label was produced with 1 mg/L TNT solution and a \(\upmu\)L syringe. The total amount of substance applied for this image was about 20 \(\upmu\)g. The color changes from black (no TNT detected) to white (high amounts of TNT detected). Therefore, the upper spectrum indicates a “black” area and the lower spectrum reveals the basis for a “white area”. For mapping purposes a LabVIEW TM script was created, which can be found in the Supplementary Material. Comparing APLD to other analytical techniques that are available to detect directly trace levels of security relevant substances like explosives, drugs, or drug precursors leads to similar results regarding the LODs, ranging mostly in the low nanogram scale. Table 3 summaries compared analytical techniques and achieved LODs. In detail, the APLD allows similar LODs compared to Raman spectroscopy [36], ion mobility spectrometry [37], and thermal desorption (TD) MS [38; 39]. Compared to APLD, desorption spray ionization (DESI) achieves better LODs for explosives [40] with the disadvantage of a more complex setup and the need of a liquid solvent supply. The LOD for TNT with DESI is about 10 fg. The achieved LODs for amphetamine using DESI is about 14 ng compared to 20 ng using APLD [41]. The low-temperature plasma (LTP) probe [42] methodology behaves similarly to DESI by also achieving a better limit of detection for explosives, but still with the need of a more complex setup due to the additional gas supply. For tetryl, the LOD of this technique was 0.25 ng. DART-TD-MS [43] is mostly comparable to APLD related to explosives and drugs. While the LOD for (3_A_-methylenedioxy-methamphetamine) hydrochloride (MDMA) is slightly better than that for DART with 2 ng compared to 10 ng, respectively, both systems can detect 2 ng tetryl. Raman spectroscopy in combination with an advanced swab system achieve a LOD of about 5 ng of 2,4-DNT, whereas APLD can measure down to 1.5 ng.