## 3 Results and Discussion
### Target Compound Enhancement  
In addition, to gain a more comprehensive impression about this system, other common explosives, like picric acid and ammonia nitrate, were determined. As can be seen in Table 2,  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Substance Measured** & **LOD Original System** & **LOD Novel System** & **Structure** \\ \hline TNT (trinitrotoluene) & 3 ng & 500 pg \\ \hline
1,3-Dinitrobenzene & 6 ng & 1.5 ng \\ \hline
2,4-DNT (Dinitrotoluene) & 3 ng & 1.5 ng \\ \hline
2,6-DNT & 6 ng & 2 ng \\ \hline Tetryl (2,4,6-trinitrophenylthylnitramine) *, \(m/z\) 241 & 30 ng & 2 ng \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of original and novel APLD systems by use of limit of detection (LOD) for different individually determined explosives. All mentioned improvements were used for the novel APLD system. Old LODs are taken from [31]. Substances that have been detected as fragments are marked with an * and the \(m/z\) fragment value. All other substances were detected as molecular ion signals. All substances were measured in negative detection mode.  
these explosives have a similar limit of detection of about 3 ng to 80 ng. The only exception is TATP (acetone peroxide) with a detection limit of about 800 ng. This can be explained due to the high thermal instability of this peroxide explosive making TATP difficult to detect for many different analyzing techniques. TATP's quite high vapor pressure limits the lowest amount of substance that can be prepared for a measurement, because the time before TATP evaporates completely decreases with the decreasing amount of substance used. Furthermore, potassium perchlorate and potassium chlorate were investigated but could not be detected within the investigated range of up to 2000 ng.  
Drug precursors were additionally chosen because they are regarded as important target compounds in forensic science as some are already regulated or may give evidence for illegal drug synthesis. The APLD allows someone to scan the outer surfaces of liquid containers for drug precursor substances, which could indicate drug synthesis in a clandestine laboratory. Beside drug precursors, drugs of abuse were investigated to enhance the system capability. Therefore, ketamine and amantadine were considered as model substances for drugs of abuse [33; 34]. In this work, ketamine represents possible sedatives and amantadine possible stimulants. Table 2 summarizes the limits of detection for these different substance classes. It has been shown that the capability of detecting drug precursors as well as drugs of abuse ranges from about 10 to 100 ng total substance amount compared to 0.5 to 800 ng for explosives. Therefore, the APLD system is also capable of detecting these compound classes and can be helpful for fast and straightforward collection of information and clues. To perform a substantial experiment to test the probability of detection, new substance deposition methods are needed. The applied \"droplet method\" is limited for such kinds of experiments because it cannot be assured that the spot density is high enough to mimic an evenly covered test surface.  
Beside drugs precursors and drugs of abuse, also common illegal drugs like cocaine or heroin were investigated. All of these measured drugs are illicit and are, therefore, common target compounds in the forensic field. As a result of these measurements, the LOD could be determined to be in the same range as the other compound classes. Most substances could be detected at less than 20 ng absolute amount at the tested surface. While the LOD of real samples may differ substantially due to several effects, it should be interpreted as the lower operation range and a performance indicator. For real world samples, the LOD is less relevant because a yes or no answer is sufficient in most cases.