## 2 Materials and Methods  
Explosives, drugs, and drug precursors investigated in this study were obtained from the German Federal Criminal Police Office (BKA, Wiesbaden, Germany). Explosives were acquired as methanolic solutions with a concentration of 1 g/L and drug precursors as well as ketamine as pure substances. Explosives were trinitrotoluene, 1,3-dinitrobenzene, 2,4-dinitrotoluene, 2,6-dinitrotoluene, 2,4,6-trintirophenylmethyl-nitramine, picric acid, 3,4-dinitrotoluene, triacetonetriperoxide, and ammonia nitrate. Drug precursors/drugs of abuse were ketamine hydrochloride, safrole, amantadine, and phenylacetone. Some 5F-Cumyl-PINACA, a cannabinoidic new psychoactive substance, coated on herbal blend and sold as \"Rollin' High\" was also received from the BKA. Potassium chlorate was obtained from Sigma Aldrich Chemie GmbH (Steinheim, Germany) and potassium perchlorate anhydrous was bought at Thermo Fisher GmbH (Kandel, Germany). Used solvents were dichloromethane and methanol from Carl Roth GmbH + Co. KG (Karlsruhe, Germany). Methane 4.5 as CI reactant gas and helium 5.0 were bought from Linde AG (Berlin, Germany). A schematic overview of the system and its parts can be seen in Figure 1. This figure describes all relevant parts of the system and is explained in detail at the end of this chapter. A Minilite neodymium-doped yttrium aluminum garnet (Nd:YAG) laser from Continuum Inc. (San Jose, CA, USA) was used for desorption in the second harmonic wavelength (\(\lambda\) = 532 nm). Mass spectra and software were acquired utilizing a Varian Inc. (Walnut Creek, CA, USA) ion trap 240-MS and Varian MS Workstation 6.9.1. The MS parameter for CI were a mass rage of 50 to 400 \(m/z\), 1.7 Hz acquisition rate, 3 \(\upmu\)s cans averaged, 3 mL/min helium cooling gas flow, and full scan for all measurements. For EI measurements, an acquisition rate of 0.8 Hz and an emission current of 25 \(\upmu\)A were chosen. Other MS parameters were similar to CI. Calibration was done for EI and CI with pure perfluoritbutylamine, as required by the instrument. Lens energy and tuning was carried out automatically by MS software. Laser light was transferred from the laser to the ambient pressure desorption head with a 1250 \(\upmu\)m Optran UV laser fiber from CeramOptec GmbH (Bonn, Germany). The ambient pressure desorption head was self-built and made out of brass and polytetrafluoroethylene for thermal separation. A stainless steel capillary with 100 \(\upmu\)m inner diameter was used as the transfer-line and was bought at IDEX Health & Science LLC (Oak Habor, WA, USA). Sample deposition on the sampling surface was done by a microliter syringe from Hamilton Bonaduz AG (Bonaduz, Switzerland). For automated and easy surface analysis, a self-build two-axis linear driving system (X-Y table) was combined with stepper motors and the MCC-2 stepper motor controller from Phytron GmbH (Grobenzell, Germany). The sample desorption surface was a steel sheet made of V2A steel. For sampling, 1 \(\upmu\)L aliquots were spotted directly onto the steel target; spots were dried under ambient conditions, and the residue carrying surface was placed on the X-Y table. The sample spots were as small as possible, with a maximum diameter of 1 mm. Subsequently, the sample surface was scanned automatically with the APLD system until the residue was detected. Therefore, an area of 5 \(\times\) 5 mm was scanned to ensure the substance was hit. Each spot was desorbed with a single laser pulse every millimeter distance at a repetition rate of 10 Hz. After all data was collected, the surface was cleaned with methanol and could be reused. The desorption head did not require further cleaning after a measurement. The APLD system was heated to 200 \({}^{\circ}\)C. For explosives and explosive precursors, negative detection mode was chosen. All other compounds were analyzed with positive detection mode. Due to the fact that ambient samples with high air intake were measured, CI with external ionization was realized.