applied sciences

Article Ambient Pressure Desorption—Chemical Spectrometry for Fast and Reliable Detection of Explosives, Drugs, and Their Precursors

René Reiss 1, Sven Ehlert 1, Jan Heide 1, Michael Pütz 2, Thomas Forster 2 and Ralf Zimmermann 1,3,* 1 Joint Centre, Chair of Analytical Chemistry, University of Rostock, Dr.-Lorenz-Weg 2, 18059 Rostock, Germany; [email protected] (R.R.); [email protected] (S.E.); [email protected] (J.H.) 2 Bundeskriminalamt-Federal Criminal Police Office (BKA), Forensic Science Institute, Äppelallee 45, 65203 Wiesbaden, Germany; [email protected] (M.P.); [email protected] (T.F.) 3 Joint Mass Spectrometry Centre, Comprehensive Molecular Analytics, Helmholtz Zentrum München, Ingolstädter Landstr. 1, 85764 Neuherberg, Germany * Correspondence: [email protected]; Tel.: +49-381-498-6460

 Received: 6 May 2018; Accepted: 2 June 2018; Published: 5 June 2018 

Featured Application: Direct detection of safety relevant, low volatile substances on surfaces, ranging from tableted drugs to suspicious remnants on surfaces like bags, luggage, or even containers.

Abstract: Fast and reliable information is crucial for first responders to draw correct conclusions at crime scenes. An ambient pressure laser desorption (APLD) mass spectrometer is introduced for this scenario, which enables detecting substances on surfaces without sample pretreatment. It is especially useful for substances with low vapor pressure and thermolabile ones. The APLD allows for the separation of desorption and ionization into two steps and, therefore, both can be optimized separately. Within this work, an improved version of the developed system is shown that achieves limits of detection (LOD) down to 500 pg while remaining fast and flexible. Furthermore, realistic scenarios are applied to prove the usability of this system in real-world issues. For this purpose, post-blast residues of a bomb from the Second World War were analyzed, and the presence of PETN was proven without sample pretreatment. In addition, the analyzable substance range could be expanded by various drugs and drug precursors. Thus, the presented instrumentation can be utilized for an increased number of forensically important compound classes without changing the setup. Drug precursors revealed a LOD ranging from 6 to 100 ng. Drugs such as cocaine hydrochloride, heroin, (3,4-methylendioxy-methamphetamine) hydrochloride (MDMA) hydrochloride, and others exhibit a LOD between 10 to 200 ng.

Keywords: APLD; drugs; explosives; post blast residue detection; mass spectrometry; ; on-line measurement; forensic chemistry; mapping

1. Introduction In real-world scenarios, the fast and selective detection of security-relevant substances can be crucial to ensure the most appropriate decision is taken as a foundation for further tactical procedures and minimization of risk for first responders and the civil population. In addition to other approaches like mobility spectrometry, mass spectrometry (MS) has been proven as a powerful and versatile technique to detect such substances. There are some MS techniques that should be mentioned, especially when focusing on explosives. In particular, gas chromatography coupled to

Appl. Sci. 2018, 8, 933; doi:10.3390/app8060933 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 933 2 of 15 mass spectrometry (GC-MS) has to be mentioned as a lab based technique and is used for more than 40 years to detect explosives and is a still improving field [1–6]. Besides an increase of reliability due to chromatographic separation, some mass spectrometers allow selective ion fragmentation of potential target substance to prove identification. This so-called function increases significantly the probability of a correct positive detection result and enables a lower false alarm rate, e.g., at luggage checks on airports. Ion trap (IT) mass spectrometers equipped with different ionization techniques are commonly deployed for such tandem mass spectrometric experiments. Depending on the analytical target, most IT-MS can be equipped with different ionization techniques like (EI), chemical ionization (CI) [7], or single photon ionization (SPI) [8]. Surface analyzing techniques allow for the observation of different potentially interesting targets in forensic science like luggage, letters, or clothes without time-consuming sample pretreatment. Direct analyses in real time (DART) [9,10], DART-Raman combination [11], desorption ionization (DESI) [12,13], solvent assisted desorption/ionization mass spectrometry (DI-MS) [14], or a variety of laser desorption/ionization (LDI) [15] systems are common techniques. Two of the most famous setups of these LDI systems are called matrix-assisted laser desorption/ionization (MALDI) [16,17] and surface-assisted laser desorption/ionization (SALDI) [10,18]. Omitting a special matrix and directly using the surface where a sample is located leads to the so called direct LDI approach. If interesting spots on bulky objects like containers, bulky luggage, or cars should be analyzed, highly mobile systems are preferable that can operate with a flexible sampling. Therefore, a separation of desorption and ionization processes can be beneficial because it allows a targeted optimization of the specific components to acquire enhanced results. One effective opportunity to realize the sampling is direct laser desorption (LD) that does not require additional media other than electricity. It is commonly used as the desorption step for various applications such as investigation of biological tissues [19–21], aerosols [22], pesticides [23], drugs [24], and explosives [25,26]. Drugs and explosives are relevant classes in forensic science. Explosives exhibit two characteristics that complicate their detection with common approaches but improves their applicability for LD: a very low vapor pressure, e.g., RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) has 4.4 × 10−9 mPa or HNS (hexanitrostilbene) with 6.2 × 10−18 mPa [27], and many of them are thermolabile. Some drugs also have a very low vapor pressure such as cocaine hydrochloride with 1.9 × 10−3 mPa or heroin hydrochloride with 1.2 × 10−3 mPa [28] and can be detected with this system even if they are not prone to a fast thermal degradation like explosives. By using a pulsed LD, instead of a continuous wave approach [29], thermal stress is lowered and leads to less fragmentation and better detection limits for explosives. Furthermore, a pulsed LD induces a shockwave-like ablation of the sample material from the surface and thus vapor pressure of the analyte is less crucial [29]. Some well-known and established vacuum ionization techniques that can be combined with LD are EI or CI. CI, for example, uses a reactant gas for a softer ionization [7]. Hence, spectra generated by CI mostly have fewer fragments and the quasi-molecular ion signal is more intense, which leads to easier interpretable spectra. Recently, we suggested ambient pressure laser desorption (APLD) [30] in combination with CI [31] for different explosives and showed the advantages of this straightforward and field-deployable system. In particular, this design is considered as a benefit for on-site applications, because only electricity is required (and methane as reactant gas if CI is used). To enable the detection of even small amounts of analyte, it was attempted to improve the limit of detection. Therefore, improvements on the transfer system and the desorption unit are done in this work to further enhance the detection characteristics for different explosives and drug precursors. In addition, there are many other substances that are of potential interest in the forensic field and that could also benefit from the APLD. Because of this, we would like to show that the APLD is not limited to explosives but also allows for the measuring of drugs and drug precursors at a comparable level to other measurement techniques. This greatly enhances the versatility of this technique because it enables it to cover different forensically-relevant compound classes with a single system. A practical example could be the rapid search of containers for smuggled synthetic drug precursors in container ports. Appl. Sci. 2018, 8, 933 3 of 15

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-trinitrophenylmethyl-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 cannabimimetic 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 5.0 were bought from Linde AG (Berlin, Germany). A schematic overview of the system and its parts can be seen in Figure1. 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 (λ = 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 µs 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 µA were chosen. Other MS parameters were similar to CI. Calibration was done for EI and CI with pure perfluorotributylamine, 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 µm 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 µm 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 (Gröbenzell, Germany). The sample desorption surface was a steel sheet made of V2A steel. For sampling, 1 µL 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 × 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 ◦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. External ionization means that are formed outside the trap in a specific ionization region inside the mass spectrometer. Although, internal ionization would lead to better limits of detection (LOD), the mass resolution and mass accuracy would strongly decrease simultaneously due to higher amounts of air. External ionization is a compromise between sensitivity and mass resolution for direct sampling of target compounds in an environmental gas matrix. Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 15

Real-world samples were held in front of the APLD, and the laser was activated. All settings were the same as for standard laboratory measurements. No cleaning was necessary after executed measurements. The LOD is defined by a signal to noise ratio (S/N) of three. Therefore, the signal intensity was not extrapolated to an S/N of three, instead it represents the lowest total amount of substance that was actually measured. The ambient pressure laser desorption, shown in Figure 1, operates by direct desorption of samples from surfaces with short laser pulses. At first, an approximately 5 ns wide Nd:YAG laser pulse with a pulse energy of about 3–5 mJ was guided via a laser fiber to the sampling surface. The laser fiber was mounted at the desorption head with approximately three millimeters distance to the surface. The pulsed laser light ablated the analyte from the surface, and, subsequently, the ablated material was transferred through a capillary into the IT-MS. Desorption head and transfer capillary Appl.were Sci. heated2018, 8, 933to 200 °C to prevent unintended surface adsorption of analytes. Methane as reactant gas4 of 15 was used for chemical ionization, generating primarily protonated quasi molecular ions.

Figure 1. (a) Laser desorption sampling for mass spectrometric analysis from a stainless steel surface Figure 1. (a) Laser desorption sampling for mass spectrometric analysis from a stainless steel surface with a self-built X-Y table; (b) Deployed ambient pressure laser desorption (APLD) head with front with a self-built X-Y table; (b) Deployed ambient pressure laser desorption (APLD) head with front view on desorption volume and polytetrafluoroethylene (PTFE) ring; (c) APLD head side view with view on desorption volume and polytetrafluoroethylene (PTFE) ring; (c) APLD head side view with attached laser fiber, heating cartridge, and thermocouple; (d) Schematic drawing of the APLD attached laser fiber, heating cartridge, and thermocouple; (d) Schematic drawing of the APLD chemical chemical ionization-mass spectrometer (APLD-CI-MS). The heating cartridge inside of the brass ionization-mass spectrometer (APLD-CI-MS). The heating cartridge inside of the brass body ensures body ensures a complete heating of the sampling head. A laser fiber transfers pulsed laser light, a completeproduced heatingby the Nd:YAG of the sampling laser, to the head. surface. A laser Th fibere ablated transfers analyte pulsed is subsequently laser light, transferred produced byinto the Nd:YAGthe transferline laser, to and the surface.afterwards The into ablated the IT-MS. analyte There, is subsequently it can be ionize transferredd by electron into theionization transferline (EI) or and afterwardschemical ionization into the IT-MS. (CI) and There, then itanalyzed. can be ionized by electron ionization (EI) or chemical ionization (CI) and then analyzed. 3. Results and Discussion Real-world samples were held in front of the APLD, and the laser was activated. All settings were3.1. Improved the same Detection as for Limits standard through laboratory Design Optimization measurements. No cleaning was necessary after executedWe measurements. recently demonstrated in a previous work [31] an ambient pressure laser desorption system for Thethe sensitive LOD is defined and flexible by a mass signal spectrometric to noise ratio detection (S/N) of of three. explos Therefore,ives. The detection the signal limits intensity were in was nota extrapolatedsuitable range to for an S/Nforensic of three, application instead itas represents presented the in lowestthis work. total Nevertheless, amount of substance an improved that was actuallydetection measured. limit can enhance the reliability of the analytical method. In the original setup, a non-heatedThe ambient front end pressure made laserout of desorption, polyether ether shown ketone in Figure(PEEK)1 ,was operates utilized. by direct desorption of samples from surfaces with short laser pulses. At first, an approximately 5 ns wide Nd:YAG laser pulse with a pulse energy of about 3–5 mJ was guided via a laser fiber to the sampling surface. The laser fiber was mounted at the desorption head with approximately three millimeters distance to the surface. The pulsed laser light ablated the analyte from the surface, and, subsequently, the ablated material was transferred through a capillary into the IT-MS. Desorption head and transfer capillary were heated to 200 ◦C to prevent unintended surface adsorption of analytes. Methane as reactant gas was used for chemical ionization, generating primarily protonated quasi molecular ions.

3. Results and Discussion

3.1. Improved Detection Limits through Design Optimization We recently demonstrated in a previous work [31] an ambient pressure laser desorption system for the sensitive and flexible mass spectrometric detection of explosives. The detection limits were in a suitable range for forensic application as presented in this work. Nevertheless, an improved detection Appl. Sci. 2018, 8, 933 5 of 15

Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 15 limit can enhance the reliability of the analytical method. In the original setup, a non-heated front end madeIn out this of study, polyether several ether modifications ketone (PEEK) to the was exis utilized.ting system were performed. These modifications are theIn thisexchange study, of several the type modifications of material, to establishi the existingng a system heatable were system, performed. shortening These the modifications transferline, areand the increasing exchange the of laser the typefiber ofdiameter. material, All establishing these changes a heatable are discussed system, in shortening the following the transferline,section. and increasing the laser fiber diameter. All these changes are discussed in the following section. 3.1.1. Sampling Head Design Optimization 3.1.1. Sampling Head Design Optimization The sampling head, crucial for a fast and sensitive detection, was modified first. The novel The sampling head, crucial for a fast and sensitive detection, was modified first. The novel design, design, utilizing a heated brass body, was aimed to further improve the sampling. Within this utilizing a heated brass body, was aimed to further improve the sampling. Within this modification, modification, the brass area exposed to the analyte is minimized. Theoretically, imaginable catalytic the brass area exposed to the analyte is minimized. Theoretically, imaginable catalytic surface processes surface processes can be neglected due to the fact that the analytes pass only a small brass surface can be neglected due to the fact that the analytes pass only a small brass surface area on their way into area on their way into the transferline and only very few applications of brass for catalytic surface the transferline and only very few applications of brass for catalytic surface processes can be found [32]. processes can be found [32]. Because of the minimized adsorption processes at the inside of this Because of the minimized adsorption processes at the inside of this desorption head, a faster analyte desorption head, a faster analyte transfer into the IT-MS is enabled. This in turn should give more transfer into the IT-MS is enabled. This in turn should give more narrow signals and better limits narrow signals and better limits of detection. Heating of the investigated sample surface is of detection. Heating of the investigated sample surface is prevented by mounting of a thin spacer prevented by mounting of a thin spacer of polytetrafluoroethylene (PTFE) at the top of the sampling of polytetrafluoroethylene (PTFE) at the top of the sampling head (Figure1b). The influence of the head (Figure 1b). The influence of the transferline was investigated by varying the length and the transferline was investigated by varying the length and the diameter of the transferline while keeping diameter of the transferline while keeping the temperature and parameters constant. The diameter the temperature and parameters constant. The diameter was adjusted to ensure a nearly similar sample was adjusted to ensure a nearly similar sample flow into the MS. As exemplarily visualized in Figure flow into the MS. As exemplarily visualized in Figure2 for the investigation of tetryl, the version with 2 for the investigation of tetryl, the version with the shortened transferline revealed an the shortened transferline revealed an approximately four-times improved signal width. This effect is approximately four-times improved signal width. This effect is caused by the higher temperature caused by the higher temperature and a shorter transferline. On that account, a minimum of ad- and and a shorter transferline. On that account, a minimum of ad- and de-sorption within the de-sorption within the transferline was achieved and thus more narrow peaks should be obtained. transferline was achieved and thus more narrow peaks should be obtained.

Figure 2. Compared smoothed desorption from spots with about 1.5 µg Tetryl. Measured at negative Figure 2. Compared smoothed desorption from spots with about 1.5 µg Tetryl. Measured at negative CI mode and fragment m/z 241. Novel system with short transferline and heated APLD head (solid CI mode and fragment m/z 241. Novel system with short transferline and heated APLD head (solid line). Original measurement system with long transferline (dashed line). Compared is the full width line). Original measurement system with long transferline (dashed line). Compared is the full width at at half maximum. half maximum.

3.1.2. Optimization of Desorption Area 3.1.2. Optimization of Desorption Area A further step that was implemented is the ability to use bigger laser fiber diameters at the same A further step that was implemented is the ability to use bigger laser fiber diameters at the same desorption volume inside the APLD head. By increasing the diameter of the laser fiber it is possible desorption volume inside the APLD head. By increasing the diameter of the laser fiber it is possible to transfer more laser light to the surface and simultaneously keeping the energy density constant. to transfer more laser light to the surface and simultaneously keeping the energy density constant. The diameter used before was about 0.6 mm, while the new fiber has a 1.25 mm diameter and thus The diameter used before was about 0.6 mm, while the new fiber has a 1.25 mm diameter and thus increased the desorption area by four times while retaining the same divergence angle of the laser increased the desorption area by four times while retaining the same divergence angle of the laser fiber. Laser fluence of both systems ranged between 0.2 to 0.4 J/cm2. This lead to a higher analyte fiber. Laser fluence of both systems ranged between 0.2 to 0.4 J/cm2. This lead to a higher analyte concentration within the gaseous phase above the sample spot. concentration within the gaseous phase above the sample spot. 3.1.3. Optimization Results As can be seen in Figure 2, the full width at half maximum (FWHM) could be significantly decreased to about 5 s compared to 22 s with the original measurement system, reported previously Appl. Sci. 2018, 8, 933 6 of 15

3.1.3. Optimization Results As can be seen in Figure2, the full width at half maximum (FWHM) could be significantly decreased to about 5 s compared to 22 s with the original measurement system, reported previously [31]. Starting point was the rise of the detector signal. At both measurements, the data acquisition was not triggered by laser shots and absolute delay was not regarded due to the lack of influence to the LOD.

The transferlineAppl. Sci. 2018 length, 8, x FOR differed PEER REVIEW from 1.5 m for the original system to 0.25 m for the novel6 of 15 one. As a Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 15 result, theAppl. length Sci. 2018 of, 8 the, x FOR transferline PEER REVIEW contributed to the peak shape and subsequently6 to of 15 the system Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 15 [31]. Starting point was the rise of the detector signal. At both measurements, the data acquisition response time.[31]. 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APLD SubstancesAll Substa mentioned systemsnces that thatbyimprovements have use have ofbeen limit been detected wereof detecteddetection used as fragments for (LOD) the as novel fragments arefor are differentAPLD system. individually Old LODs determined are taken explosives. from [31]. All Substa mentionednces that improvements have been detected were used as fragments for the novel are marked withdifferentmarkedAPLD an system. *with individually and an theOld * and LODsm /determined thez fragment arem/z takenfragment explosives. from value. value. [31]. All AllSubstaAll mentioned other nces substances substancesthat improvements have werebeen were detected were detected used as molecularfragments for the as novel molecular ionare ion APLDmarked system. with an Old * and LODs the arem/z taken fragment from value. [31]. SubstaAll othernces substances that have werebeen detected as fragmentsmolecular ionare APLDsignals.marked system. Allwith substances an Old * and LODs werethe arem measured/z taken fragment from in value.negative[31]. SubstaAll detection othernces substances that mode. have beenwere detected as fragmentsmolecular areion signals. Allmarkedsignals. substances Allwith substances an were* and measuredthewere m /measuredz fragment in negativein value. negative All detection detectionother substances mode. mode. were detected as molecular ion markedsignals. Allwith substances an * and thewere m /measuredz fragment in value. negative All detectionother substances mode. were detected as molecular ion signals. All substances were measured in negative detection mode. signals. All substances were measured in negative detectionLOD Original mode. LOD Novel SubstanceSubstance Measured Measured LOD Original SystemLOD Original LOD Novel LOD System Novel Structure Structure Substance Measured LODSystem Original LODSystem Novel Structure Substance Measured LODSystem Original LODSystem Novel Structure Substance Measured System System Structure O2N NO2 System System O2N NO2 O2N NO2 TNT (trinitrotoluene)TNT (trinitrotoluene) 3 ng 3 ng 500 pg 500 pg O2N NO2 TNT (trinitrotoluene) 3 ng 500 pg O N NO TNT (trinitrotoluene) 3 ng 500 pg 2 2 TNT (trinitrotoluene) 3 ng 500 pg TNT (trinitrotoluene) 3 ng 500 pg NO2 NO2 O2N NO2 NO2 O2N NO2 NO2 1,3-Dinitrobenzene 6 ng 1.5 ng O2N NO2 NO2 1,3-Dinitrobenzene1,3-Dinitrobenzene 6 ng 6 ng 1.5 ng 1.5 ng O2N NO2 1,3-Dinitrobenzene 6 ng 1.5 ng O2N NO2 1,3-Dinitrobenzene 6 ng 1.5 ng 1,3-Dinitrobenzene 6 ng 1.5 ng NO2 NO2 NO2 2,4-DNT (Dinitrotoluene) 3 ng 1.5 ng NO2 2,4-DNT (Dinitrotoluene) 3 ng 1.5 ng NO 2,4-DNT (Dinitrotoluene)2,4-DNT (Dinitrotoluene) 3 ng 3 ng 1.5 ng 1.5 ng 2 2,4-DNT (Dinitrotoluene) 3 ng 1.5 ng 2,4-DNT (Dinitrotoluene) 3 ng 1.5 ng NO2 NO2 NO2 NO2 O2N NO2 NO 2 2,6-DNT 6 ng 2 ng O2N NO 2 2,6-DNT 6 ng 2 ng O2N NO2 2,6-DNT 6 ng 2 ng O2N NO2 2,6-DNT2,6-DNT 6 ng 6 ng 2 ng 2 ng O2N NO2 2,6-DNT 6 ng 2 ng O2N O2N O2N O NNOO2N 2 O N 2 Tetryl (2,4,6-trinitrophenylmethylnitramine) *, O2NNO2 2 Tetryl (2,4,6-trinitrophenylmethylnitramine) *, 30 ng 2 ng O2NNO2 Tetryl (2,4,6-trinitrophem/z 241nylmethylnitramine) *, 30 ng 2 ng O2NNO2 Tetryl (2,4,6-trinitropheTetryl m/z nylmethylnitramine)241 *, 30 ng 2 ng O2NNO2 Tetryl (2,4,6-trinitrophem/z nylmethylnitramine)241 *, 30 ng 2 ng (2,4,6-trinitrophenylmethylnitramine)m/z 241 *, 30 ng30 ng 2 ng 2 ng m/z 241 NO 2 m/z 241 NO 2 NO 2 NO 2 NO 2 3.2. Target Compound Enhancement 3.2. Target Compound Enhancement 3.2. Target Compound Enhancement 3.2. TargetIn addition, Compound to Enhancementgain a more comprehensive impression about this system, other common In addition, to gain a more comprehensive impression about this system, other common 3.2. Targetexplosives, CompoundIn addition, like Enhancement picric to gain acid aand more ammonia comprehensive nitrate, we impressionre determined. about As thiscan besystem, seen inother Table common 2, these explosives,In addition, like picric to gain acid aand more ammonia comprehensive nitrate, we impressionre determined. about As thiscan besystem, seen inother Table common 2, these explosives,explosives havelike picric a similar acid limitand ammonia of detection nitrate, of about were determined.3 ng to 80 ng. As Thecan beonly seen exception in Table is 2, TATP these In addition,explosives,explosives to havelike gain picric a similar a acid more andlimit comprehensiveammonia of detection nitrate, of about we impressionre determined.3 ng to 80 ng. aboutAs Thecan beonly this seen exception system, in Table is 2, other TATPthese common explosives(acetone peroxide) have a similarwith a limitdetection of detection limit of ofabou aboutt 800 3 ng.ng Thisto 80 can ng. beThe explained only exception due to theis TATP high explosives(acetone peroxide) have a similarwith a limitdetection of detection limit of ofabou aboutt 800 3 ng.ng Thisto 80 can ng. beThe explained only exception due to isthe TATP high explosives,(acetone like picricperoxide) acid with and a detection ammonia limit nitrate,of about 800 were ng. determined.This can be explained As can due beto the seen high in Table2, (acetone peroxide) with a detection limit of about 800 ng. This can be explained due to the high Appl. Sci. 2018, 8, 933 7 of 15 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. Table2 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.

3.3. Real Sample Measurements

3.3.1. Shrapnel of Defused Bomb Besides standard substances, samples from real-world scenarios were investigated to exploit the practical capabilities for forensic analysis. For this purpose, post-blast residues of a defused Second World War bomb are investigated. This bomb was defused by controlled explosion carried out by the Berlin Police. After the explosion, shrapnel were gathered up by hand, sent to our laboratory and stored in a cardboard box at room temperature inside the laboratory. This scenario is a good example of a real world scenario where only little information about the exploded bomb is available. No further information about the bomb is available because it was defused as part of a batch of old bombs. Figure3 shows the results of the investigation of one of the pictured shrapnel with the APLD system. In sum, four different shrapnel pieces were investigated and 2 to 4 locations per shrapnel measured. It turned out that all spectra exhibit a clear signal at m/z 227, which can be correlated to the molecular ion signal M¯ of TNT. The presence of TNT can be assumed by the observed signals in the lower including the molecular ion signal of TNT at m/z 227, as well as two significant signals of respective fragments. As a comparison, the embedded mass spectrum shows an additional measurement of a 10 ng TNT standard sample measured under the same conditions. As can be observed in the mass spectrum, strong indications of TNT attached to the bomb shrapnel were found. In addition, fragmentation seems to be caused by the substrate. Therefore, results confirm that the APLD measurement system is not only suitable for laboratory measurements but also for answering realistic forensic questions. The APLD delivers valuable information about the used explosive. Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 15 thermal instability of this peroxide explosive making TATP difficult to detect for many different thermal instability of this peroxide explosive making TATP difficult to detect for many different analyzingthermal instability techniques. of TATP’sthis peroxide quite highexplosive vapor making pressure TATP limits difficult the lowest to detectamount for of many substance different that analyzing techniques. TATP’s quite high vapor pressure limits the lowest amount of substance that cananalyzing be prepared techniques. for a measurement, TATP’s quite highbecause vapor the pressu time beforere limits TATP the evaporateslowest amount completely of substance decreases that can be prepared for a measurement, because the time before TATP evaporates completely decreases withcan be the prepared decreasing for aamount measurement, of substance because used. the Furthermore, time before TATP potassium evaporates perchlorate completely and potassium decreases with the decreasing amount of substance used. Furthermore, potassium perchlorate and potassium chloratewith the weredecreasing investigated amount but of could substance not be used. detected Furthermore, within the pota investigatedssium perchlorate range of andup to potassium 2000 ng. chlorate were investigated but could not be detected within the investigated range of up to 2000 ng. chlorateDrug were precursors investigated were but additionally could not be chosen detected because within theythe investigated are regarded range as ofimportant up to 2000 target ng. Drug precursors were additionally chosen because they are regarded as important target compoundsDrug precursors in forensic werescience additionally as some are chosen already because regulated they or mayare regarded give evidence as important for illegal target drug compounds in forensic science as some are already regulated or may give evidence for illegal drug synthesis.compounds The in forensicAPLD allowsscience someone as some areto scanalready the regulated outer surfaces or may of give liquid evidence containers for illegal for drug synthesis. The APLD allows someone to scan the outer surfaces of liquid containers for drug precursorsynthesis. substances,The APLD whichallows could someone indicate to scandrug thesynthesis outer insurfaces a clandestine of liquid laboratory. containers Beside for drug precursor substances, which could indicate drug synthesis in a clandestine laboratory. Beside drug precursors,precursor substances, drugs of abuse which were could investigated indicate drug to en syhancenthesis the in system a clandestine capability. laboratory. Therefore, Beside ketamine drug precursors, drugs of abuse were investigated to enhance the system capability. Therefore, ketamine andprecursors, amantadine drugs were of abuse considered were investigated as model tosubstances enhance thefor systemdrugs ofcapability. abuse [33,34]. Therefore, In this ketamine work, and amantadine were considered as model substances for drugs of abuse [33,34]. In this work, ketamineand amantadine represents were possible considered sedatives as modeland amantadine substances possible for drugs stimulants. of abuse Table [33,34]. 2 summarizes In this work, the ketamine represents possible sedatives and amantadine possible stimulants. Table 2 summarizes the limitsketamine of detectionrepresents for possible these sedativesdifferent substanceand amantadine classes. possible It has stimulants.been shown Table that 2the summarizes capability the of limits of detection for these different substance classes. It has been shown that the capability of detectinglimits of detectiondrug precursors for these as welldifferent as drugs substance of abus cleasses. ranges It fromhas been about shown 10 to 100that ng the total capability substance of detecting drug precursors as well as drugs of abuse ranges from about 10 to 100 ng total substance amountdetecting compared drug precursors to 0.5 to as 800 well ng as for drugs explosives. of abuse Therefore, ranges from the about APLD 10 system to 100 ngis alsototal capable substance of amount compared to 0.5 to 800 ng for explosives. Therefore, the APLD system is also capable of detectingamount compared these compound to 0.5 to classes800 ng andfor explosives.can be help Therefore,ful for fast the and APLD straightforward system is also collection capable of detecting these compound classes and can be helpful for fast and straightforward collection of informationdetecting these and compoundclues. To perform classes aand substantial can be experimenthelpful for tofast test and the straightforwardprobability of detection, collection new of information and clues. To perform a substantial experiment to test the probability of detection, new substanceinformation deposition and clues. methods To perform are needed. a substantial The applie experimentd “droplet to test method” the probability is limited of for detection, such kinds new of substance deposition methods are needed. The applied “droplet method” is limited for such kinds of experimentssubstance deposition because methodsit cannot arebe assuredneeded. Thethat appliethe spdot “droplet density method”is high enough is limited to mimicfor such an kinds evenly of experiments because it cannot be assured that the spot density is high enough to mimic an evenly coveredexperiments test surface.because it cannot be assured that the spot density is high enough to mimic an evenly covered test surface. coveredBeside test drugs surface. precursors and drugs of abuse, also common illegal drugs like cocaine or heroin Appl. Sci. 2018, 8, 933 Beside drugs precursors and drugs of abuse, also common illegal drugs like cocaine or heroin 8 of 15 wereBeside investigated. drugs precursors All of these and measureddrugs of abuse, drugs also are common illicit and illegal are, drugs therefore, like cocaine common or herointarget were investigated. All of these measured drugs are illicit and are, therefore, common target compoundswere investigated. in the forensic All of field.these As measured a result of drugs these measurements,are illicit and theare, LOD therefore, could becommon determined target to compounds in the forensic field. As a result of these measurements, the LOD could be determined to becompounds in the same in therange forensic as the field. other As compound a result of classes. these measurements, Most substances the could LOD becould detected be determined at less than to Moreover, even longerbe in the periods same range between as the other the compound collection classes. of samples Most substances and the could related be detected investigation at less than seem to 20be ngin theabsolute same amountrange as at the the other tested compound surface. Whil classes.e the Most LOD substances of real samples could may be detected differ substantially at less than 20 ng absolute amount at the tested surface. While the LOD of real samples may differ substantially be acceptable. due20 ng to absolute several amounteffects, atit theshould tested be surface.interpreted Whil ase the the LOD lower of realoperation samples range may and differ a performancesubstantially due to several effects, it should be interpreted as the lower operation range and a performance indicator.due to several For real effects, world it samples, should bethe interpreted LOD is less asrelevant the lower because operation a yes or range no answer and a is performance sufficient in indicator. For real world samples, the LOD is less relevant because a yes or no answer is sufficient in mostindicator. cases. For real world samples, the LOD is less relevant because a yes or no answer is sufficient in Table 2. Limitmost of detection cases. for different individually determined drug precursors, drugs, drugs of abuse, most cases. and some additional explosives measured with the novel APLD system. It is mutually assumed that the Table 2. Limit of detection for different individually determined drug precursors, drugs, drugs of Table 2. Limit of detection for different individually determined drug precursors, drugs, drugs of amount of substanceabuse,Table is 2. toand Limit be some seen of detectionadditional as a total for explamount differentosives andindividuallymeasured the sample with determined the spot novel isdrug consideredAPLD precursors, system. as drugs,It totally is mutuallydrugs desorbed of abuse, and some additional explosives measured with the novel APLD system. It is mutually at the measurementassumedabuse, as and described that some the additionalamount in the of materialsexplsubstaosivesnce ismeasured andto be methods seen with as thea section.total novel amount APLD Substances andsystem. the sampleIt thatis mutually spot have is been assumed that the amount of substance is to be seen as a total amount and the sample spot is consideredassumed that as thetotally amount desorbed of substa at thence measurement is to be seen as as described a total amount in the andmaterials the sample and methods spot is detected as fragmentsconsidered are markedas totally withdesorbed an *at along the measurement with the mas/ zdescribedfragment in the value. materials All explosivesand methods were section.considered Substances as totally that desorbed have been at detectedthe measurement as fragments as describedare marked in with the anmaterials * along and with methods the m/z detected as molecularsection. ion Substances signals that and have all otherbeen detected substances as fragments as protonated are marked molecular with an * along ions. with All the explosives m/z fragmentsection. Substances value. All thatexplosives have b eenwere detected detected as as fragments molecular are ion marked signals with and anall * otheralong substances with the m as/z fragment value. All explosives were detected as molecular ion signals and all other substances as were measured inprotonatedfragment negative value. molecular detection All explosives ions. mode, All wereexplosives all detected other were substances as molecularmeasured in ionin positive negativesignals and detection all other mode, mode.substances all other as protonated molecular ions. All explosives were measured in negative detection mode, all other substancesprotonated inmolecular positive detectionions. All mode.explosives were measured in negative detection mode, all other substances in positive detection mode. substancesSubstances in Measured positive detection at Novel mode. Systems LOD Structure Substances Measured at Novel Systems LOD Structure Substances Measured at Novel Systems LOD Structure Substances Measured Explosivesat NovelExplosives Systems LOD Structure Explosives OH Explosives OH O2N OH NO 2 O2N NO 2 PA (picric acid)PA (picric acid) 15 ng 15 ng O2N NO 2 PA (picric acid) 15 ng PA (picric acid) 15 ng NO 2 NO 2 NO 2 3,4-DNT 3 ng 3,4-DNT 3 ng 3,4-DNT 3 ng NO2 3,4-DNT 3 ng NO2 NO2 NO2 NO2 NO O2 O O O O O O TATP (triacetonetriperoxide) 800 ng O TATP (triacetonetriperoxide) 800 ng OO O O O TATP (triacetonetriperoxide)TATP (triacetonetriperoxide) 800 ng 800 ng O O O O Appl. Sci. 2018, 8, x FOR PEER REVIEW O 8 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW - 8 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW NO3 8 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW NO - 8 of 15 Appl. Sci. 2018AN, 8 (ammonia, x FOR PEERAN nitrate) (ammoniaREVIEW *, m /z nitrate)63 *, m/z 63 78 ng 78 ng NH + NO3 - 8 of 15 Appl. Sci. 2018, 8, x FOR ANPEER (ammonia REVIEW nitrate) *, m/z 63 78 ng +4 3- 8 of 15 Appl. Sci. 2018, 8, x FOR PEERAN (ammoniaREVIEW nitrate) *, m/z 63 78 ng NH4 + NO3 - 8 of 15 NH4 NO AN (ammonia nitrate)Drug *, m/ zprecursors 63 78 ng NH +NO -3 AN (ammonia nitrate)DrugDrug precursors *, m precursors/z 63 78 ng 4 + NO3 - AN (ammonia nitrate) *, m/z 63 78 ng ONH+4 3- Drug precursors NH NO3 AN (ammonia nitrate) *, m/z 63 78 ng O NH4 + NO - AN (ammoniaSafrole nitrate) Drug *, m/z precursors 63 78100 ng ng O 4+ 3 AN (ammonia nitrate)Drug *, m/ zprecursors 63 78 ng NH4 + SafroleSafrole Drug precursors 100 ng100 ng OONH4 Safrole Drug precursors 100 ng OO Safrole Drug precursors 100 ng OO Safrole Drug precursors 100 ng O Safrole 100 ng OO Phenylacetone 50 ng O Safrole 100 ng O O PhenylacetoneSafrole 10050 ng ng O O PhenylacetonePhenylacetoneSafrole 50 ng10050 ng ng O O Phenylacetone 50 ng O O PhenylacetoneDrugs /drugs of abuse 50 ng O PhenylacetoneDrugs /drugs of abuse 50 ng O PhenylacetoneDrugs/drugsDrugs of/drugs abuse of abuse 50 ng O∗ PhenylacetoneDrugs /drugs of abuse 50 ng ∗O AmphetaminePhenylacetoneDrugs /drugs of abuse 2050 ng O∗ AmphetamineDrugs /drugs of abuse 20 ng ∗NH 2 AmphetamineDrugs /drugs of abuse 20 ng NH∗2 ∗ NH2 Drugs/drugs of abuse ∗ AmphetamineAmphetamine 20 ng20 ng ∗ Drugs/drugs of abuse NH2 Amphetamine 20 ng ∗ ∗ ∗NH2 Amphetamine 20 ng ∗ Methamphetamine 10 ng NH2 Amphetamine 20 ng NH∗ Methamphetamine 10 ng NH2 Amphetamine 20 ng NH ∗ Methamphetamine 10 ng NH2 Amphetamine 20 ng NH∗ O NH2 Methamphetamine 10 ng ∗ MethamphetamineMDMAMethamphetamine (3,4-methylendioxy- 10 ng10 ng NH O NH∗ Methamphetamine 10 ng O MDMA (3,4-methylendioxy- 20 ng NH ∗ HN MDMAMethamphetamine (3,4-methylendioxy- 10 ng OO methamphetamine) hydrochloride 20 ng NHHN MDMAMethamphetamine (3,4-methylendioxy- 1020 ng ng O methamphetamine)Methamphetamine hydrochloride 10 ng O O NH HN methamphetamine)MDMA (3,4-methylendioxy- hydrochloride 20 ng O O NH MDMA (3,4-methylendioxy- O O HN methamphetamine) hydrochloride 20 ng O O HN MDMA (3,4-methylendioxy- O O MDMA (3,4-methylendioxy-methamphetamine) hydrochloride 20 ng20 ng O methamphetamine) hydrochloride O O HN MDMA (3,4-methylendioxy- 20 ng O methamphetamine) hydrochloride O O HN methamphetamine)MDMA (3,4-methylendioxy-Heroin hydrochloride 20200 ng ng O O O HN 20 ng OOO methamphetamine)Heroin hydrochloride 200 ng O O N HN methamphetamine)Heroin hydrochloride 200 ng O O O O N O O Heroin 200 ng O O N O Heroin 200 ng O O O O O N Heroin 200 ng O O O O O N HeroinHeroin 200 ng200 ng O O O + O O N HNO Heroin 200 ng O N O O + O O Heroin 200 ng HN O O N Cocaine hydrochloride 17 ng + O HN O O N Cocaine hydrochloride 17 ng O O O + O Cocaine hydrochloride 17 ng HN O O O O O HN+ O O OO Cocaine hydrochloride 17 ng + O HN ClO O O Cocaine hydrochloride 17 ng HN+ O O Cocaine hydrochloride 17 ng + OCl O HN O OO Cocaine hydrochloride 17 ng +O Cl HN Cocaine hydrochlorideCocaine hydrochloride 17 ng 17 ng O Cl O O KetamineCocaine hydrochloride hydrochloride 17 6 ng ng O O O Cl O + Ketamine hydrochloride 6 ng OCl NH2 - O+ Cl Ketamine hydrochloride 6 ng Cl NH - O 2ClO + Ketamine hydrochloride 6 ng OCl NH2 - +Cl Ketamine hydrochloride 6 ng O Cl NH - 2Cl+ Ketamine hydrochloride 6 ng O NH - Ketamine hydrochloride 6 ng +2Cl NH2 +- Ketamine hydrochloride 6 ng NHCl - 3.3. Real Sample Measurements +2Cl Ketamine hydrochlorideKetamine hydrochloride 6 ng 6 ng NH - 3.3. Real Sample Measurements 2Cl+ 3.3. Real Sample Measurements NH2Cl- 3.3. Real Sample Measurements 3.3.3.3.1. Real Shrapnel Sample ofMeasurements Defused Bomb 3.3.1.3.3.3.3. Real RealShrapnel Sample Sample of Measurements Measurements Defused Bomb 3.3.3.3.1. Real Shrapnel Sample ofMeasurements Defused Bomb 3.3.1.3.3. Real BesidesShrapnel Sample standard of Measurements Defused substances, Bomb samples from real-world scenarios were investigated to exploit 3.3.1.Besides Shrapnel standard of Defused substances, Bomb samples from real-world scenarios were investigated to exploit 3.3.1.the3.3.1. practical BesidesShrapnel Shrapnel standardcapabilities of of Defused Defused substances, Bombfor Bomb forensic samples analysis. from For real-w this orldpurpose, scenarios post-blast were investigatedresidues of ato defused exploit the3.3.1. practical BesidesShrapnel capabilitiesstandard of Defused substances, for Bomb forensic samples analysis. from For real-w this purpose,orld scenarios post-blast were residuesinvestigated of a todefused exploit theSecond3.3.1. practicalBesides Shrapnel World standardcapabilities Warof Defused bomb substances, for areBomb forensicinvestigated. samples analysis. Thisfrom Forbomb real-w this was orldpurpose, defused scenarios post-blast by werecontrolled investigatedresidues explosion of ato defused carriedexploit Secondthe practicalBesidesBesides World standard standardcapabilitiesWar bomb substances, substances, arefor investigated.forensic samples samples analysis. Thisfrom from bombFor real-w real-w this wasorld orldpurpose, defused scenarios scenarios post-blast by werecontrolled were investigated investigatedresidues explosion of to ato defusedcarriedexploit exploit Secondtheout practicalBesidesby Worldthe Berlinstandard capabilities War Police. bomb substances, forareAfter forensicinvestigated. th samplese explosion, analysis. fromThis For shrapnelbombreal-w this wasorld purpose, were defusedscenarios gathered post-blast by were controlled up investigated residuesby hand, explosion of sent toa defused exploitcarriedto our outtheSecondthe by practicalpracticalBesides the World Berlin capabilitiesstandardcapabilities War Police. bomb substances, for Afterforare forensic forensicinvestigated. the samplesexplosion, analysis.analysis. Thisfrom Forshrapnel Forbomb real-w thisthis was purpose,orldwerepurpose, defused scenarios gathered post-blastpost-blast by werecontrolled up residuesbyinvestigatedresidues hand, explosion ofofsent a a to defused defusedto carriedexploit our theoutSecondlaboratory practicalby Worldthe and Berlincapabilities Warstored Police. bomb in afor arecardboardAfter forensic investigated. the explosion,boxanalysis. at Thisroom For shrapnelbomb temperature this was purpose, were defused inside gathered post-blast by the controlled laboratory.up residuesby hand, explosion This of sent ascenario defused carriedto our is laboratorySecondoutSecondthe bypractical World theWorld and Berlin capabilitiesWarstored War Police.bomb bomb in a cardboard are forareAfter investigated. forensicinvestigated. the boxexplosion, analysis. at roomThis This bomb Forshrapneltemperaturebomb this was was purpose,were defused defused inside gathered post-blastby theby controlled controlledlaboratory. up byresidues hand,explosion explosion This of sentscenario a carrieddefused carriedto our is Secondlaboratoryouta good by Worldexamplethe and Berlin War stored of Police.bomba real in a are worldcardboardAfter investigated. scenariothe explosion,box atwhere This room bombshrapnelonly temperature little was were informationdefused inside gathered by the controlledabout laboratory. up bythe hand,explodedexplosion This sent scenario bombcarried to our is aoutlaboratory outSecondgood byby example the Worldthe and BerlinBerlin storedWarof Police.aPolice. bombreal in aworld AftercardboardareAfter investigated. scenario ththee explosion, boxexplosion, where at roomThis only shrapnel shrapnelbomb temperature little was wereinformationwere defused inside gatheredgathered bythe about controlled laboratory. upup the byby explodedhand,hand, explosion This sentsent scenario bomb tocarriedto our ouris is outalaboratoryavailable. good by theexample No andBerlin further stored of Police. a real informationin a world Aftercardboard thscenario eabout explosion, box theatwhere room bomb shrapnelonly temperature is littleavailable were information inside gatheredbecause the about itlaboratory.up wa bysthe defused hand, exploded This sentas scenario part bombto ourof isa available.laboratoryalaboratoryout good by examplethe No and and Berlinfurther stored stored of aPolice. informationrealin in a a worldcardboard cardboardAfter scenario thaboute box explosion,box the at whereat room bombroom only shrapneltemperature temperatureis availablelittle wereinformation inside becauseinside gathered the the aboutit laboratory. laboratory. waups thebydefused explodedhand, This This as sentscenario scenariopart bomb to of our isa isis laboratoryavailable.abatch good of example old Noand bombs. furtherstored of a realininformation a worldcardboard scenario about box atthewhere room bomb only temperature is littleavailable information inside because the about laboratory.it wa sthe defused exploded This as scenario part bomb of is isa batchaavailable.alaboratory good good of example oldexample No andbombs. further storedof of a a real real informationin aworld world cardboard scenario scenario about box wherethe whereat room bomb only only temperature is little littleavailable information information insidebecause the about aboutit laboratory. wa the sthe defused exploded exploded This as scenario partbomb bomb of is isa abatchavailable. goodFigure of example old No 3bombs. showsfurther of a realthe information resultsworld scenarioof aboutthe investigation wherethe bomb only is oflittle available one information of the because pictured about it washrapnel thes defused exploded with as the partbomb APLD of is a available.batchavailable.a goodFigure of exampleold No No3 bombs. showsfurther further of athe information realinformation results world of scenario theabout about investigation the thewhere bomb bomb only is ofis available littleoneavailable of information the because because pictured itabout it washrapnel was s thedefused defused exploded with as asthe part part bombAPLD of of a is a available.batchsystem.Figure of Inold No sum, 3bombs. furthershows four the informationdifferent results shrapnel of aboutthe investigation thepieces bomb we reis of investigatedavailable one of the because picturedand 2 it to wa shrapnel4 slocations defused with peras the partshrapnel APLD of a system.batchbatchavailable.Figure of of Inold old sum,No bombs.3 bombs. showsfurther four thedifferent information results shrapnel of theabout investigation pieces the bomb were isofinvestigated availableone of the because andpictured 2 to it 4washrapnel locationss defused with per as theshrapnel part APLD of a batchsystem.measured.Figure of oldIn sum,It bombs.3 turnedshows four outthe different resultsthat all shrapnel ofspectra the investigation exhibitpieces wea clearre of investigated onesignal of atthe m pictured/andz 227, 2 towhich shrapnel4 locations can bewith percorrelated the shrapnel APLD to measured.system.batchFigureFigure of In old It sum,3 3turned bombs.shows shows four out the thedifferent thatresults results all shrapnelofspectra of the the investigation investigationexhibit pieces awe clearre of ofinvestigated signalone one of of atthe the m /pictured zpicturedand 227, 2 whichto shrapnel shrapnel4 locations can bewith with correlated per the the shrapnel APLD APLD to measured.system.the molecularFigure In Itsum,3 showsturned ion four signal theout different resultsM¯that of all TNT. shrapnelof spectra the The investigation exhibitpiecespresence wea clearofre of TNTinvestigated onesignal can of thebeat m assumedpictured /andz 227, 2 towhich byshrapnel 4 thelocations can observed withbe percorrelated the signalsshrapnel APLD toin thesystem.measured.system. molecularFigure In In sum, Itsum, 3 turnedion shows four signalfour outdifferentthe different M¯ thatresults of allTNT. shrapnel shrapnel spectraof theThe investigation presencepieces 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the delivers collection for valuable laboratory of samples information measurements and the about related but the investigationalsousedalsoconfirm for forexplosive. answering answeringthat seem the Moreover, APLD torealistic realistic be acceptable.measurement forensiceven forensic longer questions. questions. system periods isThe Thebenot tweenAPLD APLDonly thesuitabledelivers delivers collection for valuable valuable laboratory of samples information information measurements and theabout about related the butthe alsoinvestigationused for explosive. answering seem Moreover, realisticto be acceptable. forensiceven longer questions. periods The be tweenAPLD thedelivers collection valuable of samples information and theabout related the usedinvestigationusedalso explosive. forexplosive. answering seem Moreover, Moreover, to realistic be acceptable. even evenforensic longer longer questions. periods periods Thebe betweentween APLD the the delivers collection collection valuable of of samples samples information and and the the about related related the usedinvestigation explosive. seem Moreover, to be acceptable. even longer periods between the collection of samples and the related investigationinvestigationused explosive. seem seem Moreover, to to be be acceptable. acceptable. even longer periods between the collection of samples and the related investigation seem to be acceptable. investigation seem to be acceptable. Appl. Sci. 2018, 8, 933 9 of 15

Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 15

Figure 3. Desorption in negative CI mode of TNT from the surface of shrapnel recovered from a Figure 3. Desorption in negative CI mode of TNT from the surface of shrapnel recovered from a defused bomb of the Second World War. All shrapnel in the upper image show clear signals of TNT defused bomb of the Second World War. All shrapnel in the upper image show clear signals of TNT when investigated. A A part part of of one one of of those those single single io ionn traces traces can can be be seen seen in in the the mass mass profile profile where where the the TNTTNT signal signal at at mm/z/ z227227 is isplotted. plotted. The The full full size size mass mass spectrum spectrum shows shows the first the firstpeak peak at about at about 1 min. 1 min.The panelThe panel in the in lower the lower left leftshows shows the thespectrum spectrum of ofa 10 a 10ng ng TNT TNT standard standard sample, sample, measured measured under under the the samesame conditions. conditions. The The signals signals look look similar similar for for every every detected detected TNT TNT trace trace and and shrapnel. shrapnel.

3.3.2. New Psychoactive Substance on Herbal Mixture 3.3.2. New Psychoactive Substance on Herbal Mixture Another practical application is illustrated in Figure 4. It shows the ability to detect synthetic Another practical application is illustrated in Figure4. It shows the ability to detect synthetic cannabinoid species directly on herbal mixtures. These herbal blends are commonly coated with cannabinoid species directly on herbal mixtures. These herbal blends are commonly coated with so-called new psychoactive substances (NPS). The investigated compound belongs to these NPS and so-called new psychoactive substances (NPS). The investigated compound belongs to these NPS and is a cannabimimetic aminoalkyl indazole with a carboxamide-linked cumyl moiety is a cannabimimetic aminoalkyl indazole with a carboxamide-linked cumyl moiety (5F-Cumyl-PINACA). (5F-Cumyl-PINACA). For better comparability and reliable substance identification, through fragment identification, EI, was applied as the ionization technique. Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 15

Strong EI fragmentation signal at low m/z are typical when investigating complex natural samples. Direct measurement of the investigated herbal mixture revealed the usage of 5F-Cumyl-PINACA as an active substance at m/z 367 by comparing the molecular ion signal as well as the fragment pattern [35]. For direct measurement of the herbal mixture, a S/N ratio of 50.9 was calculated for the base peak at m/z 352 and a S/N ratio of 3.7 for the molecular ion signal at m/z 367. For the respective measurement of the emptied bag, the S/N ratio for the base peak at m/z 352 was 45.6Appl. and Sci. 2018 for, the8, 933 molecular ion signal at m/z 367 the S/N ratio was 3.1. Even an emptied bag, in which10 of 15 the mixture was sold, showed a clear presence of 5F-Cumyl-PINACA. Therefore, a fast decision can be made whether the target substance is illicit or not simply by measuring suspect or confiscated For better comparability and reliable substance identification, through fragment identification, objects. A reasonable scenario could be the rapid identification of designer drugs and related EI, was applied as the ionization technique. products found in conspicuous parcels in postal distribution centers.

Figure 4. APLD EI-mass spectra of the herbal blend material. Direct laser desorption exhibits intense 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 signals at higher m/z, tentatively assigned to 5F-Cumyl-PINACA from an herbal blend sold as “Rollin’ “Rollin’ High”. This new psychoactive substance could be detected directly from herbal blend as High”. This new psychoactive substance could be detected directly from herbal blend as well as well as residues sticking on the bag. The upper part shows a mass spectrum achieved by desorbing residues sticking on the bag. The upper part shows a mass spectrum achieved by desorbing the inner the inner side of the bag and the lower one a direct measurement of the herbal blend. A similar side of the bag and the lower one a direct measurement of the herbal blend. A similar spectrum spectrum compared to the inside of the empty bag is revealed. compared to the inside of the empty bag is revealed. 3.4. Additional Capabilities of the APLD Approach Strong EI fragmentation signal at low m/z are typical when investigating complex natural samples. DirectThe measurement LODs and of the the investigatedmeasurement herbal speed mixture of the revealed APLD the system usage ofallows 5F-Cumyl-PINACA not only for asthe an measurementactive substance of atsamplesm/z 367 on by surfaces comparing at trace the molecularlevels, but ion it also signal enables as well an as automated the fragment measurement pattern [35]. ofFor surfaces direct measurement without sample of the pretreatment. herbal mixture, For ath S/Nis purpose, ratio of 50.9a computer was calculated controlled for the X-Y base table peak was at mountedm/z 352 and in front a S/N of ratio the APLD of 3.7 forhead. the In molecular consequence, ion signal the automated at m/z 367. mapping For the respective of flat surfaces measurement can be performed,of the emptied which bag, reveals the S/N APLD ratio forspectra the base with peak sub-millimeter at m/z 352 was lateral 45.6 resolution. and for the Analysis molecular of ion non-flat signal surfacesat m/z 367 could the S/Nbe difficult ratio was due 3.1. to delocalization Even an emptied of target bag, in compounds which the mixture by touching was sold, the PTFE showed ring. a clearThe actualpresence system of 5F-Cumyl-PINACA. performance is illustrated Therefore, in aFigure fast decision 5. This canfigure be shows made whetherthe possibility the target to detect substance the spatialis illicit distribution or not simply of an by artificially measuring prepared suspect substa or confiscatednce trace objects. on a surface. A reasonable An imaginable scenario scenario could bein contextthe rapid of identificationa preliminary of investigation designer drugs that and benefits related from products APLD found capabilities in conspicuous could be parcels the analysis in postal of distribution centers.

3.4. 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 Appl. Sci. 2018, 8, 933 11 of 15 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 Figure5. This figure shows the possibility to detect the spatial

Appl.distribution Sci. 2018, 8 of, x anFOR artificially PEER REVIEW prepared substance trace on a surface. An imaginable scenario in context11 of 15 of a preliminary investigation that benefits from APLD capabilities could be the analysis of fingerprint fingerprintresidues from residues plastic from explosives. plastic explosives. In this case, In the this identification case, the identification can be challenging can be challenging due to the strong due to thepresence strong of presence matrix signals of matrix in thesignals plastic in explosive.the plastic explosive.

Figure 5. APLD mapping capabilities shown at a TNT example generated with CI negative detection 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 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 intensity is combined at any point of the surface the image on the top right can be achieved. This image image shows a prepared stainless steel surface that was labeled with the word TNT. This label was shows a prepared stainless steel surface that was labeled with the word TNT. This label was produced produced with 1 mg/L TNT solution and a µL syringe. The total amount of substance applied for this with 1 mg/L TNT solution and a µL syringe. The total amount of substance applied for this image was imageabout 20wasµ g.about The color20 µg. changes The color from changes black (no from TNT black detected) (no TNT to white detected) (high amountsto white of(high TNT amounts detected). of TNTTherefore, detected). the upper Therefore, spectrum the indicates upper spectrum a “black” indicates area and the a “black” lower spectrum area and reveals the lower the basis spectrum for a reveals“white the area”. basis For for mapping a “white purposes area”. 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 Appl. Sci. 2018, 8, 933 12 of 15

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. Table3 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 (3,4-methylendioxy-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. Moreover, Raman detection limits for cocaine-hydrochloride [44] can be found at about 1 µg whereas APLD achieves a LOD of 17 ng. According to the author, this high LOD is due to the low analyte cross section and affects all Raman measurements [36]. Ion mobility spectrometry is capable of detecting 350 pg total amount of TNT and 5 ng of cocaine. TD MS allows for the detection of 5 ng of TNT while APLD can detect 500 pg. For cocaine [45], a LOD of 388 ng and 2 ng for MDMA [38] is reported.

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.

LOD LOD Compared Analytical Technique Measured Substance Ref. Technique/ng APLD/ng 2,4-DNT 5 1.5 Raman spectroscopy Cocaine HCl 1000 17 TNT 0.35 0.5 Ion mobility spectrometry Cocaine HCl 5 5 TNT 5 0.5 Thermal desorption MS Cocaine HCl 388 17 MDMA 2 10 TNT 10 fg 0.5 Desorption spray ionization Amphetamine 14 20 Low-temperature plasma probe Tetryl 0.25 2 Tetryl 2 2 DART-TD-MS MDMA 2 10

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. Appl. Sci. 2018, 8, 933 13 of 15

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.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/8/6/933/s1. LabVIEW script for generated heat map, used in Figure5. Author Contributions: J.H. and R.R. conceived and designed the experiments; J.H. and R.R. performed the experiments; R.R. analyzed the data; M.P. and T.F. contributed reagents, samples, measurement tools, and expert knowledge for those; R.R. wrote the paper; S.E. and R.Z. contributed significantly to the discussion and revision. Acknowledgments: 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”. We want to especially thank the German Federal Criminal Police Office (BKA) for providing the samples and the opportunity of a measurement campaign with real samples in Wiesbaden. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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