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Detection of nitro-organic and peroxide explosives in latent fingermarks by DART- and SALDI-TOF-mass spectrometry
Forensic Science International 221 (2012) 84–91
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Detection of nitro-organic and peroxide explosives in latent fingermarks by DART- and SALDI-TOF-mass spectrometry Frederick Rowell a,*, John Seviour b, Angelina Yimei Lim c,e, Cheryl Grace Elumbaring-Salazar c, Jason Loke d, Jan Ma e a
Fingerprinting Analytics Ltd., 35, Park Road South, Chester le Street, DH3 3LS, United Kingdom Forensics Ltd., Malvern Hills Science Park, Great Malvern, Worcestershire WR14 3SZ, United Kingdom Nanofrontier Pte Ltd., Research TechnoPlaza, 50 Nanyang Drive, Singapore 637553, Singapore d Forensic Management Branch, CID, Singapore 088762, Singapore e The School of Materials Science and Engineering, 50 Nanyang Avenue, Nanyang Technological University, Singapore 639798, Singapore b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 June 2011 Received in revised form 4 April 2012 Accepted 9 April 2012 Available online 30 April 2012
The ability of two mass spectrometric methods, surface-assisted laser desorption/ionization-time of flight-mass spectrometry (SALDI-TOF-MS) and direct analysis in real time (DART-MS), to detect the presence of seven common explosives (six nitro-organic- and one peroxide-type) in spiked latent fingermarks has been examined. It was found that each explosive could be detected with nanogram sensitivity for marks resulting from direct finger contact with a glass probe by DART-MS or onto stainless steel target plates using SALDI-TOF-MS for marks pre-dusted with one type of commercial black magnetic powder. These explosives also could be detected in latent marks lifted from six common surfaces (paper, plastic bag, metal drinks can, wood laminate, adhesive tape and white ceramic tile) whereas no explosive could be detected in equivalent pre-dusted marks on the surface of a commercial lifting tape by the DART-MS method due to high background interference from the tape material. The presence of TNT and Tetryl could be detected in pre-dusted latent fingermarks on a commercial lifting tape for up to 29 days sealed and stored under ambient conditions. ß 2012 Elsevier Ireland Ltd. All rights reserved.
Keywords: DART-mass spectrometry SALDI-TOF-mass spectrometry Nitro-organic and peroxide explosives Lifted latent fingermarks Dusting powder Contact residues Stability
1. Introduction There are several well established methods for detection of nitro-organic and peroxide explosives in a variety of analytical matrices [1,2]. For security and forensic applications detection of such explosives directly on the surface of fingers or in latent fingermarks deposited on surfaces at crime scenes or at suspicious locations is of obvious interest. There are several reports of the use of mass spectrometric (MS) methods to detect the presence of explosives directly on the surface of fingers resulting from contact deposition following handling of explosives. These include DESIMS (direct electrospray ionization) [3–5] and DART-MS (direct analysis in real time) techniques [6,7]. To date there appear to be no reports of detection of explosives in developed latent fingermarks lifted from surfaces at crime scenes or at suspicious locations. Such development leading to visualization of latent fingermarks at these sites is generally achieved through use of commercial developing agents such as dusting powders. Their location must be the first step in any subsequent analysis of latent marks for the presence of explosives or other contact residues
resulting from handling chemicals. Following visualization the marks are then photographed and generally lifted using adhesivecoated plastic sheets and sealed for later use as evidence. We have described use of novel magnetizable doped silicabased sub-micron particles as dual agents which can be used to locate latent marks on surfaces following conventional dusting using a magnetic wand and which then act as a signal enhancing agent in surface-assisted laser desorption/ionization-time of flight-mass spectrometry (SALDI-TOF-MS) analysis when the developed marks are lifted using conventional lifting tape then subjected to direct MS analysis on the tape’s surface. This approach has been used for detection of drugs and their metabolites in lifted marks following contact or oral use [8] and nicotine in smokers [9,10]. We now describe the use of these particles for detection of six commonly used nitro-organic explosives and one peroxidetype explosive in latent fingermarks and compare the SALDI-TOFMS and DART systems with samples lifted from six common contact surfaces of differing surface characteristics. 2. Methods 2.1. Materials
* Corresponding author. Tel.: +44 191 388 2283; fax: +44 191 515 2698. E-mail address: [email protected] (F. Rowell). 0379-0738/$ – see front matter ß 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.forsciint.2012.04.007
The explosive standards used were TNT (2,4,6-trinitrotoluene), Tetryl (N-methylN,2,4,6-tetranitroaniline), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane),
F. Rowell et al. / Forensic Science International 221 (2012) 84–91 RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) [from Ultra Scientific Analytical Solutions (North Kingston, Rhode Island, USA)], TNG/Nitroglycerin (propane-1,2,3triyltrinitrate), and PETN/Pentaerythritol tetranitrate (1,3-dinitrato-2,2-bis(nitratomethyl)propane) [from Cerillant Analytical (Round Rock, TX, USA)] and TATP/ triacetone triperoxide (3,3,6,6,9,9-hexamethyl-1,2,4,5,7,8-hexaoxonane) [from AccuStandards Inc. (New Haven, CT, USA)]. They were supplied in acetonitrile as 1 mg/ml solutions except for TATP which was supplied as a 100 mg/ml solution. The calibration standard used for DART-MS was PEG 600 from Jeol Inc. (Jeol ASIA Pte. Ltd., Corporation Road, Singapore) and those for SALDI-TOF-MS including 2,5-dihydroxy benzoic acid (DHB) from Sigma Aldrich (St. Louis, MO, USA) and calibration was performed as earlier [7,8]. ROAR Black magnetic powder [11] and lifting tape were gifts from ROAR Particles plc, Sedgefield, UK (now Air Analytics Ltd., Chester le Street, UK). Magnetic wands used were from CSI [CSI Ltd. (Woburn Sands, UK)] and Sirchie Laboratories [Sirchie Fingerprint Laboratories (Youngsville, NC, USA)]. Stainless steel MALDI target plates (type DE 2115TA and DE1580TA) were from Shimadzu Biotech [Shimadzu (ASIA Pacific) Pte. Ltd., Science Park 1, Singapore] and glass DART dip-it probes were from Jeol/Ion Sense [Jeol Ltd. (Akishima, Japan/Ionsense Inc./Saugus, MA, USA)]. 2.2. Mass spectrometry SALDI-TOF-MS was performed following calibration using an Axima TOF2 mass spectrometer [Shimadzu (ASIA Pacific) Pte. Ltd., Science Park 1, Singapore] with settings in negative mode (nitro-organic explosives) or positive mode (TATP) whilst DART-MS was performed following calibration using an atmospheric pressure Ionization source from IonSense (Ionsense Inc., Saugus, MA, USA) and a JMS T100LP mass spectrometer from Jeol (Jeol Ltd., Akishima, Japan) with settings in negative or positive modes for the explosives as before. DART calibration was carried out using neat polyethylene glycol (PEG 600) on glass rods. In positive mode a series of [M+H]+ and [M+H H2O]+ peaks from m/z 45 to 1000 were used. In the negative mode a series of {M+O2 H] and [(C2H4O)n+O2 H] peaks were used for calibration from m/z 43 to 1000. Instrument settings were a needle voltage of 2000 V, discharge electrode 250 V, grid electrode 150 V, He flow rate 2.5 l/min. SALDI calibration was carried out by adding 1 ml of a DHB solution (10 mg/ml in 20:80 (v/v) acetonitrile; deionized water) to a 1 ml mixture of reserpine (1 mg/ml in acetonitrile), cesium iodide (1 mg/ml in deionized water) and papaverine (1 mg/ml in methanol) to a well on a MALDI target plate. The resulting calibration mixture applied consisted of 25 ml papaverine, 25 ml resperpine and 50 ml cesium iodide. The instrument settings were laser 85–115 (power = 2.83–3.83) with 100 accumulated profiles at 1 laser shot per profile; calibration range in positive mode
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from m/z 132 to 607 and in negative mode from 153 to 646 for calibrant peaks and those from DHB. 2.3. MS analysis of reference samples Reference spectra were obtained by applying a 1 ml aliquot of standards for the seven explosives at concentrations of 10 mg/mL, 100 mg/mL and 1 mg/mL (except for TATP which was used at 10 mg/mL and 100 mg/mL) onto a dip-it glass rod (DART-MS) or onto metal target plates (SALDI-MS), followed by their direct analysis following solvent evaporation under ambient conditions. 2.4. MS analysis of spiked un-lifted latent fingermarks Fingermarks were obtained from volunteers in Singapore by rubbing three middle fingers of one hand over their brow. To serve as control, a finger was pressed onto a clean portion of the SALDI-MS plate or onto the tip of a clean glass dip-it probe for DART-MS. The other two fingers were applied to dip-it glass probes. Likewise for SALDI-MS, the first finger was pressed onto a clean portion of the SALDI-MS plate or and the other two onto surfaces pre-coated with explosives. For SALDI-MS, aliquots [1 ml of standard (equivalent to 1 mg of each explosive except for TATP where 100 ng was applied)] of the explosive were applied onto two wells adjacent to the control deposit on the SALDI target plate. After drying, the other two fingers were pressed on the plate covering the two wells where the explosives were spotted. For DART-MS, 1 ml aliquots of explosive standards were applied directly onto the surface of a clean dip-it glass probe. After drying a fingermark was deposited onto the probe over the explosives. For both methods each deposited fingermark was then developed using ROAR Black magnetic powder within 1 h followed by immediate MS analysis. 2.5. MS analysis of spiked lifted latent fingermarks Fingermarks were obtained as described above. For both SALDI and DART MS analyses, 5 ml of explosive standard was applied to three discrete areas of approximately 2.5 cm 2.5 cm on the smooth surface of a white ceramic tile. After drying, a new fingertip was pressed onto each of the spiked areas. The contaminated finger was then pressed onto another clean tile. Fingermarks were developed using the ROAR Black magnetic powder and the developed latent fingermarks were then lifted using commercial lifting tape. Analysis by SALDI-MS was made by sticking the lifted latent fingermarks with the adhesive portion facing up, onto the SALDI metal target plate using strips of cellotape. For each lifted mark five different areas were analyzed and the average signal intensities calculated. For DART-MS, the lifted
Table 1 Summary of major peaks observed following MS of standards of explosives on spiked surfaces (metal target plate for SALDI-MS and a glass dip-it probe for DART-MS). Explosive
Identifying fragments at m/z (italics-major peak)
SALDI-MS 100 ng
SALDI-MS 10 ng
DART-MS 100 ng
DART-MS 10 ng
TATP
116 178 172 223a 228 241 207 213 286a 226a 197 181 165 190 46 62 131 147 314a 46 102 129 133 221a 46 102 129 133 295a
Detected Detected Detected nd Detected Detected Detected Detected nd Detected Detected Detected Detected Detected Detected Detected Detectedb Detected nd Detected Detected Detected Detected nd Detected Detected Detected Detectedb nd
Detected Detected Detected nd Detected Detected Detected Detected nd Detected Detected Detected Detected Detected Detected Detected nd Detected nd Detected Detectedb Detected Detected nd Detected Detected Detected Detectedb nd
nd Detected nd Detected Detected Detected Detected Detected nd Detected Detected Detected Detected Detected nd nd Detected Detected Detected nd Detected Detectedb nd Detected nd Detectedb Detectedb nd Detected
nd nd nd nd Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd nd nd Detected Detected nd Detected Detectedb nd Detected nd Detectedb Detectedb nd Detected
Tetryl
TNT
TNG PETN
RDX
HMX
Detected (Signal: noise ratio, S/N > 50), nd (not detected; S/N < 3). Bold numbers: molecular ion (M H) [or (M+H)+ for TATP]. low intensity peaks observed (S/N 4–50).
a
b
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fingermark was manually and directly introduced to the ion source. Signal to noise ratios were calculated from the average intensities of the signals for doped and undoped spectrums at the same m/z values. The above method was repeated for analysis of explosives on spiked fingerprints lifted from six surfaces. These were obtained as described above from a white tile and the resulting contaminated fingers then used to deposit fingermarks onto a second white ceramic tile, the non-sticky surface of a brown adhesive tape, from the upper surface of a laminated wood sample, a metal drinks can, a transparent plastic bag and a sheet of A4 printing paper. The same procedure was followed as described above for analysis of explosives by SALDI-MS and DART-MS in dusted lifted prints taken from these surfaces. 2.6. Stability of explosives in spiked latent fingermarks The procedure for SALDI-MS analysis of spiked un-lifted latent fingermarks was repeated for TNT and tetryl for contaminated marks on metal target plates for spiked un-lifted marks or spiked marks on lifting tape (5 mg and 10 mg of explosive were used for the spiking in both cases). Three sets of spiked fingermarks were used for each the four sets of samples (5 mg and 10 mg, and lifted and un-lifted). Unspiked marks (1 lifted and 1 un-lifted) were also used as controls. The samples were stored together in a sealed container under ambient laboratory conditions for up to 28 days prior to their analysis by SALDI-MS. Five discrete areas on each mark were subjected to SALDI-MS at each time point and the average intensity values used to obtain S/N values as described above.
2.7. Effect of dusting agents on MS sensitivity The effect of a range of 11 commercial dusting agents (from Sirchie Fingerprint Laboratories, CSI and Armor Forensics (now Safariland) used to develop latent fingermarks on surfaces was examined by SALDI-MS analysis as described above for latent fingermarks spiked with 1 mg of TNT or PETN on a metal target plate and for lifted developed spiked fingermarks, on lifting tape. The dusting agents were aluminium (1), magnetic (3), bichromatic (1), white (1), silver (1), black (2), coloured (3) and fluorescent (2) powders (some were combinations of these such as magnetic and fluorescent). The non-magnetic powders were applied using a commercial squirrel hair dusting brush (CSI) and the magnetic ones using a commercial magnetic wand. The intensities of the TNT peak at m/z 226 or the PETN peak at m/z 147 were compared with that obtained using the ROAR Black dusting powder.
3. Results 3.1. MS analysis of reference samples Detection of characteristic peaks was observed for standards of each explosive deposited directly onto clean surfaces by both DART-MS and SALDI-MS as shown in Table 1. Generally both
Fig. 1. DART-MS of explosive-spiked fingermarks for marks deposited directly onto dip-it glass probes pre-spiked with TNT (a) and Tetryl (b). Characteristic peaks were observed for (a) at m/z 226 and 197 and for (b) at 241 and 228.
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Table 2 Major peaks obtained by SALDI-TOF-MS observed in spiked latent fingermarks lifted from ceramic tile, parcel tape and wood laminate surfaces. Explosive
Identifying fragments (m/z)
Direct on metal target plate
Lifted from ceramic tile
Lifted from parcel tape
Lifted from wood laminate
TATP
116 178 172 228 241 207 213 226 197 181 165 190 46 62 131 147 46 102 129 133 46 102 129 133
Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detectedb Detected Detected Detected Detected Detected Detected Detected Detected Detectedb
Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd Detected Detected Detectedb Detected Detected Detected Detected Detected Detectedb
Detected nda nd Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd Detected Detected Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb
Detected nd nd Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd Detected Detected Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb
Tetryl
TNT
TNG PETN
RDX
HMX
Detected: S/N > 50. a nd – not detected; S/N < 3. b Detected with low intensity; S/N 4–50.
methods are of similar sensitivity being able to detect 10 ng of explosive on the surface except for TATP where only peaks at m/z 178 and 223 peaks were observed at 100 ng for DART-MS whereas peaks at m/z 116, 178 and 172 were seen for the 10 ng sample using SALDI-MS. DART-MS detected nitro-explosives as deprotonated ions in the negative mode and TATP as protonated ion in the positive mode in addition to similar ion fragments to those detected for the corresponding SALDI-MS spectrums. 3.2. MS analysis of spiked un-lifted latent fingermarks The same characteristic peaks were observed for both methods for spiked marks applied directly by contact of contaminated fingers onto a dip-it probe for DART-MS or onto a metal target plate for SALDI-MS. In both cases additional peaks due to endogenous
fingermark constituents were also observed. The DART-MS spectrums of fingermarks spiked with TNT and tetryl are shown in Fig. 1 which show characteristic peaks for the explosives. 3.3. MS analysis of spiked lifted latent fingermarks In lifted developed fingermarks on commercial lifting tape from metal target plates that were subjected to SALDI-MS, characteristic peaks of explosive were seen but with lower intensity than the corresponding spectrums of spiked marks on the metal target plate. A summary of the peaks in dusted marks lifted from 4 surfaces (metal target plate, ceramic tile, parcel tape and wood laminate) for the 7 explosives is shown in Table 2 and the spectrums for tetryl and TNT in lifted developed spiked marks lifted from a ceramic tile is shown in Fig. 2 together with the
Fig. 2. Explosive-doped fingermarks dusted, lifted and analysed via SALDI-MS on lifting tape lifted from a ceramic tile. Characteristic peaks for the respective explosive were observed in each case. Spectrum 1c: Blank undoped fingermark (bottom). Spectrum 2c: Tetryl-doped fingermark (middle) (peaks at m/z 228 and 241). Spectrum 3c: TNTdoped fingermark (top) (peaks at m/z 197 and 226).
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Fig. 3. Detection of TNT on different surfaces from lifted fingermarks by SALDI-MS with observed increasing intensity of TNT fragment m/z 226.1 m/z from spectrum 1 to 6. Spectrum 1c: Plastic bag (bottom). Spectrum 2c: Non-sticky side of parcel tape. Spectrum 3c: Metal drinks can. Spectrum 4c: Tile. Spectrum 5c: Paper. Spectrum 6c: Laminated wood (upper).
spectrum for an un-spiked fingermark. Characteristic peaks for the two explosives are observed. Fig. 3 shows the relative intensities of the m/z 226 peak of TNT in lifted fingermarks from six surfaces (plastic bag, non-sticky side of parcel tape, metal can, ceramic tile,
paper and laminated wood). It was observed that the peak intensity increased progressively from the samples lifted from plastic bag through to the laminated wood and this was generally seen for the other six explosives. The SALDI-MS spectrum for TATP lifted from paper is shown in Fig. 4a and that lifted from metal in Fig. 4b. Both show characteristics peaks which are not present in the corresponding un-spiked fingermark. Fig. 4b also shows the presence of endogenous cholesterol at m/z 369. The characteristic spectrums for explosives in lifted spiked marks on lifting tape were not observed in the DART-MS spectra. Fig. 5a shows the DART-MS spectrum of a dusted lifted mark spiked with tetryl on the lifting tape and Fig 5b shows the corresponding spectrum for a fingermark spiked with TNT. As seen in these spectrums the major peaks for explosives by DART-MS were of very weak intensity or were not detected. In each case the spectrums were nearly identical comprising a complex spectrum possibly originating from the adhesive present on the surface of the lifting tape. Different temperature settings of 120, 200 and 250 8C were also used to test optimum temperature for analysis of marks on lifted prints. The resulting complex spectra (not shown) were similar at all temperatures with background peak intensities increasing with increasing temperatures. It was noted that at 200 8C and above the tape started melting and became distorted. 3.4. Stability of explosives in spiked latent fingermarks
Fig. 4. SALDI-MS of TATP-spiked latent fingermark. (a) lifted from paper and (b) lifted from metal. Characteristic peaks at m/z 116 and 172 seen in (a) (2c, upper spectrum; 1c, un-doped fingermark lower spectrum), and at m/z 178 in (b) with a cholesterol peak at m/z 369.
It was observed that for TNT (peaks at m/z 226, 197 and 181) and tetryl (m/z 241, 228, 213 and 207) characteristic peaks were detected for 28 days and that generally their intensity decreased throughout this period for spiked un-lifted samples for both explosives. For lifted TNT samples a general increase was observed from weeks 0 to 2, followed by a progressive decrease in intensities for the majority of the characteristic peaks. For tetryl a progressive increase in intensity for the major peaks at m/z 241 and 228 and the minor peaks at 213 and 207 was observed for both un-lifted and lifted marks. The intensities for lifted marks were again lower than seen for the corresponding un-lifted marks for both explosives but were detectable throughout the time course. Fig. 6 shows this trend for the m/z peak at 197 for TNT for spiked marks on a metal target plate and for corresponding marks on lifting tape. The lowest signal: noise ratio for any peak was 3:1 seen
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Fig. 5. Explosive-doped fingermarks dusted, lifted and analysed via DART-MS. No characteristic peaks at m/z 228 and 241 (a) and m/z 197 and 226 (b) were observed on the lifted fingermarks. (a) Tetryl-doped fingermark. (b) TNT-doped fingermark.
in the lifted sample for the lowest concentration of TNT at time zero for the m/z 181 peak. This rose to 36 after 1 week. The most intense peaks were seen at m/z 197 for TNT and at 228 for tetryl in all samples and the corresponding S/N values were 9 and 49 (TNT) and 13 and 129 (tetryl).
produced peaks of highest intensity. In contrast, only the ROAR Black powder produced detectable peaks for both explosives with lifted samples on commercial lifting tape. The results for TNT are shown in Fig. 7. It was observed that the presence of ROAR Black dusting agent had no effect on the intensity of peaks in the DARTMS spectra on dip-it probes for standards or spiked fingermarks.
3.5. Effect of dusting agents on MS sensitivity 4. Discussion Eleven commercial dusting agents and the ROAR Black powder were used as dusting agents with latent fingermarks from the same individual for marks deposited on a metal target plate and spiked with 1 mg of TNT or PETN. The intensities of the m/z peaks at 226 or 147 were examined by SALDI-MS. The dusted marks were also lifted and the intensity of the m/z 226 or 147 peaks also determined. It was observed that 3 commercial powders (Lightning Silver Grey, Lightning non-Magnetic Black and CSI Magneta Flake) and ROAR Black produced detectable signals for both explosives in un-lifted samples but that the ROAR Black powder
It was found possible to detect standards for all seven explosives using the DART system when they were deposited from solution onto a glass probe which was then inserted into the beam of ionized helium following established protocols. Likewise it was possible to detect these explosives by SALDI-TOF-MS following their deposition onto metal target plates and treatment with the ROAR Black powder. The only other report of the use of the related technique, Matrix Assisted Laser desorption/ionizationTOF-MS (MALDI-TOF-MS) for detection of explosives used
Fig. 6. Detection of TNT in spiked fingermarks stored under ambient conditions over a 28 day period for the characteristic fragment at m/z 197 for un-lifted and lifted samples (5 ml and 10 ml are equivalent to contact of fingers with 5 mg and 10 mg respectively of TNT prior to deposition and lifting). The points correspond to the mean intensities from 3 fingermarks for each of the four samples with 5 areas analysed per mark.
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Fig. 7. Intensities of SALDI-MS TNT peaks at m/z 226 with ROAR Black and commercial powders. ROAR Black powder (bottom) and three of the eleven commercial powders examined that provided peaks at m/z 226 in un-lifted marks. All spectrums are for lifted fingermarks. The two upper spectrums are from doped marks dusted with commercial powders that gave no TNT peaks in the un-lifted marks.
cyclodextrin as an adsorbing agent to capture the explosives over a 72-h period at 50 8C prior to MS analysis of the resulting explosive/ cyclodextrin complex [12]. In our experiments it is assumed that similar adsorption of the explosives onto the ROAR particles occurred rapidly to produce a complex of sufficient stability to resist evaporation of the explosives under the high vacuum conditions within the mass spectrometer and that under laser irradiation the residues present on the surface of the particles assisted ionization of the adsorbed explosives producing the negative ions detected for the nitro-organic explosives or positive ions for the peroxide explosive TATP. The mechanism for Surface Assisted Laser Desorption/Ionization of polar and non-polar analytes using these particles has recently been described [13]. The SALDI-MS of the aliphatic explosives were characterized by a lack of the de-protonated molecular ions and peaks probably due
loss of nitro groups and the presence of low mass peaks at m/z 46 and/or 62 due to (NO2+) and (NO3+) ions, respectively. In contrast DART-MS generally produced the de-protonated molecular ion plus these peaks reflecting the milder conditions associated with the DART system. The characteristic spectral patterns associated with both DART and SALDI-systems can be used identify these explosives with good sensitivity and use of MS–MS via collision induced dissociation (CID) can add further un-ambiguous evidence for their identity. In contrast screens for explosives in fingermarks using IR/Raman spectroscopy [14,15] require further complex sample processing to obtain such MS identification. An example of the SALDI-MS–MS spectrum for PETN is shown in Fig. 8. For SALDI-MS signal intensity was greatly improved by the presence of ROAR Black powder as 11 other commercial powders
Fig. 8. MS–MS (collision induced dissociation) of the m/z 147 SALDI-TOF-MS peak for PETN showing the characteristic fragmentation pattern for the explosive.
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gave either no (n = 8) or considerably reduced signals (n = 3). The enhanced signal intensity with this powder compared with other commercial dusting agents has also been observed for drugs [8] and nicotine [9,10] in pre-dusted lifted latent fingermarks using SALDI-MS. Its effect on the spectrums of endogenous constituents in latent fingrmarks has also been described [13]. In a second set of experiments transfer via contact was carried out for deposition of explosives onto a metal target plate or a glass probe. Following dusting with the ROAR Black powder DART- or SALDI-MS was performed as for the standards. In both cases the explosives were again detected indicating that the presence of dusting powder or print material did not interfere with the ionization process in the DART-MS system and that the presence of fingerprint material did not adversely affect explosive detection in the SALDI-MS system. For both systems the presence of endogenous constituents such as fatty acids and squalene within the marks was observed. In the third set of experiments, the finger tips made contact with deposited explosive but the finger then made contact with one of six common surfaces. The resulting latent fingermarks were dusted with ROAR Black powder and then lifted from the surface with a commercial lifting tape. The surfaces were chosen to represent non-porous and porous surfaces with flexible and nonflexible, and smooth and non-smooth surfaces. The latent marks on the lifted tape were then placed into the ionized helium stream (DART-MS) so that the stream was reflected from the surface into the entrance to the mass spectrometer, or positioned onto the metal target plate for SALDI-MS. In the case of the DART system no signals due to explosives could be un-ambiguously distinguished from the complex background signal. This background spectrum also masked the peaks due to endogenous compounds within the lifted fingermarks. A range of helium gas temperatures between 120 and 250 8C was used and it was noted that as temperatures increased the intensity of the background peaks also increased and in addition at temperatures above 200 8C, the tape distorted and eventually charred. Repeat of this experiment with a second type of adhesive tape [J-LAr from 3 M (USA)] produced only identical background spectrums to those seen with the CSI lifting tape with the DART-MS system but with characteristic explosive peaks and those from endogenous components observed with the SALDI-MS analysis (data not shown). This indicates that thermal degradation of the surface coating of adhesive is likely to be a problem with lifting tapes that employ commonly used pressure sensitive adhesives which are known to undergo thermal degradation since their pyrolysis coupled to GC-MS has been used for their identification [16]. In contrast to the DART-MS results each explosive was detected by SALDI-MS in the lifted marks from each of the six surfaces together with peaks from endogenous constituents. For these surfaces good signals were obtained for the seven explosives from samples lifted from laminated wood, paper, ceramic tile and metal can with lower signal intensities from the parcel tape and the clear plastic bag. The results may indicate that explosives become adsorbed into plastic surfaces resulting in less effective lifting of the explosive from the surface. Additional experiments with a wider range of plastic materials and surfaces are needed to investigate this possibility. Stability studies with spiked samples demonstrated that TNT and tetryl could be detected by SALDI-MS on un-lifted and lifted latent fingermarks throughout a 28-day period. For both explosives the intensities of the peaks progressively decreased with time for the un-lifted marks with the exception of the tetryl peak at m/z 228which showed an increase from week 1 to 3. For lifted marks both TNT and tetryl exhibited a progressive increase in peak intensities for the non-parent ions on storage and this may result
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from accumulation of the chemically degraded explosives on the surface of the lifting tape. Further studies are needed to identify the origin of this phenomenon. In conclusion it has been shown that the nitro-aromatic, nitroaliphatic and peroxide-type explosives investigated can be detected using DART-MS when they are pre-dispensed directly onto a glass probe used to place the analyte in the helium stream prior to entry into the mass spectrometer, or when contact residues in fingermarks are deposited directly onto glass probes using this technique. These explosives can also be detected in standards applied directly onto metal target plates and in contact residues in fingermarks deposited onto metal target plates when in contact with the ROAR Black magnetic powder and subsequently analysed by SALDI-TOF-MS. When these explosives in fingermarks are deposited onto surfaces (laminated wood, plastic tape, paper, plastic bag, metal can, and ceramic tile) which are dusted with the ROAR Black powder then lifted onto a commercial lifting tape and the lifted marks subjected to SALDI-TOF-MS characteristic signals due to the explosives are observed. In contrast when equivalent lifted fingermarks are analyzed by DART-MS, no, characteristic peaks due to the explosives are seen in the resulting spectra. Acknowledgements This work was partly funded by the Government of Singapore through the Office of the Prime Minister through a TECProject (P00656/1409) and was largely undertaken in laboratories of the Forensic Management Branch of the Criminal Investigation Department of the Singapore Police Force. References [1] D.S. Moore, Recent advances in trace explosion detection instrumentation, Sens. Imaging 8 (2007) 9–38. [2] L. Song, J.E. Bartmess, Liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry: a highly sensitive method for the analysis of organic explosives, Rapid Commun. Mass Spectrom. 23 (2009) 77–84. [3] D.R. Ifa, N.E. Manicke, A.L. Dill, R.G. Cooks, Latent fingerprint chemical imaging by mass spectrometry, Science 321 (2008) 805. [4] D.R. Justes, N. Talaty, I. Cotte-Rodriguez, R.G. Cooks, Detection of explosives on skin using ambient mass spectrometry, Chem. Commun. (2007) 2142–2144. [5] I. Cotte-Rodrigues, H. Chen, R.G. Cooks, Trace detection of triacetone triperoxide (TATP) by complexation reactions during desorption electrospray ionization, Chem. Commun. (2006) 953–955. [6] J. Larame´e, H. Dupont Durst, T.R. Connell, J.M. Nilles, Detection of peroxide and tetrazine explosives on surfaces by Direct Analysis in Real Time mass spectrometry, Am. Lab. 2 (2) (2009) 1–5. [7] Joel on-line information in AccuTOFDART Applications Notebook, 4th Addition, March 2010. www.jeolusa.com. [8] F. Rowell, K. Hudson, J. Seviour, Detection of drugs and their metabolites in dusted latent fingermarks by mass spectrometry, Analyst 134 (2009) 701–707. [9] M. Benton, F. Rowell, L. Sundar, M. Jan, Direct detection of nicotine and cotinine in dusted latent fingermarks of smokers using hydrophobic silica particles and MS, Surf. Interface Anal. 42 (2009) 339–343. [10] M. Benton, M.J. Chua, F. Gu, F. Rowell, J. Ma, Environmental nicotine contamination in latent fingermarks from smoker contacts and passive smoking, Forensic Sci. Int. 200 (2010) 28–34. [11] B. Theaker, K. Hudson, F. Rowell, Doped hydrophobic silica nano- and microparticles as novel agents for developing latent finger marks, Forensic Sci. Int. 174 (2008) 26–34. [12] M. Zhang, Z. Shi, Y. Bai, Y. Gao, R. Hu, F. Zhao, Using molecular recognition of betacyclodextrin to determine low-molecular-weight explosives by MALDI-TOF mass spectrometry, J. Am. Soc. Mass Spectrom. 17 (2006), 198–193. [13] A. Lim, F. Gu, Z. Ma, F. Rowell, Doped amorphous silica nanoparticles as enhancing agents for surface-assisted time-of-flight mass spectrometry, Analyst 136 (2011) 2775–2785. [14] Y. Mou, J. Rabalais, Detection and identification of explosive particles in fingerprints using attenuated total reflection-Fourier transformation infrared spectromicroscopy, J. Forensic Sci. 54 (2009) 846–850. [15] E. Emmons, A. Tripathi, J.S. Christesen, A. Fountain, Raman chemical imaging of explosive-contaminated fingerprints, Appl. Spectrosc. 63 (2009) 1197–2003. [16] S. Nakamura, M. Takino, S. Daishima, Analysis of pressure sensitive adhesives by GC/MS and GC/AED with temperature programmable pyrolyzer, Anal. Sci. 16 (2000) 627–631.
Contents lists available at SciVerse ScienceDirect
Forensic Science International journal homepage: www.elsevier.com/locate/forsciint
Detection of nitro-organic and peroxide explosives in latent fingermarks by DART- and SALDI-TOF-mass spectrometry Frederick Rowell a,*, John Seviour b, Angelina Yimei Lim c,e, Cheryl Grace Elumbaring-Salazar c, Jason Loke d, Jan Ma e a
Fingerprinting Analytics Ltd., 35, Park Road South, Chester le Street, DH3 3LS, United Kingdom Forensics Ltd., Malvern Hills Science Park, Great Malvern, Worcestershire WR14 3SZ, United Kingdom Nanofrontier Pte Ltd., Research TechnoPlaza, 50 Nanyang Drive, Singapore 637553, Singapore d Forensic Management Branch, CID, Singapore 088762, Singapore e The School of Materials Science and Engineering, 50 Nanyang Avenue, Nanyang Technological University, Singapore 639798, Singapore b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 June 2011 Received in revised form 4 April 2012 Accepted 9 April 2012 Available online 30 April 2012
The ability of two mass spectrometric methods, surface-assisted laser desorption/ionization-time of flight-mass spectrometry (SALDI-TOF-MS) and direct analysis in real time (DART-MS), to detect the presence of seven common explosives (six nitro-organic- and one peroxide-type) in spiked latent fingermarks has been examined. It was found that each explosive could be detected with nanogram sensitivity for marks resulting from direct finger contact with a glass probe by DART-MS or onto stainless steel target plates using SALDI-TOF-MS for marks pre-dusted with one type of commercial black magnetic powder. These explosives also could be detected in latent marks lifted from six common surfaces (paper, plastic bag, metal drinks can, wood laminate, adhesive tape and white ceramic tile) whereas no explosive could be detected in equivalent pre-dusted marks on the surface of a commercial lifting tape by the DART-MS method due to high background interference from the tape material. The presence of TNT and Tetryl could be detected in pre-dusted latent fingermarks on a commercial lifting tape for up to 29 days sealed and stored under ambient conditions. ß 2012 Elsevier Ireland Ltd. All rights reserved.
Keywords: DART-mass spectrometry SALDI-TOF-mass spectrometry Nitro-organic and peroxide explosives Lifted latent fingermarks Dusting powder Contact residues Stability
1. Introduction There are several well established methods for detection of nitro-organic and peroxide explosives in a variety of analytical matrices [1,2]. For security and forensic applications detection of such explosives directly on the surface of fingers or in latent fingermarks deposited on surfaces at crime scenes or at suspicious locations is of obvious interest. There are several reports of the use of mass spectrometric (MS) methods to detect the presence of explosives directly on the surface of fingers resulting from contact deposition following handling of explosives. These include DESIMS (direct electrospray ionization) [3–5] and DART-MS (direct analysis in real time) techniques [6,7]. To date there appear to be no reports of detection of explosives in developed latent fingermarks lifted from surfaces at crime scenes or at suspicious locations. Such development leading to visualization of latent fingermarks at these sites is generally achieved through use of commercial developing agents such as dusting powders. Their location must be the first step in any subsequent analysis of latent marks for the presence of explosives or other contact residues
resulting from handling chemicals. Following visualization the marks are then photographed and generally lifted using adhesivecoated plastic sheets and sealed for later use as evidence. We have described use of novel magnetizable doped silicabased sub-micron particles as dual agents which can be used to locate latent marks on surfaces following conventional dusting using a magnetic wand and which then act as a signal enhancing agent in surface-assisted laser desorption/ionization-time of flight-mass spectrometry (SALDI-TOF-MS) analysis when the developed marks are lifted using conventional lifting tape then subjected to direct MS analysis on the tape’s surface. This approach has been used for detection of drugs and their metabolites in lifted marks following contact or oral use [8] and nicotine in smokers [9,10]. We now describe the use of these particles for detection of six commonly used nitro-organic explosives and one peroxidetype explosive in latent fingermarks and compare the SALDI-TOFMS and DART systems with samples lifted from six common contact surfaces of differing surface characteristics. 2. Methods 2.1. Materials
* Corresponding author. Tel.: +44 191 388 2283; fax: +44 191 515 2698. E-mail address: [email protected] (F. Rowell). 0379-0738/$ – see front matter ß 2012 Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.forsciint.2012.04.007
The explosive standards used were TNT (2,4,6-trinitrotoluene), Tetryl (N-methylN,2,4,6-tetranitroaniline), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane),
F. Rowell et al. / Forensic Science International 221 (2012) 84–91 RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) [from Ultra Scientific Analytical Solutions (North Kingston, Rhode Island, USA)], TNG/Nitroglycerin (propane-1,2,3triyltrinitrate), and PETN/Pentaerythritol tetranitrate (1,3-dinitrato-2,2-bis(nitratomethyl)propane) [from Cerillant Analytical (Round Rock, TX, USA)] and TATP/ triacetone triperoxide (3,3,6,6,9,9-hexamethyl-1,2,4,5,7,8-hexaoxonane) [from AccuStandards Inc. (New Haven, CT, USA)]. They were supplied in acetonitrile as 1 mg/ml solutions except for TATP which was supplied as a 100 mg/ml solution. The calibration standard used for DART-MS was PEG 600 from Jeol Inc. (Jeol ASIA Pte. Ltd., Corporation Road, Singapore) and those for SALDI-TOF-MS including 2,5-dihydroxy benzoic acid (DHB) from Sigma Aldrich (St. Louis, MO, USA) and calibration was performed as earlier [7,8]. ROAR Black magnetic powder [11] and lifting tape were gifts from ROAR Particles plc, Sedgefield, UK (now Air Analytics Ltd., Chester le Street, UK). Magnetic wands used were from CSI [CSI Ltd. (Woburn Sands, UK)] and Sirchie Laboratories [Sirchie Fingerprint Laboratories (Youngsville, NC, USA)]. Stainless steel MALDI target plates (type DE 2115TA and DE1580TA) were from Shimadzu Biotech [Shimadzu (ASIA Pacific) Pte. Ltd., Science Park 1, Singapore] and glass DART dip-it probes were from Jeol/Ion Sense [Jeol Ltd. (Akishima, Japan/Ionsense Inc./Saugus, MA, USA)]. 2.2. Mass spectrometry SALDI-TOF-MS was performed following calibration using an Axima TOF2 mass spectrometer [Shimadzu (ASIA Pacific) Pte. Ltd., Science Park 1, Singapore] with settings in negative mode (nitro-organic explosives) or positive mode (TATP) whilst DART-MS was performed following calibration using an atmospheric pressure Ionization source from IonSense (Ionsense Inc., Saugus, MA, USA) and a JMS T100LP mass spectrometer from Jeol (Jeol Ltd., Akishima, Japan) with settings in negative or positive modes for the explosives as before. DART calibration was carried out using neat polyethylene glycol (PEG 600) on glass rods. In positive mode a series of [M+H]+ and [M+H H2O]+ peaks from m/z 45 to 1000 were used. In the negative mode a series of {M+O2 H] and [(C2H4O)n+O2 H] peaks were used for calibration from m/z 43 to 1000. Instrument settings were a needle voltage of 2000 V, discharge electrode 250 V, grid electrode 150 V, He flow rate 2.5 l/min. SALDI calibration was carried out by adding 1 ml of a DHB solution (10 mg/ml in 20:80 (v/v) acetonitrile; deionized water) to a 1 ml mixture of reserpine (1 mg/ml in acetonitrile), cesium iodide (1 mg/ml in deionized water) and papaverine (1 mg/ml in methanol) to a well on a MALDI target plate. The resulting calibration mixture applied consisted of 25 ml papaverine, 25 ml resperpine and 50 ml cesium iodide. The instrument settings were laser 85–115 (power = 2.83–3.83) with 100 accumulated profiles at 1 laser shot per profile; calibration range in positive mode
85
from m/z 132 to 607 and in negative mode from 153 to 646 for calibrant peaks and those from DHB. 2.3. MS analysis of reference samples Reference spectra were obtained by applying a 1 ml aliquot of standards for the seven explosives at concentrations of 10 mg/mL, 100 mg/mL and 1 mg/mL (except for TATP which was used at 10 mg/mL and 100 mg/mL) onto a dip-it glass rod (DART-MS) or onto metal target plates (SALDI-MS), followed by their direct analysis following solvent evaporation under ambient conditions. 2.4. MS analysis of spiked un-lifted latent fingermarks Fingermarks were obtained from volunteers in Singapore by rubbing three middle fingers of one hand over their brow. To serve as control, a finger was pressed onto a clean portion of the SALDI-MS plate or onto the tip of a clean glass dip-it probe for DART-MS. The other two fingers were applied to dip-it glass probes. Likewise for SALDI-MS, the first finger was pressed onto a clean portion of the SALDI-MS plate or and the other two onto surfaces pre-coated with explosives. For SALDI-MS, aliquots [1 ml of standard (equivalent to 1 mg of each explosive except for TATP where 100 ng was applied)] of the explosive were applied onto two wells adjacent to the control deposit on the SALDI target plate. After drying, the other two fingers were pressed on the plate covering the two wells where the explosives were spotted. For DART-MS, 1 ml aliquots of explosive standards were applied directly onto the surface of a clean dip-it glass probe. After drying a fingermark was deposited onto the probe over the explosives. For both methods each deposited fingermark was then developed using ROAR Black magnetic powder within 1 h followed by immediate MS analysis. 2.5. MS analysis of spiked lifted latent fingermarks Fingermarks were obtained as described above. For both SALDI and DART MS analyses, 5 ml of explosive standard was applied to three discrete areas of approximately 2.5 cm 2.5 cm on the smooth surface of a white ceramic tile. After drying, a new fingertip was pressed onto each of the spiked areas. The contaminated finger was then pressed onto another clean tile. Fingermarks were developed using the ROAR Black magnetic powder and the developed latent fingermarks were then lifted using commercial lifting tape. Analysis by SALDI-MS was made by sticking the lifted latent fingermarks with the adhesive portion facing up, onto the SALDI metal target plate using strips of cellotape. For each lifted mark five different areas were analyzed and the average signal intensities calculated. For DART-MS, the lifted
Table 1 Summary of major peaks observed following MS of standards of explosives on spiked surfaces (metal target plate for SALDI-MS and a glass dip-it probe for DART-MS). Explosive
Identifying fragments at m/z (italics-major peak)
SALDI-MS 100 ng
SALDI-MS 10 ng
DART-MS 100 ng
DART-MS 10 ng
TATP
116 178 172 223a 228 241 207 213 286a 226a 197 181 165 190 46 62 131 147 314a 46 102 129 133 221a 46 102 129 133 295a
Detected Detected Detected nd Detected Detected Detected Detected nd Detected Detected Detected Detected Detected Detected Detected Detectedb Detected nd Detected Detected Detected Detected nd Detected Detected Detected Detectedb nd
Detected Detected Detected nd Detected Detected Detected Detected nd Detected Detected Detected Detected Detected Detected Detected nd Detected nd Detected Detectedb Detected Detected nd Detected Detected Detected Detectedb nd
nd Detected nd Detected Detected Detected Detected Detected nd Detected Detected Detected Detected Detected nd nd Detected Detected Detected nd Detected Detectedb nd Detected nd Detectedb Detectedb nd Detected
nd nd nd nd Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd nd nd Detected Detected nd Detected Detectedb nd Detected nd Detectedb Detectedb nd Detected
Tetryl
TNT
TNG PETN
RDX
HMX
Detected (Signal: noise ratio, S/N > 50), nd (not detected; S/N < 3). Bold numbers: molecular ion (M H) [or (M+H)+ for TATP]. low intensity peaks observed (S/N 4–50).
a
b
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fingermark was manually and directly introduced to the ion source. Signal to noise ratios were calculated from the average intensities of the signals for doped and undoped spectrums at the same m/z values. The above method was repeated for analysis of explosives on spiked fingerprints lifted from six surfaces. These were obtained as described above from a white tile and the resulting contaminated fingers then used to deposit fingermarks onto a second white ceramic tile, the non-sticky surface of a brown adhesive tape, from the upper surface of a laminated wood sample, a metal drinks can, a transparent plastic bag and a sheet of A4 printing paper. The same procedure was followed as described above for analysis of explosives by SALDI-MS and DART-MS in dusted lifted prints taken from these surfaces. 2.6. Stability of explosives in spiked latent fingermarks The procedure for SALDI-MS analysis of spiked un-lifted latent fingermarks was repeated for TNT and tetryl for contaminated marks on metal target plates for spiked un-lifted marks or spiked marks on lifting tape (5 mg and 10 mg of explosive were used for the spiking in both cases). Three sets of spiked fingermarks were used for each the four sets of samples (5 mg and 10 mg, and lifted and un-lifted). Unspiked marks (1 lifted and 1 un-lifted) were also used as controls. The samples were stored together in a sealed container under ambient laboratory conditions for up to 28 days prior to their analysis by SALDI-MS. Five discrete areas on each mark were subjected to SALDI-MS at each time point and the average intensity values used to obtain S/N values as described above.
2.7. Effect of dusting agents on MS sensitivity The effect of a range of 11 commercial dusting agents (from Sirchie Fingerprint Laboratories, CSI and Armor Forensics (now Safariland) used to develop latent fingermarks on surfaces was examined by SALDI-MS analysis as described above for latent fingermarks spiked with 1 mg of TNT or PETN on a metal target plate and for lifted developed spiked fingermarks, on lifting tape. The dusting agents were aluminium (1), magnetic (3), bichromatic (1), white (1), silver (1), black (2), coloured (3) and fluorescent (2) powders (some were combinations of these such as magnetic and fluorescent). The non-magnetic powders were applied using a commercial squirrel hair dusting brush (CSI) and the magnetic ones using a commercial magnetic wand. The intensities of the TNT peak at m/z 226 or the PETN peak at m/z 147 were compared with that obtained using the ROAR Black dusting powder.
3. Results 3.1. MS analysis of reference samples Detection of characteristic peaks was observed for standards of each explosive deposited directly onto clean surfaces by both DART-MS and SALDI-MS as shown in Table 1. Generally both
Fig. 1. DART-MS of explosive-spiked fingermarks for marks deposited directly onto dip-it glass probes pre-spiked with TNT (a) and Tetryl (b). Characteristic peaks were observed for (a) at m/z 226 and 197 and for (b) at 241 and 228.
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Table 2 Major peaks obtained by SALDI-TOF-MS observed in spiked latent fingermarks lifted from ceramic tile, parcel tape and wood laminate surfaces. Explosive
Identifying fragments (m/z)
Direct on metal target plate
Lifted from ceramic tile
Lifted from parcel tape
Lifted from wood laminate
TATP
116 178 172 228 241 207 213 226 197 181 165 190 46 62 131 147 46 102 129 133 46 102 129 133
Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detectedb Detected Detected Detected Detected Detected Detected Detected Detected Detectedb
Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd Detected Detected Detectedb Detected Detected Detected Detected Detected Detectedb
Detected nda nd Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd Detected Detected Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb
Detected nd nd Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected Detected nd Detected Detected Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb Detectedb
Tetryl
TNT
TNG PETN
RDX
HMX
Detected: S/N > 50. a nd – not detected; S/N < 3. b Detected with low intensity; S/N 4–50.
methods are of similar sensitivity being able to detect 10 ng of explosive on the surface except for TATP where only peaks at m/z 178 and 223 peaks were observed at 100 ng for DART-MS whereas peaks at m/z 116, 178 and 172 were seen for the 10 ng sample using SALDI-MS. DART-MS detected nitro-explosives as deprotonated ions in the negative mode and TATP as protonated ion in the positive mode in addition to similar ion fragments to those detected for the corresponding SALDI-MS spectrums. 3.2. MS analysis of spiked un-lifted latent fingermarks The same characteristic peaks were observed for both methods for spiked marks applied directly by contact of contaminated fingers onto a dip-it probe for DART-MS or onto a metal target plate for SALDI-MS. In both cases additional peaks due to endogenous
fingermark constituents were also observed. The DART-MS spectrums of fingermarks spiked with TNT and tetryl are shown in Fig. 1 which show characteristic peaks for the explosives. 3.3. MS analysis of spiked lifted latent fingermarks In lifted developed fingermarks on commercial lifting tape from metal target plates that were subjected to SALDI-MS, characteristic peaks of explosive were seen but with lower intensity than the corresponding spectrums of spiked marks on the metal target plate. A summary of the peaks in dusted marks lifted from 4 surfaces (metal target plate, ceramic tile, parcel tape and wood laminate) for the 7 explosives is shown in Table 2 and the spectrums for tetryl and TNT in lifted developed spiked marks lifted from a ceramic tile is shown in Fig. 2 together with the
Fig. 2. Explosive-doped fingermarks dusted, lifted and analysed via SALDI-MS on lifting tape lifted from a ceramic tile. Characteristic peaks for the respective explosive were observed in each case. Spectrum 1c: Blank undoped fingermark (bottom). Spectrum 2c: Tetryl-doped fingermark (middle) (peaks at m/z 228 and 241). Spectrum 3c: TNTdoped fingermark (top) (peaks at m/z 197 and 226).
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Fig. 3. Detection of TNT on different surfaces from lifted fingermarks by SALDI-MS with observed increasing intensity of TNT fragment m/z 226.1 m/z from spectrum 1 to 6. Spectrum 1c: Plastic bag (bottom). Spectrum 2c: Non-sticky side of parcel tape. Spectrum 3c: Metal drinks can. Spectrum 4c: Tile. Spectrum 5c: Paper. Spectrum 6c: Laminated wood (upper).
spectrum for an un-spiked fingermark. Characteristic peaks for the two explosives are observed. Fig. 3 shows the relative intensities of the m/z 226 peak of TNT in lifted fingermarks from six surfaces (plastic bag, non-sticky side of parcel tape, metal can, ceramic tile,
paper and laminated wood). It was observed that the peak intensity increased progressively from the samples lifted from plastic bag through to the laminated wood and this was generally seen for the other six explosives. The SALDI-MS spectrum for TATP lifted from paper is shown in Fig. 4a and that lifted from metal in Fig. 4b. Both show characteristics peaks which are not present in the corresponding un-spiked fingermark. Fig. 4b also shows the presence of endogenous cholesterol at m/z 369. The characteristic spectrums for explosives in lifted spiked marks on lifting tape were not observed in the DART-MS spectra. Fig. 5a shows the DART-MS spectrum of a dusted lifted mark spiked with tetryl on the lifting tape and Fig 5b shows the corresponding spectrum for a fingermark spiked with TNT. As seen in these spectrums the major peaks for explosives by DART-MS were of very weak intensity or were not detected. In each case the spectrums were nearly identical comprising a complex spectrum possibly originating from the adhesive present on the surface of the lifting tape. Different temperature settings of 120, 200 and 250 8C were also used to test optimum temperature for analysis of marks on lifted prints. The resulting complex spectra (not shown) were similar at all temperatures with background peak intensities increasing with increasing temperatures. It was noted that at 200 8C and above the tape started melting and became distorted. 3.4. Stability of explosives in spiked latent fingermarks
Fig. 4. SALDI-MS of TATP-spiked latent fingermark. (a) lifted from paper and (b) lifted from metal. Characteristic peaks at m/z 116 and 172 seen in (a) (2c, upper spectrum; 1c, un-doped fingermark lower spectrum), and at m/z 178 in (b) with a cholesterol peak at m/z 369.
It was observed that for TNT (peaks at m/z 226, 197 and 181) and tetryl (m/z 241, 228, 213 and 207) characteristic peaks were detected for 28 days and that generally their intensity decreased throughout this period for spiked un-lifted samples for both explosives. For lifted TNT samples a general increase was observed from weeks 0 to 2, followed by a progressive decrease in intensities for the majority of the characteristic peaks. For tetryl a progressive increase in intensity for the major peaks at m/z 241 and 228 and the minor peaks at 213 and 207 was observed for both un-lifted and lifted marks. The intensities for lifted marks were again lower than seen for the corresponding un-lifted marks for both explosives but were detectable throughout the time course. Fig. 6 shows this trend for the m/z peak at 197 for TNT for spiked marks on a metal target plate and for corresponding marks on lifting tape. The lowest signal: noise ratio for any peak was 3:1 seen
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Fig. 5. Explosive-doped fingermarks dusted, lifted and analysed via DART-MS. No characteristic peaks at m/z 228 and 241 (a) and m/z 197 and 226 (b) were observed on the lifted fingermarks. (a) Tetryl-doped fingermark. (b) TNT-doped fingermark.
in the lifted sample for the lowest concentration of TNT at time zero for the m/z 181 peak. This rose to 36 after 1 week. The most intense peaks were seen at m/z 197 for TNT and at 228 for tetryl in all samples and the corresponding S/N values were 9 and 49 (TNT) and 13 and 129 (tetryl).
produced peaks of highest intensity. In contrast, only the ROAR Black powder produced detectable peaks for both explosives with lifted samples on commercial lifting tape. The results for TNT are shown in Fig. 7. It was observed that the presence of ROAR Black dusting agent had no effect on the intensity of peaks in the DARTMS spectra on dip-it probes for standards or spiked fingermarks.
3.5. Effect of dusting agents on MS sensitivity 4. Discussion Eleven commercial dusting agents and the ROAR Black powder were used as dusting agents with latent fingermarks from the same individual for marks deposited on a metal target plate and spiked with 1 mg of TNT or PETN. The intensities of the m/z peaks at 226 or 147 were examined by SALDI-MS. The dusted marks were also lifted and the intensity of the m/z 226 or 147 peaks also determined. It was observed that 3 commercial powders (Lightning Silver Grey, Lightning non-Magnetic Black and CSI Magneta Flake) and ROAR Black produced detectable signals for both explosives in un-lifted samples but that the ROAR Black powder
It was found possible to detect standards for all seven explosives using the DART system when they were deposited from solution onto a glass probe which was then inserted into the beam of ionized helium following established protocols. Likewise it was possible to detect these explosives by SALDI-TOF-MS following their deposition onto metal target plates and treatment with the ROAR Black powder. The only other report of the use of the related technique, Matrix Assisted Laser desorption/ionizationTOF-MS (MALDI-TOF-MS) for detection of explosives used
Fig. 6. Detection of TNT in spiked fingermarks stored under ambient conditions over a 28 day period for the characteristic fragment at m/z 197 for un-lifted and lifted samples (5 ml and 10 ml are equivalent to contact of fingers with 5 mg and 10 mg respectively of TNT prior to deposition and lifting). The points correspond to the mean intensities from 3 fingermarks for each of the four samples with 5 areas analysed per mark.
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Fig. 7. Intensities of SALDI-MS TNT peaks at m/z 226 with ROAR Black and commercial powders. ROAR Black powder (bottom) and three of the eleven commercial powders examined that provided peaks at m/z 226 in un-lifted marks. All spectrums are for lifted fingermarks. The two upper spectrums are from doped marks dusted with commercial powders that gave no TNT peaks in the un-lifted marks.
cyclodextrin as an adsorbing agent to capture the explosives over a 72-h period at 50 8C prior to MS analysis of the resulting explosive/ cyclodextrin complex [12]. In our experiments it is assumed that similar adsorption of the explosives onto the ROAR particles occurred rapidly to produce a complex of sufficient stability to resist evaporation of the explosives under the high vacuum conditions within the mass spectrometer and that under laser irradiation the residues present on the surface of the particles assisted ionization of the adsorbed explosives producing the negative ions detected for the nitro-organic explosives or positive ions for the peroxide explosive TATP. The mechanism for Surface Assisted Laser Desorption/Ionization of polar and non-polar analytes using these particles has recently been described [13]. The SALDI-MS of the aliphatic explosives were characterized by a lack of the de-protonated molecular ions and peaks probably due
loss of nitro groups and the presence of low mass peaks at m/z 46 and/or 62 due to (NO2+) and (NO3+) ions, respectively. In contrast DART-MS generally produced the de-protonated molecular ion plus these peaks reflecting the milder conditions associated with the DART system. The characteristic spectral patterns associated with both DART and SALDI-systems can be used identify these explosives with good sensitivity and use of MS–MS via collision induced dissociation (CID) can add further un-ambiguous evidence for their identity. In contrast screens for explosives in fingermarks using IR/Raman spectroscopy [14,15] require further complex sample processing to obtain such MS identification. An example of the SALDI-MS–MS spectrum for PETN is shown in Fig. 8. For SALDI-MS signal intensity was greatly improved by the presence of ROAR Black powder as 11 other commercial powders
Fig. 8. MS–MS (collision induced dissociation) of the m/z 147 SALDI-TOF-MS peak for PETN showing the characteristic fragmentation pattern for the explosive.
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gave either no (n = 8) or considerably reduced signals (n = 3). The enhanced signal intensity with this powder compared with other commercial dusting agents has also been observed for drugs [8] and nicotine [9,10] in pre-dusted lifted latent fingermarks using SALDI-MS. Its effect on the spectrums of endogenous constituents in latent fingrmarks has also been described [13]. In a second set of experiments transfer via contact was carried out for deposition of explosives onto a metal target plate or a glass probe. Following dusting with the ROAR Black powder DART- or SALDI-MS was performed as for the standards. In both cases the explosives were again detected indicating that the presence of dusting powder or print material did not interfere with the ionization process in the DART-MS system and that the presence of fingerprint material did not adversely affect explosive detection in the SALDI-MS system. For both systems the presence of endogenous constituents such as fatty acids and squalene within the marks was observed. In the third set of experiments, the finger tips made contact with deposited explosive but the finger then made contact with one of six common surfaces. The resulting latent fingermarks were dusted with ROAR Black powder and then lifted from the surface with a commercial lifting tape. The surfaces were chosen to represent non-porous and porous surfaces with flexible and nonflexible, and smooth and non-smooth surfaces. The latent marks on the lifted tape were then placed into the ionized helium stream (DART-MS) so that the stream was reflected from the surface into the entrance to the mass spectrometer, or positioned onto the metal target plate for SALDI-MS. In the case of the DART system no signals due to explosives could be un-ambiguously distinguished from the complex background signal. This background spectrum also masked the peaks due to endogenous compounds within the lifted fingermarks. A range of helium gas temperatures between 120 and 250 8C was used and it was noted that as temperatures increased the intensity of the background peaks also increased and in addition at temperatures above 200 8C, the tape distorted and eventually charred. Repeat of this experiment with a second type of adhesive tape [J-LAr from 3 M (USA)] produced only identical background spectrums to those seen with the CSI lifting tape with the DART-MS system but with characteristic explosive peaks and those from endogenous components observed with the SALDI-MS analysis (data not shown). This indicates that thermal degradation of the surface coating of adhesive is likely to be a problem with lifting tapes that employ commonly used pressure sensitive adhesives which are known to undergo thermal degradation since their pyrolysis coupled to GC-MS has been used for their identification [16]. In contrast to the DART-MS results each explosive was detected by SALDI-MS in the lifted marks from each of the six surfaces together with peaks from endogenous constituents. For these surfaces good signals were obtained for the seven explosives from samples lifted from laminated wood, paper, ceramic tile and metal can with lower signal intensities from the parcel tape and the clear plastic bag. The results may indicate that explosives become adsorbed into plastic surfaces resulting in less effective lifting of the explosive from the surface. Additional experiments with a wider range of plastic materials and surfaces are needed to investigate this possibility. Stability studies with spiked samples demonstrated that TNT and tetryl could be detected by SALDI-MS on un-lifted and lifted latent fingermarks throughout a 28-day period. For both explosives the intensities of the peaks progressively decreased with time for the un-lifted marks with the exception of the tetryl peak at m/z 228which showed an increase from week 1 to 3. For lifted marks both TNT and tetryl exhibited a progressive increase in peak intensities for the non-parent ions on storage and this may result
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from accumulation of the chemically degraded explosives on the surface of the lifting tape. Further studies are needed to identify the origin of this phenomenon. In conclusion it has been shown that the nitro-aromatic, nitroaliphatic and peroxide-type explosives investigated can be detected using DART-MS when they are pre-dispensed directly onto a glass probe used to place the analyte in the helium stream prior to entry into the mass spectrometer, or when contact residues in fingermarks are deposited directly onto glass probes using this technique. These explosives can also be detected in standards applied directly onto metal target plates and in contact residues in fingermarks deposited onto metal target plates when in contact with the ROAR Black magnetic powder and subsequently analysed by SALDI-TOF-MS. When these explosives in fingermarks are deposited onto surfaces (laminated wood, plastic tape, paper, plastic bag, metal can, and ceramic tile) which are dusted with the ROAR Black powder then lifted onto a commercial lifting tape and the lifted marks subjected to SALDI-TOF-MS characteristic signals due to the explosives are observed. In contrast when equivalent lifted fingermarks are analyzed by DART-MS, no, characteristic peaks due to the explosives are seen in the resulting spectra. Acknowledgements This work was partly funded by the Government of Singapore through the Office of the Prime Minister through a TECProject (P00656/1409) and was largely undertaken in laboratories of the Forensic Management Branch of the Criminal Investigation Department of the Singapore Police Force. References [1] D.S. Moore, Recent advances in trace explosion detection instrumentation, Sens. Imaging 8 (2007) 9–38. [2] L. Song, J.E. Bartmess, Liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry: a highly sensitive method for the analysis of organic explosives, Rapid Commun. Mass Spectrom. 23 (2009) 77–84. [3] D.R. Ifa, N.E. Manicke, A.L. Dill, R.G. Cooks, Latent fingerprint chemical imaging by mass spectrometry, Science 321 (2008) 805. [4] D.R. Justes, N. Talaty, I. Cotte-Rodriguez, R.G. Cooks, Detection of explosives on skin using ambient mass spectrometry, Chem. Commun. (2007) 2142–2144. [5] I. Cotte-Rodrigues, H. Chen, R.G. Cooks, Trace detection of triacetone triperoxide (TATP) by complexation reactions during desorption electrospray ionization, Chem. Commun. (2006) 953–955. [6] J. Larame´e, H. Dupont Durst, T.R. Connell, J.M. Nilles, Detection of peroxide and tetrazine explosives on surfaces by Direct Analysis in Real Time mass spectrometry, Am. Lab. 2 (2) (2009) 1–5. [7] Joel on-line information in AccuTOFDART Applications Notebook, 4th Addition, March 2010. www.jeolusa.com. [8] F. Rowell, K. Hudson, J. Seviour, Detection of drugs and their metabolites in dusted latent fingermarks by mass spectrometry, Analyst 134 (2009) 701–707. [9] M. Benton, F. Rowell, L. Sundar, M. Jan, Direct detection of nicotine and cotinine in dusted latent fingermarks of smokers using hydrophobic silica particles and MS, Surf. Interface Anal. 42 (2009) 339–343. [10] M. Benton, M.J. Chua, F. Gu, F. Rowell, J. Ma, Environmental nicotine contamination in latent fingermarks from smoker contacts and passive smoking, Forensic Sci. Int. 200 (2010) 28–34. [11] B. Theaker, K. Hudson, F. Rowell, Doped hydrophobic silica nano- and microparticles as novel agents for developing latent finger marks, Forensic Sci. Int. 174 (2008) 26–34. [12] M. Zhang, Z. Shi, Y. Bai, Y. Gao, R. Hu, F. Zhao, Using molecular recognition of betacyclodextrin to determine low-molecular-weight explosives by MALDI-TOF mass spectrometry, J. Am. Soc. Mass Spectrom. 17 (2006), 198–193. [13] A. Lim, F. Gu, Z. Ma, F. Rowell, Doped amorphous silica nanoparticles as enhancing agents for surface-assisted time-of-flight mass spectrometry, Analyst 136 (2011) 2775–2785. [14] Y. Mou, J. Rabalais, Detection and identification of explosive particles in fingerprints using attenuated total reflection-Fourier transformation infrared spectromicroscopy, J. Forensic Sci. 54 (2009) 846–850. [15] E. Emmons, A. Tripathi, J.S. Christesen, A. Fountain, Raman chemical imaging of explosive-contaminated fingerprints, Appl. Spectrosc. 63 (2009) 1197–2003. [16] S. Nakamura, M. Takino, S. Daishima, Analysis of pressure sensitive adhesives by GC/MS and GC/AED with temperature programmable pyrolyzer, Anal. Sci. 16 (2000) 627–631.