INTRODUCTION
A fingermark possesses unique patterns, acting as an individual’s primary identification source. It is essential forensic evidence due to the reproduction of complex ridge patterns of fingers from the transfer of substances from the skin on surfaces.[ 1 ] Such evidence can be collected at a crime scene or recovered from surfaces to identify suspects, victims, and other individuals in forensic science applications.[ 2 ]
Over the years, various detection techniques for latent fingermarks have been developed and established implementing both physical and chemical means. Various types of fingerprint powders which are currently available, i.e., colored, magnetic, luminescence, and metallic powders have increased the success rate and sensitivity in the detection of latent fingermarks.[ 1 , 2 ] In addition to the conventional fingerprint powdering and chemical development procedure, microscopic instrumentations have been used to detect fingermarks and their residues.[ 3 ] Phase-contrast imaging and environmental scanning electron microscopy coupled with energy dispersive X-ray spectrometry were suggested to be essential tools to determine latent fingermarks.[ 3 ] Scanning electron microscope (SEM) was used to study fingermarks after development procedures, including postcyanoacrylate fuming and physical powdering.[ 4 , 5 ] More recently, SEM together with scanning Kelvin probe allowed the recovery of fingermarks from metallic surfaces.[ 6 ] Scanning electrochemical microscopy (SECM) was also utilized in imaging biological compounds present together with fingermarks such as bloody fingermarks. Diluted bloody fingermarks were successfully captured and found to be superior to the optical microscope.[ 7 ]
Despite the various detection techniques mentioned above, there are limitations, for example, the capability to detect minutiae, age of marks, and identification of foreign residuals that do not originate from the individual leaving the fingermarks. Pollen, environmental dust, soil, and water residues were examples of foreign residuals which commonly reported as “unknown” or “unidentified,” reducing the likelihood of establishing a solid chain of evidence. One of this current study’s aims is to reduce such limitations using Field Emission SEM (FESEM) due to its selection of high magnifications, high resolutions, and depth of focus.
Another interest in this research is the possible recovery of fingermarks containing residual illicit substances from clandestine laboratories and its implication in a police investigation. The manufacturing and production of illicit substances can be located either in highly organized “super” laboratories or smaller scale “Beavis and Butthead” laboratories.[ 8–10 ] Regardless of the type of clandestine laboratories, operators within a structure tend to deposit their fingermarks onto any surfaces, and the successful recovery of fingermarks could aid in linking suspects to a drug-related offense. There are also cases where an individual handled a substance and subsequently deposited contaminated fingermarks on a surface at a crime scene.[ 1 ] In other instances, fingermarks can be firstly deposited on an apparently clean surface which was later contaminated with one or more substances. The ability to differentiate the sequence of fingermark deposition and methamphetamine contamination, especially on nonporous surfaces such as glassware is important to account for a suspect’s statement on his/her involvement in illicit drug handling or manufacturing.
The current study is aimed to reduce the gaps in knowledge and improve forensic detection technology by differentiating two fingermarks bearing methamphetamine present on a surface as to whether (i) the fingermark was deposited on an “already methamphetamine-contaminated” glass surface or (ii) the fingermark was deposited on an apparently clean surface but was subsequently contaminated by methamphetamine. For the sake of clarity, we define the fingermark, in relation to the state of contamination of the surface, in (i) as “prior-deposition contaminated fingermark ” indicating that the fingermark deposited on the surface which has already been contaminated by trace methamphetamine and it was not subjected to further contamination after its deposition. On the contrary, fingermark in (ii) is defined as “postdeposition contaminated fingermark ” indicating that the fingermark was deposited on a clean surface but the surface bearing the fingermark was subjected to subsequent methamphetamine contamination. It is hoped that such differentiation can assist the investigator in predicting the timeline instance when a fingermark was deposited on a surface, and subsequently determining the sequence of an activity such as the handling of surfaces bearing trace methamphetamine.
MATERIALS AND METHODS
Preparation of methamphetamine solution
Methamphetamine hydrochloride (≥99.2% purity) was obtained from the Department of Chemistry, Malaysia. A methamphetamine solution was prepared by dissolving methamphetamine hydrochloride powder in methanol (Merck, Kenilworth, NJ) at 1 mg/mL concentration.
Preparation of fingermark samples
All latent fingermarks, provided by one of the authors, were prepared in three replicates by wiping the thumb across the forehead and nose.[ 11 ] The groomed finger was pressed onto a glass coverslip (Sail, China) at approximately 300 g for 5 s as adapted from the Fieldhouse.[ 12 ] Ethical approval was obtained from the institutional human research ethics committee with approval number USM/JEPeM/18050228 on July 5, 2018.
Samples were prepared according to 10 different scenarios. The control samples were consisted of latent fingermark only and methamphetamine drug only, respectively. For the latter, 1 mL of the methamphetamine solution (1 mg/mL) was sprayed onto the coverslip and allowed to dry before FESEM analysis. Next, latent fingermarks dusted with white and black fingerprint powders were prepared separately. For each, fingermarks deposited on the coverslips were developed using Hi-Fi Volcano Latent Print White Powder (Sirchie, Youngsville, NC, USA) and Hi-Fi Volcano Latent Print Black Powder (Sirchie, Youngsville, NC, USA), respectively.
Two categories of fingermarks were prepared. The prior-deposition contaminated fingermark was prepared by firstly spraying 1 mL of the prepared methamphetamine solution (1 mg/mL) onto the coverslip. Once dried, the groomed fingermark was then deposited onto the same coverslip. On the other hand, the postdeposition contaminated fingermark was prepared by depositing a groomed fingermark followed by gently spraying 1 mL of the methamphetamine solution (1 mg/mL) onto the coverslip bearing fingermark and allowed to dry. Both categories of the prior-deposition contaminated fingermark and the postdeposition contaminated fingermark were then undergone the following procedures, i.e., (i) left untreated (without powdering procedure) and subject to FESEM analysis and (ii) treated with fingerprint powders using fingerprint brush (black and white fingerprint powders, respectively) before FESEM analysis.
Sample analysis
All prepared samples were subjected to FESEM (Quanta FEG 450, FEI, Hillsboro, OR) analysis. All the samples were firstly gold-coated under vacuum at 20 mA for 2 min (Leica EM SCD005 Coating System) to allow conductivity. All samples were then visualized and captured using FESEM operated at a ~ 9–10 mm working distance under 5.00 kV. Four different magnifications, namely, ×50, ×250, ×500, and ×2000 magnifications, were utilized for each sample. Any potential discrimination among samples prepared for the 10 different scenarios was evaluated, considering the overall appearance and observable differences between prior-deposition contaminated fingermarks and postdeposition contaminated fingermarks. At each magnification, the crystal structure of methamphetamine was also visualized.
RESULTS
Both categories of the prior-deposition contaminated fingermarks and postdeposition contaminated fingermarks, with or without fingerprint powder treatment, were observed under FESEM. The utilization of secondary electron imaging allowed the morphological visualization of fingermarks. Secretion originated from the skin of a finger was transferred onto the substrate and presenting the details of the ridges. Figure 1 illustrates fingermarks from the same thumb deposited on the glass and observed with FESEM with four different magnifications, namely, (a) ×50, (b) ×250, (c) ×500, and (d) ×2000. As shown in Figure 1a , a relatively lower magnification was found sufficient to visualize the ridge pattern and differentiating the ridges and nonridges areas of the latent fingermark. In addition, the minutiae (bifurcation) could also be determined from all the fingermark samples as they were contributed by the same individual [red circle; Figure 1a ]. At ×250 and ×500, separations of the ridge and nonridge areas could be clearly observed, where the darker areas were the ridges, and the lights were the nonridge areas. At × 2000, tiny white spots of the glass coverslip surface were evident, while the separation of ridges and nonridges could be further magnified. A comparison of the four observed magnifications used in the study evidenced that lower magnification was more suitable to visualize fingermarks.
Figure 1: FESEM micrographs of a latent fingermark under (a) ×50, (b) ×250, (c) ×500, and (d) 2000 × magnifications. The red circle shows the presence of bifurcation. FESEM: Field Emission Scanning Electron Microscope
Methamphetamine is commonly found in crystal and powder forms. Crystal methamphetamine is known for its crystalline structures appeared similarly to glass fragments or shiny, bluish-white rocks in photonegative and microscope in either square, rectangle, rhombus, or other forms and sizes.[ 13–16 ] It distinctly appears with sharp edges in shapes as shown in Figure 2 . At ×50, they are seen as tiny white clusters [Figure 2a ], where the crystalline structure of methamphetamine was only evident with greater magnifications, of at least 250× [Figure 2b ]. It was also noted that the crystal structure of methamphetamine shall be interpreted with care, where crack cocaine was reported to appear similar to methamphetamine under microscopic examination.[ 1 ]
Figure 2: FESEM micrographs of methamphetamine salt under (a) ×50, (b) ×250, (c) ×500, and (d) ×2000 magnifications. The red circles show the presence of square-like methamphetamine crystals. FESEM: Field Emission Scanning Electron Microscope
Fingermarks varied in their instants of contamination and demonstrated different observations on their respective appearances in the ridge areas, as evident in Figure 3 . Fingermarks present on a surface that were subsequently contaminated by methamphetamine, i.e., the postdeposition contaminated fingermarks, appeared in smudge conditions where the ridge and nonridge areas could not be well-distinguished.
Figure 3: FESEM micrographs of fingermark deposited on contaminated surfaces (a and c), and a fingermark subjected to further contamination by methamphetamine (b and d) under ×50 and ×250 magnifications. The red arrows show the clear separations between the ridge and non-ridge areas. FESEM: Field Emission Scanning Electron Microscope
On the contrary, the prior-deposition contaminated fingermarks demonstrated distinct separations between ridges and nonridges even with the application of fingerprint powders. This is probably due to the force exerted during the deposition of fingermark onto a surface contaminated with drug substance have pushed the methamphetamine crystal toward the outer areas of the fingermark, causing an accumulation of contaminants around the exterior ridge areas. The above observations were evident in all the three replicate samples of each scenario prepared in this study.
Powdering to detect the presence of fingermarks is frequently carried out by forensic investigators at the crime scene or from any forensic materials. On application of fingerprint powders onto the latent fingermarks, the ridge and nonridge areas can be clearly distinguishable, in which the ridge areas were covered with the respective powders while the nonridge areas remained clear [Figure 4a and 4b ]. However, the pore area of these fingermarks could not be determined based on the photomicrograph as opposed to the studies conducted by Zhang and Girault.[ 17 ] Compared to the study involving the examination of inked fingermarks using SECM, the utilization of fingerprint powders in the current study could have covered or concealed the ridges, restricting the determination of pore areas. In general, the dusting procedure did not affect the visualization of fingermarks, suggesting its usefulness toward the development of latent fingermarks.
Figure 4: FESEM micrographs of the effects of fingerprint powdering on (a and b) latent fingermarks, (c and d) fingermarks deposited on contaminated surfaces, and (e and f) fingermarks subjected to further contamination by methamphetamine under ×50 magnification. FESEM: Field Emission Scanning Electron Microscope
Fingerprint powders were also applied to recover the prior-deposition contaminated fingermarks [Figure 4c and d ] and postdeposition contaminated fingermarks [Figure 4e and 4f ]. The ridge details of fingermarks were clearly visualized under the FESEM regardless of the immediacy of their depositions. However, whether a contamination on a surface occurred before or after the deposition of fingermarks could not be confidently determined through microscopic observation. Accumulation of contaminants at the exterior ridge areas as shown in Figure 3b was not seen, probably due to concealment by the fingerprint powders. From the observation, the application of fingerprint powders might cause the latent fingermarks (uncontaminated and contaminated) to be slightly smudged compared to their original state, but such procedure did not affect their visualization with naked eyes (without the microscope).
The presence of methamphetamine crystals on drug-contaminated fingermarks could hardly be determined at the suggested magnification as shown in Figure 3c and d . The fingermark powder had masked the methamphetamine crystals which imposed some difficulties in observing the drugs at lower magnifications. Increasing magnification to 2000× sufficiently enabled the visualization of the clear to the white square-like appearance of methamphetamine crystals at an average size of around 1.3 μm [Figure 5 ] from the fingerprint powders which appeared in clumps.
Figure 5: FESEM micrographs of methamphetamine crystals on (a and b) fingermarks deposited on contaminated surfaces; and (c and d) fingermarks subjected to further contamination by methamphetamine, on powdering procedure under ×2000. The red circles show the presence of methamphetamine crystals. FESEM: Field Emission Scanning Electron Microscope
DISCUSSION
Exogenous substances, such as methamphetamine could present in latent fingermarks. In such cases, forensic investigators shall decide the recovery and preservation strategies to maximize the value of such forensic evidence. This study found that both the fingermarks and methamphetamine can be visualized under the FESEM. It was worth noting that samples were prone to be burnt by electron at high magnification if the electron was focused on a specific area for too long. Hence, it was suggested to search for the crystal at a lower magnification for example at 250× (as of this study) and increase the magnification whenever required.
Lower magnification could be applied to visualize the ridge details of fingermarks while increasing the magnification could help determine the presence of methamphetamine crystals. As samples analyzed using FESEM should be in conductive state, coating procedure is necessary depending on the types of the samples. In this study, all samples were coated with gold, and the procedure did not disrupt the fingermark structures. The vacuum state of FESEM could also be another potential factor that might disrupt the molecules of fingermarks. However, the fingermarks prepared under our experimental conditions remained intact for observation and such factors shall be taken into account in future studies. Past studies have shown that the detection of drugs with fingermarks could be conducted using various techniques despite the application of fingerprint powder.[ 18 , 19 ] The presence of drugs are also persistent in fingermarks for hours,[ 19 ] making the use of FESEM appropriate.
Visualization of fingermarks which were contaminated by methamphetamine under FESEM could serve as an alternative technique for determining the presence of contaminant. This is especially useful to determine the deposition states of the marks as to whether the fingermarks were deposited on a priorly methamphetamine-contaminated surface or on an essentially clean surface but subsequently contaminated by the foreign substances. This technique did not require any complex preparations before the visualization; hence such technique could be conducted in a quick manner and after basic training. As the visualization of samples depends on the conductivity of the surfaces, one of the study’s limitations was the type of surface materials where nonconductive surfaces were required to be coated with conductive materials such as gold or silver. This study also emphasized the application of conventional fingermark powders and further studies using magnetic powder were recommended to determine the conductivity and visualization of fingermarks under FESEM. Successfully visualization of latent fingermarks contaminated by methamphetamine under FESEM could aid in determining the presence of methamphetamine residue and the immediacy of fingermark deposition during forensic investigations.
CONCLUSION
Methamphetamine-contaminated fingermarks can be visualized and differentiated using microscopic study. Crystalline methamphetamine was clearly observed from ×250 magnification. Fingermarks present on a surface which was subsequently contaminated by methamphetamine appeared in smudge conditions with not well-distinguished ridge and nonridge areas, whereas fingermarks were deposited on a drug-contaminated surface but without any subsequent contamination demonstrated distinct separations between these areas. The ability to differentiate between the two types of fingermarks shown in this study is important, especially to answer whether the person has contact with the surface before or after methamphetamine surface contamination occurs.
Financial support and sponsorship
Universiti Sains Malaysia RUI grant (1001/PPSK/8012236).
Conflicts of interest
There are no conflicts of interest.
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