Detection and mapping of illicit drugs and their metabolites in fingermarks by MALDI MS and compatibility with forensic techniques

Abstract

Despite the proven capabilities of Matrix Assisted Laser Desorption Ionisation Mass Spectrometry (MALDI MS) in laboratory settings, research is still needed to integrate this technique into current forensic fingerprinting practice. Optimised protocols enabling the compatible application of MALDI to developed fingermarks will allow additional intelligence to be gathered around a suspect's lifestyle and activities prior to the deposition of their fingermarks while committing a crime. The detection and mapping of illicit drugs and metabolites in latent fingermarks would provide intelligence that is beneficial for both police investigations and court cases. This study investigated MALDI MS detection and mapping capabilities for a large range of drugs of abuse and their metabolites in fingermarks; the detection and mapping of a mixture of these drugs in marks, with and without prior development with cyanoacrylate fuming or Vacuum Metal Deposition, was also examined. Our findings indicate the versatility of MALDI technology and its ability to retrieve chemical intelligence either by detecting the compounds investigated or by using their ion signals to reconstruct 2D maps of fingermark ridge details.

Figure 1. MALDI-QTOF-MS spectrum of a solution containing 10 illicit drugs at a concentration of 10 μg/mL mixed with an equal volume of matrix solution.

Using the predominant [M+H]+ parent ions formed, the compounds can be identified based on MS and MS/MS experiments. Methamphetamine and 4-methylamphetamine were not differentiated in MS mode owing to mass-to-charge overlap (m/z 150.1).

Figure 2

MALDI MS images of fingermarks simultaneously spiked with drugs and their metabolites at defined ratios to show the distribution of samples containing (A) cocaine, BZE and EME in a 5:1:1 ratio; (B) heroin, 6-MAM, morphine in a 5:1:1 ratio; (C) amphetamine and MDA in a 1:1 ratio; and (D) THC and THCA in a 1:1 ratio.

Figure 3. MALDI MSI of serial dilutions ranging from 10 μg/mL to 1 ng/mL of the different drug classes (drug/metabolites) spotted on top of a fingermark and subsequently spray-coated.

LODs were determined from (A) the amphetamine mixture, (B) the cocaine mixture, (C) the heroin mixture and (D) the THC mixture. The ion signals of the compounds are shown as an overlay (green) on top of the ion signal originating from an endogenous species (red) depicting the ridge pattern.

Figure 4. MALDI MS images of ‘high’ (“handling” scenario) and ‘low’ (“abuse” scenario) spiked fingermarks contaminated with amphetamine, cocaine, BZE, EME, heroin, 6-MAM, morphine, MDA, THC and THCA. All images were normalised against the matrix peak at m/z 190.

Figure 5. MALDI MS images of split fingermarks spiked with a mixture of 4 drugs and analysed with and without prior FET development. All images were normalised to total ion current, but each entire fingermark image was adjusted to optimal brightness and contrast.

A: MALDI MS images acquired after CAF, CAF + acetone vapour exposure and CAF + BY40 staining. The last row of the panel shows the brightness increased to reveal enhanced 2D maps of cocaine, heroin and cholesterol. B: MALDI MS images acquired after VMD, VMD + CAF and CAF + BY40 staining + VMD. For both panels A and B, the last row of the panels shows brightness increased to reveal enhanced 2D maps of cocaine, heroin and cholesterol.