The images and MS data presented here were provided by Dr. Ron Heeren , Florian Barré, Silvia Mas, and Anton Skriba from the M4i Institute at Maastricht University.
Application & Background
Matrix assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) allows for the generation of molecular maps from a variety of materials. While MALDI MSI has thus far been most heavily utilized in the fields of pharmacology, analytical chemistry, and "omics" level studies, this technology offers tremendous opportunities in the field of forensic science as well. MALDI MSI can be used to examine a variety of types of evidence, such as fingerprints, hair, and fibers, from crime scenes at a deep biomolecular level while preserving important patterns in the analytical data that may be important factors in the events leading up to a crime. Here, we aimed to demonstrate the depth of biomolecular information that can be gained from single fingerprints of five different individuals using a robust and fast MALDI MSI protocol that could be easily adopted in forensic laboratories.
Experimental
Sample Preparation
A single fingerprint was provided from five test subjects at the M4i Institute at Maastricht University, The Netherlands. All research participants provided informed consent prior to the start of the study. The fingerprints were collected on 2 non-coated glass slides, with three fingerprints on one slide and two on another.
Matrix Application: HTX Sublimator
To apply matrix to the sample slide with two fingerprints prior to MALDI MSI, 50 mg of DHB matrix was dissolved in 2 mL MeOH. The matrix mixture was pipetted onto the trough in the HTX Sublimator vacuum chamber. The trough was pre-heated to 65ºC to evaporate all of the MeOH solvent. A vacuum chamber was then drawn, and sublimation occurred at 160ºC over 300 seconds in the HTX Sublimator.
Matrix Application: HTX TM-Sprayer
CHCA was applied to the slide at a concentration (C) of 5 mg/mL in 70% acetonitrile using the HTX TM-Sprayer. The slide was coated using the following parameters:
Table 1. Spray parameters.
MALDI Mass spectrometry imaging
Imaging of the slide with two fingerprints and sublimated DHB matrix was performed on a Bruker rapifleX MALDI Tissuetyper operated in positive ion mode with a 70 μm pixel spot size. All ion images were generated using flexImaging v4.0 (Bruker Dalontik GmbH). Imaging of the slide with three fingerprints and sprayed CHCA matrix was performed on a Waters SYNAPT HDMS G2-Si mass spectrometer fitted with a prototype uMALDI source operated in positive ion mode with a 80 μm pixel spot size. All ion images were generated using Waters® High Definition Imaging Software (HDI®).
Figure 1. Two workflows for fingerprint imaging.
Results
Using MALDI MSI, we were able to detect many distinct compounds present in all five subjects' fingerprints. Furthermore, we able to identify several peaks as materials the subjects confirmed to have used throughout the day (i.e. pen ink, hand lotion, and shampoo). Applying this technique in forensic science could greatly expand the information provided from a fingerprint from a simple confirmation of a subject's presence to an extensive biomolecular profile.
Figure 2. MALDI MSI analysis of fingerprints was able to distinguish between Subject 1 who used both a blue ink pen (m/z 478.3) and hand lotion (m/z 537.4) during the day versus Subject 2 who only used hand lotion (m/z 537.4).
Figure 3. Three unique compounds were identified in the fingerprints of the three different subjects whose fingerprints were collected on a single glass slide: m/z 324.2 was identified as the green ink preferred by Subject 4; m /z 439.3 was identified as the hand cream favored by Subject 5; m/z 365.1 was identified as the shampoo used by Subject 6. Subject 5 was found to prefer a different brand of hand lotion than that used by Subjects 1 and 2.
Conclusions
MALDI MSI is an analytical technique that presents new opportunities in the field of forensic science. In one simple experiment, hundreds of compounds were able to be associated with five different fingerprints. This technology allows for forensic scientists to utilize a non-targeted approach to investigate trace evidence from a single fingerprint found at a crime scene while preserving the fingerprint pattern for suspect identification. In addition, MALDI MSI is not limited to fingerprint analysis, but can be used on hair, fiber, and many other evidence samples. As the technology expands into the field of forensic science, reproducibility and standardized sample preparation protocols will be of utmost importance in order to guarantee that the evidence is admissible in a court of law.
Figure 4. (A) m/z 691 was identified only in the fingerprints of subjects 4 and 6. (B) m/z 298.3 was also identified only in the fingerprints of subjects 4 and 6. (C) m/z 1142 was identified in all three subjects' fingerprints. It is possibly an exogenous lipid compound from the soap at M4i.
Figure 5. Average spectrum of 200 spectra extracted from the general chromatogram.