A Comparative Study of MALDI MSI Versus DESI MSI Applied to Questioned Document Examination

In forensic investigations, questioned document examination (QDE) provides critical information for document authentication and the detection of forgeries. Conventional QDE approaches often require separate analyses of inks and of any potential fingerprints. This study compares two mass spectrometry imaging (MSI) techniques for QDE, enabling the simultaneous analysis of inks and fingerprints deposited onto paper, providing chemical and biometric information from a single sample.

Two mock forgery samples containing two ballpoint pen inks, printed ink and latent fingerprints were each analysed using either desorption electrospray ionisation mass spectrometry imaging (DESI MSI) or matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI MSI) and the results were compared. Analyses were conducted using state-of-the-art multi reflection time-of-flight (MRT) mass spectrometry instrumentation, providing high resolving power and sub-ppm mass accuracy.

Both techniques successfully differentiated between two blue ballpoint pen inks by detecting ink components present in one pen but absent from the other (despite them being optically indistinguishable). Chemical information associated with the deposited fingerprints was also successfully detected and mapped using both DESI MSI and MALDI MSI. However, MALDI MSI outperformed DESI MSI in terms of the clarity of biometric information obtained.

Overall, the strengths and limitations of DESI MSI and MALDI MSI for QDE were evaluated, demonstrating the potential of each technique for QDE. This assessment helps identify the most suitable approach for the analysis of this type of forensic casework by showing the benefits and reliability of each technique.