A New Tool for Forensic Analysis: How Can DESI/PI Mass Spectrometry Imaging Help Resolve Challenges in These Three Types of Cases?

‌Foreword

This instrument is equipped with a desorption electrospray ionisation (DESI)/ photochemical ionisation (PI) integrated ionisation source for ambient pressure mass spectrometry imaging. It offers the advantages of in situ, minimally invasive and visual molecular imaging. Without the need for complex pre-treatment, it can directly characterise the spatial distribution of chemical components on the surfaces of physical evidence such as paper and fingerprints. It covers core forensic scenarios such as the examination of documentary and trace evidence, and is suited to the requirements of court evidence and standardised laboratory testing.

(1) Chronological imaging analysis of red and black ink (authentication of documentary evidence)

Application requirements: One of the key challenges in document forgery cases is distinguishing, in the overlapping areas between handwriting and seals, whether the writing was made before or after the seal was applied; traditional optical and spectroscopic methods are prone to disputes regarding the determination.

Test Results: Mass spectrometry imaging can visually demonstrate differences in molecular signals at the intersection of red ink and black ink, yielding uncontested conclusions.

Stamp applied before writing: The ink penetrates the seal paste layer, resulting in a distinct break in the molecular signals of the ink at the intersection; Writing first, then stamping: the ink penetrates the layer of seal paste, resulting in a marked discontinuity in the molecular signals of the ink at the intersection;

The imaging of multiple characteristic ions is clearly stratified, enabling the production of visual spectra that serve as valid evidence in court, thereby resolving the issue of insufficient evidential weight associated with traditional identification methods.

(2) Traceability and Identification of Ballpoint Pen Ink (Forensic Examination of Tampered Documents)

Application Requirements: To rapidly distinguish between different brands of gel-ink pens and identify added or altered handwriting on forged documents such as contracts, promissory notes and bills of exchange.

Test Results: Successfully distinguished ink marks from mainstream gel-ink pen brands such as Baixue, Deli A12 and Deli 009A;

Characteristic ions (m/z 150, m/z 349, m/z 701, etc.) in the ink of different ballpoint pen brands are visualised in separate layers; the overlaid image allows for an intuitive distinction between writing areas produced by different pens.

(3) Molecular Fingerprint Imaging (Extraction of Biochemical Characteristics from Trace Evidence)

Application requirements: Conventional optical fingerprinting only captures the morphological features of the ridges; it cannot extract human biochemical information, making it difficult to link the fingerprint to a suspect’s individual characteristics.

Test results: In DESI/PI mode, complete visualisation of fingerprint ridges and qualitative analysis of sebum characteristic substances can be achieved. High-definition reconstruction of complete fingerprint ridges and bifurcation details compensates for defects in faint fingerprints that are unrecognisable to the naked eye. Qualitative and quantitative detection of characteristic markers: m/z 369 cholesterol, m/z 411 squalene, and m/z 523, m/z 537, m/z 549 triglyceride fragments.

Multi-ion-channel layered imaging simultaneously captures dual forensic evidence—fingerprint morphology and human metabolic molecules—expanding the dimensions of fingerprint traceability.

Summary

The DESI/PI mass spectrometry imaging source is suitable for the authentication of documents, the tracing of ink origins, and the collection of evidence for forensic analysis of fingerprints and biochemical traces. It addresses the shortcomings of traditional optical and physico-chemical examination methods—which are often limited in scope and highly contentious—and provides objective, visual scientific evidence for criminal cases involving contract forgery, the alteration of financial instruments, and personal trace evidence. It is widely applicable in police forensic laboratories and third-party forensic centres.