Chemical Imaging of Latent Fingerprints by Mass Spectrometry
Based on Laser Activated Electron Tunneling
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Abstract
Identification of endogenous and
exogenous chemicals
contained in latent fingerprints is important for forensic science
in order to acquire evidence of criminal identities and contacts with
specific chemicals. Mass spectrometry has emerged as a powerful technique
for such applications without any derivatization or fluorescent tags.
Among these techniques, MALDI (Matrix Assisted Laser Desorption Ionization)
provides small beam size but has interferences with MALDI matrix materials,
which cause ion suppressions as well as limited spatial resolution
resulting from uneven distribution of MALDI matrix crystals with different
sizes. LAET (Laser Activated Electron Tunneling) described in this
work offers capabilities for chemical imaging through electron-directed
soft ionization. A special film of semiconductors has been designed
for collection of fingerprints. Nanoparticles of bismuth cobalt zinc
oxide were compressed on a conductive metal substrate (Al or Cu sticky
tape) under 10 MPa pressure. Resultant uniform thin films provide
tight and shining surfaces on which fingers are impressed. Irradiation
of ultraviolet laser pulses (355 nm) on the thin film instantly generates
photoelectrons that can be captured by adsorbed organic molecules
and subsequently cause electron-directed ionization and fragmentation.
Imaging of latent fingerprints is achieved by visualization of the
spatial distribution of these molecular ions and structural information-rich
fragment ions. Atomic electron emission together with finely tuned
laser beam size improve spatial resolution. With the LAET technique,
imaging analysis not only can identify physical shapes but also reveal
endogenous metabolites present in females and males, detect contacts
with prohibited substances, and resolve overlapped latent fingerprints