Reagents for infrared chemical imaging of fingerprints on difficult surfaces
- Publication Type:
- Thesis
- Issue Date:
- 2008
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Fingerprints continue to be an important form of forensic evidence for individual
identification. A number of techniques are currently available for the detection and
enhancement of invisible or latent fingerprints. While these techniques perform well on
many surfaces, there are a number of surfaces that pose problems. On such surfaces, Fourier
transform infrared (FTIR) chemical imaging can provide superior results. FTIR chemical
imaging involves the simultaneous collection of thousands of mid-infrared spectra across a
sample using a focal plane array (FPA) detector. This allows for the collection of chemically
specific spectral data while maintaining spatial information. Images can then be generated
based on spectral / chemical contrast between components within a sample. A key aim of
this project was to further investigate the use of FTIR chemical imaging for the detection and
enhancement of latent (untreated) and developed (treated) fingerprints on a number of
‘difficult’ surfaces.
During the initial development of an infrared chemical imaging technique for fingerprints, an
un-optimised set of image collection parameters was used. Using these settings, the
collection of an entire fingerprint image was time consuming (often several hours or days). A
systematic method for the optimisation of the image collection parameters has been
developed. This method allows the optimisation of parameters such as spectral resolution,
number of co-added scans, spectral range, pixel aggregation and image formation parameters
in order to minimise image collection time and file size while maintaining the quality of the
fingerprint image produced.
A commonly-used fingerprint detection technique for latent fingerprints on non-porous or
semi-porous surfaces involves fuming samples with monomeric ethyl 2-cyanoacrylate
(superglue). This reagent leaves a white residue (polymeric cyanoacrylate) on the ridges of
latent fingerprints, rendering them visible under white light. On some surfaces, such as
polymer banknotes, however, the contrast between cyanoacrylate-developed fingerprints
and the background is poor. FTIR chemical imaging of cyanoacrylate fumed fingerprints on
polymer banknotes and other difficult surfaces has been shown to provide better results than
optical techniques alone. During this project, further investigations into the use of FTIR
chemical imaging for latent fingerprints treated with commercial cyanoacrylate monomer on
a range of difficult surfaces were conducted. While excellent results were obtained on many
samples, the need for novel cyanoacrylates containing infrared absorbance in specific parts of
the spectrum was identified.
A major focus of this project has been the identification, synthesis and characterisation of
modified cyanoacrylates which may be used as reagents for FTIR chemical imaging of
fingerprints. Monomers that contained particular functional groups that show vibrational
modes in the range from 2500 – 1800 cm-1 were sought. This region typically contains very
few vibrational bands and therefore a reagent that could be used to give fingerprints
absorptions in this range is desirable. This would provide the necessary contrast between the
ridge details of the treated fingerprint and the background on which it may be located.
In total four novel cyanoacrylates were prepared. These included 2-cyanoethyl 2-
cyanoacrylate (2-CECA), 1-cyanoethyl 2-cyanoacrylate (1-CECA), trideuteromethyl 2-
cyanoacrylate (MCA-d3) and pentadeuteroethyl 2-cyanoacrylate (ECA-d5).
Each of the four novel monomers was tested a reagent for the detection and enhancement of
latent fingerprints on a number of surfaces via FTIR chemical imaging. The 2-CECA monomer
was found to be less volatile than conventional cyanoacrylate and thermally decomposed at
the temperatures required to vaporise it. Treating latent fingerprints with this monomer at a
reduced pressure yielded better results on reflective surfaces. On less reflective surfaces,
such as polymer banknotes, however, the nitrile band of 2-CECA was almost undetectable
and therefore could not be used for imaging the treated prints.
Fingerprints treated with the deuterated monomers (MCA-d3 and ECA-d5) showed
characteristic bands in the region from 2300 – 1900 cm-1 owing to C–D stretching vibrations.
Once again, however, the relatively low intensity of these bands meant that they were only
detected from prints on reflective surfaces.
The monomer that produced the best results was 1-CECA. Surprisingly the contrast between
the ridge detail and the background, was not generated by the nitrile band at 2250 cm-1 as
anticipated. Instead it appears that the absorption band for the carbonyl group in poly-1-
CECA is sufficiently resolved from any absorption within this region from the background
surface (such as polymer banknotes) to provide good contrast images of the treated
fingerprint. High quality fingerprint images were obtained of prints treated with 1-CECA on
all difficult surfaces tested including white opaque acrylic sheets, fluorescent acrylic sheets
and all areas of polymer banknotes including areas containing raised intaglio printing.
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