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Thermal paper poses a significant challenge to latent print development as it tends to change color when traditional fingerprint development formulations are applied to it. In this study, the optimal components of ninhydrin, 1,2-indanedione, 1,8-diazafluoren-9-one (DFO), and 5-methylthioninhydrin (5-MTN) for yielding clear fingerprints on thermal paper were determined by systematically adjusting the relative amounts of the reagents, polar solvents, and the nonpolar diluent petroleum ether, followed by validation on text-printed thermal paper. Specifically, 3.0% ethyl acetate as the polar solvent in petroleum ether was found to be the optimal combination; the optimal dilution ratios of ninhydrin, DFO, and 5-MTN original solutions with petroleum ether were 1 to 2, 11, and 7, respectively. The optimal concentration of 1,2-indanedione in petroleum ether was 0.125 g/L, with a string of 0.5% ethyl acetate in petroleum ether.

During the past two decades, the great increase of the use of photocopying machines has given the forensic scientists many practical difficulties in identifying the origins of copied documents. Analysis of chemical properties of toners can be helpful for photocopier discrimination. Several techniques including infrared spectroscopy (IR) pyrolysis gas chromatography/mass spectrometry (Py-GC-MS) and scanning electron microscope-energy dispersive X-ray analysis (SEM-EDX) have been extensively used. In this study, a Curie-point pyrolyser, a dual wide-bore capillary column GC equipped with dual flame ionization detectors (FIDs) and a personal computer were connected online. The differentiation of photocopier toners, however, was based both on the comparison of retention indices and on conventional pattern recognition. The proposed method including the library search is quite simple, reliable and low-cost.

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