Fingerprint Reagents with Dual Action: Color and Fluorescence

We define “dual fingerprint reagents” as chemical formulations that produce with latent fingerprints in one stage impressions that are both colored and fluorescent. Solutions containing ninhydrin and group IIb metal salts appear to be true dual reagents. Application of these formulations to latent fingerprints on paper is as efficient as the two‐step process beginning with ninhydrin and followed by treatment with metal salt. In the color mode, fingerprint detectability with the two ninhydrin–metal salt reagents (one with zinc chloride and the other with cadmium chloride) is comparable with that of ninhydrin itself, in spite of the difference in color. The sensitivity is significantly higher in the fluorescence mode. To view the latent impressions the exhibits are treated with ninhydrin–metal salt reagents and observed under white light illumination and under fluorescence conditions. Cooling to liquid nitrogen temperature enhances the fluorescence considerably. In the shorter wavelength domain, ninhydrin–metal salt reagents exhibit higher sensitivity than the recently reported dual reagent, genipin. The latter is advantageous, however, in the longer wavelength domain, on paper items with strong self‐fluorescence, such as brown wrapping paper or paper printed with fluorescent ink. Upon reduction of the ninhydrin concentration 10‐fold, ninhydrin–metal salt formulations become purely fluorogenic reagents; no color is noticed but the fluorescence is as intense as with concentrated solutions. Working at lower concentrations is an advantage from ecological and economical viewpoints.