Solvent-free method for developing latent fingerprints by applying superfine ninhydrin powder

Ninhydrin is one of the most commonly used reagents for developing latent fingerprints on porous surfaces. Although its working solution is typically pre-prepared, it has several drawbacks: it can dissolve printing ink and handwritten text, and it often causes thermal paper to darken. In light of these drawbacks, this study introduces and evaluates a solvent-free fingerprint development method by applying superfine ninhydrin powder (ASNP). In this approach, finely ground ninhydrin powder is directly applied to paper surfaces, followed by controlled heating and humidification to facilitate the diffusion of ninhydrin molecules into the paper fibers for fingerprint development. Seven types of paper were evaluated, including kraft paper, newspaper, carbonless copy paper, magazine, copying paper, thermal paper, and coated art paper. The ASNP method was compared with the traditional ninhydrin solution under two conditions: 60°C at 40% relative humidity (RH) and 50°C at 65% RH. The condition of 50°C and 65% RH yielded improved fingerprint development, with most ASNP prints becoming visible within 6 h. After 24 h, the performance of the ASNP method was comparable to that of the traditional solution. This ASNP technique offers several advantages: it eliminates the need for a pre-mixed working solution, reduces solvent costs, avoids solvent-induced damage and environmental hazards, and results in only minimal background staining. Moreover, it can be effectively applied to thermal and carbonless copy papers.