For which type of material is Ninhydrin primarily used?

Ninhydrin is primarily used for developing latent fingerprints on porous materials. This is because ninhydrin reacts chemically with amino acids present in the secretions of the skin that are deposited when a person leaves a fingerprint on a surface. When ninhydrin is applied to porous materials, it forms a colored compound known as Ruhemann's purple, which allows the fingerprint to become visible.

Porous materials, such as paper, cardboard, and untreated wood, can absorb the sweat and oils from fingerprints, making them particularly suitable for ninhydrin treatment. The reaction can take time and may require heat to expedite the development process, but the ninhydrin method is a standard practice in forensic fingerprint analysis specifically because of its effectiveness on these types of surfaces.