Which technique is least effective for enhancing latent fingerprints?

Rubbing alcohol is the least effective technique for enhancing latent fingerprints because it is not specifically designed to react with the components of fingerprint residue. While it can serve to clean surfaces or remove oils, it does not produce a chemical reaction that creates a visible contrast with the fingerprint residue.

In contrast, techniques such as super glue fuming, ninhydrin, and fuming iodine are chemically reactive processes specifically tailored to develop and visualize latent fingerprints. Super glue fuming works by depicting prints through a vapor that adheres to the moisture and oils left by the fingerprint. Ninhydrin reacts with amino acids in the sweat left on surfaces to produce a purple-blue color, enhancing visibility. Fuming iodine additionally reacts with the lipids in the fingerprint and produces a brown color, making prints detectable on non-porous surfaces. The focused chemical interactions of these methods enable greater effectiveness in enhancing latent fingerprints compared to rubbing alcohol, which lacks such properties.