CLPEX Chat Board

I was asked by an examiner to ask this question. If you process something with Ninhydrin and Zinc Chloride can you then reprocess with Ninhydrin?

My understanding is no - ninhydrin reacts with all available amino acids to form Ruhemann's Purple. Zinc chloride then attaches itself to the Ruhemann's Purple compound to provide fluorescence, but does not develop new ridge detail.

Reprocessing with ninhydrin would only develop any new fingerprints placed on the paper after the first treatment. You would have further issues with purple on purple, limiting your contrast.

If you treat with DFO or indanedione first, then ninhydrin, then zinc chloride - that is the correct sequencing. DFO and indanedione only react with some amino acids, leaving the 'left overs' for ninhydrin to react with afterward. Zinc chloride can then be used to enhance weak ninhydrin prints - don't forget to use liquid nitrogen to cool the ninhydrin-zinc chloride prints to optimal fluorescence temperature.

If you are concerned that the first round of ninhydrin may not have developed all of the possible ridge detail, you may want to change your procedures to allow the ninhydrin more time to develop. This would be a better technique than processing with zinc chloride and trying to go backward in the sequence.

You can always photograph anything that develops immediately with nynhydrin, put in in your evidence locker over night, examine it again in the morning, and photograph anything new that has developed. I've seen ninhydrin continue to develop on an item for five days. But that was without any heat/humidity acceleration.