While fingerprint detection on non-porous surfaces is fairly simple, fingerprint detection on porous surfaces is more complicated. Cyanoacrylate (CA) is fumed over non-porus surfaces within a basic enclosure at ambient to elevated temperatures. For porous materials, however, high temperatures, and often high humidity, are required for timely development of most of the commonly-used reagents:
Ninhydrin – 80C, 65% RH, 3 minutes duration
DFO – 100C, ambient RH, 20 minutes duration
Nickel Nitrate – 80C, 65% RH, 20 minutes duration
5-MTN – 80C, 65% RH, 3 minutes duration
1,2-Indanedione – 100C, ambient RH, 10 minute duration
Zinc Chloride – 80C, 65% RH, 20-40 minute duration
Most of these reagents are toxic and highly flammable when in their working solution, so great care should be exercised when applying them to samples. Samples should be dried in a fume hood prior to development in the chamber. While it is possible to accelerate sample development using an ordinary steam iron, instead of a controlled environment chamber, this method is inherently variable, with a high potential for human error. In particular, chambers deliver humidity much more uniformly to both sides of the sample.
Chambers capable of producing both high temperature and humidity need to be tightly sealed, with extremely rigid chambers, to resist the internal pressure generated by these conditions, and prevent constant moisture migration into the lab. Temperature uniformity is also key, to prevent condensation that can ruin samples.
It is also important to pay close attention to development cycle timing, to avoid under-exposing samples. A chamber that provides both cycle time and end of cycle notification, as well as offering good sample viewability during the cycle is desirable. Examine prints carefully; depending on results, it may be necessary to re-run the cycle or use more than one reagent to intensify faint prints.