Thursday, February 27, 2014

Environmental stability test show freezing PE bag is bad for cyanoacrylate fingerprints

Things don’t always go the way you expect. I’ve been working on a side by side environmental stability test of fingerprints developed on plastic bags using: AI Red, Carbon Black and 2-Ethylcyanoacrylate (CA). Most of the results are as expected. AI Red has greater environmental stability than the other two systems under test. But I expected this. I have samples left over from the first study Enhancing Contrast of Fingerprints on Plastic Tape (Journal of Forensic Science, November 2003, Vol. 48, No. 6) that are still as clear as the day they were made. AI Red dyes everything but the print. Under good storage it should last as long as the printed patter on the shirt left alone in the back of a closet.

What I didn’t expect is that some of the CA and Carbon Black prints got worse in the freezer. Some of the CA prints fell off the baggies in the freezer and all of the black prints got lighter. I guess, I take for granted that cold is good. In Forced Condensation of Cyanoacrylate with Temperature Control of the Evidence Surface to Modify Polymer Formation and Improve Fingerprint Visualization (Journal of Forensic Identification, July/August 2012, Vol. 62) we showed that chilling the evidence properly improves CA development. And the mechanism does indeed produce more easily visualized prints.

However, what the current study is showing is that if the CA prints are left at extremely cold temperatures the CA cracks and in some cases loses its bond. Now there are a few possibly explanations that are currently being discussed. For one thing the expansion coefficient of the PE bags is 2:1 vs the CA. For another the temperature is substantially lower than most freezers (-19°C). This is below the recommended working temperatures for CA and well below the temperatures used in the IAI paper. But I suppose the take away on CA uses is that while chilling improves CA print development and visualization, freezing in extreme cold will is deleterious.  

Now for the Carbon Black prints, the fact that they lightened in the freezer didn’t actually change the value of any of the prints under test. Either the print lost moisture or the change is lipid viscosity or fluid volume knocked some particles free. But in no case, did the samples lose any detail. Even prints where the naked eye could no longer see the ridges, the camera still captured them just fine.

In the near future I will discuss the incubator findings.




No comments: