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.

 

Peace   

Charles

Me, William and A Gel Lifter Part 2,

By way of reminder, back in August my eldest nephew William and I were playing with a way of increasing resolution of gel lifters with the Axis Inversion Dyes.

Well, the first few trials were miserable failures. The heat needed to activate the dyes distorted or outright melted the gel. However, as the wizard said said, 99% perspiration etc… So, the work continued to the point today whereby incorporating the dye into a fuming cartridge the heat needed for sublimation does not impact the lifter.

Essentially what we have come up with is this: the dye colors the gel, but where the gel has picked up residue, the dye is blocked from entry. The result is a yellowing of the gel -- up to a few shade units -- without coloring the impression. (See the top picture.)

So who cares you may ask? Well I do, (and possibly others … possibly people who use gel lifters) because, the result is a net bluing of the impression making it easier to view. (Blue and yellow are diametrically opposed on the visual axis. So if the background is yellower the foreground is defacto bluer.)

To be honest, in most cases while we are able to increase the contrast of the image, we weren’t able to achieve an image that couldn’t also be obtained with oblique lighting and optical filters. But, there were cases where the increase in contrast pushed the image just enough to resolve detail that weren’t achievable by conventional means. (See the bottom Picture)

Obviously, this is very preliminary. I may work it into a study if there is time in 2012, who knows, maybe I can get my nephew to work with me on a validation project.

But for now, I am simply content to have figured finished the proof of concept steps begun with William.

But it is Sunday, and I think I have put enough time in the lab tonight so with that …

Until Later

Charles

Me, William and A Gel Lifter

When I was a kid, my father spent a lot of time with me taking apart and reassembling equipment. I think in many ways it’s why I do what I do. And now that I’m the adult, I get to experience this from the other side, working with my nephews. This weekend Jon and I played with the Green Screen and a stuffed moose [poor poor moose…). And William and I worked on bad weather driving and designing a new forensic product.

In this case, we were working with gel lifters. For those of you who haven’t used them, gel lifters are used to pull up pattern impressions left on some surface. They work well and the ones we were using have such a low level of tack that they pull of only part of the impression. As a result they can separate one layer from another. But the draw-back is the impression is feint, so William and I were working on a contrasting aid that could be added to the lifter to increase resolution.

William told me he prefers proof of concept testing. He was speaking in comparison to the “work” of developing a finished product. And the truth is I couldn’t agree more. As I say often, it’s like day-camp without the adult supervision.

Our first attempt actually melted the lifter, the second washed away the print, but the third shows promise. I won’t say too much about the technology since, we’re just starting with it. And anyway my point today is how cool it was to spend some small time at the bench with my nephew. If you get the chance today, show someone younger the cool parts of what you do for a living. Trust me, their enjoyment is infectious.

But the hubbub of the trade show is behind me and I’m back to business as usual.

Until Later
- Charles

Cross-Pollinating

So, I am working on a paper I intend to submit to the society of plastics engineers on fingerprinting. Ok when I say it like that it seems like I should be rethinking my audience. But, in addition, to other professional associations, I am a member of the Color and Appearance Division of the Society of Plastics Engineers (SPE). And there is more cross-over that you would think.

Ok, in general, SPE members are concerned with consumer products where color is an aesthetic property. In forensics, color is a functional property that aids in visualization. But, the technology for the actual coloration crosses over from one application to the other. Sometimes the failure in one arena is a success in the other. So, it behooves me to follow other industries.

For example, the first official use of fingerprinting in western culture that I am aware of is a British Officer who was trying to verify payment receipt. And I believe he got the idea from a Japanese pottery journal.

For myself, the axis-inversion colors, were born out of a sublimation printing product and work on the old principals of printers resists. They sublime into the substrate to form a colored in the matrix like textile printing. But, they do not penetrate where the fingerprint blocks migration.

But, that’s the point, there are a lot of innovations to be found in other industries and arts.

Well, that’s all for today.

Introduction: How do I do that?

I often wonder how many criminalists actually watch cop shows like CSI or NCIS. People often tend to scoff at fiction that pretends to emulate what they know. It’s a question of where the suspension of disbelief kicks in. We look at what the characters are doing and go – “yea right!”

But, liberties aside, I enjoy detective and crime scene fiction, even when the science goes astray. Does the fact that warp drive doesn’t exist make Star Trek less enjoyable? No – and lets be honest, aside from the lighting, is there anyone out there who wouldn’t want Grissom’s lab?

As a product developer, I also have to admit I find a lot of fodder in fiction. While it’s true that the people who produce television forensic television shows or books based on criminalist characters try to add as much authentic science as then can, the pacing of the stories requires some liberties. Sometimes, I see that prop guys have devised easier delivery systems than the original manufacture had. Sometimes the writers have presented “real life” problems for which there is no off the shelf solution. And sometimes, you see a piece of evidence that can only be read by a really cool piece of equipment that most labs couldn’t hope to afford. And I wonder what I would do if confronted with the same problem.

And that I guess is the essence of research science. So, in this blob space, I plan to look at the developing and using of the tools that help us answer the question “how do I do that?”

Original Post June 2009