Identifying materials with FT-IR

“Please let that be chocolate.” “I hope that’s ketchup.” “Tell me that’s wine.” Whether you’re picking up your kid or doing your spouse’s laundry, we have all uttered one of those phrases while looking at a stain at some point. Few things in the laundry room are more terrifying than an unknown stain. While some people would rather just throw everything in the washing machine with a splash of bleach and hope for the best, more inquisitive minds may want to know beyond a shadow of a doubt what that stain actually is.

For one distressed client of Polymer Solutions, this question was a bit more dire. A few years ago, the materials testing firm was asked to identify a red stain on a shirt that appeared to be lipstick. This was an atypical case for the company, but they took it on as a challenge and an opportunity to experiment with material identification at their independent testing laboratory.

In order to evaluate the stain and determine whether it was in fact lipstick or something more innocuous, the team first had to extract the stain with two solvents. This way, they had a relatively pure sample of the substance, which they could then evaluate using FT-IR.

Reading radiation
Fourier transform infrared spectrometry, or FT-IR is a method of identifying unknown materials. According to Cal Tech, this procedure is done by exposing a sample of material to infrared radiation. As the IR radiation passes through the sample, some of it is absorbed by the material and the rest is transmitted to a detector on the other side. The resultant spectrum represents molecular absorption and transmission, which gives you what amounts to a molecular fingerprint of the sample. And, like a fingerprint, the FT-IR signature is unique for each material since no two different molecular structures produce the same infrared spectrum.

Once the Polymer Solutions scientists acquired the FT-IR signature of the substance, they cross referenced it with their IR library. An IR library is basically a massive archive of hundreds of thousands of FT-IR signatures for different materials. Like a police database full of criminal records, the IR library can be searched to identify a material based on its molecular fingerprint. So what material did the substance match? Unfortunately, the stain was in fact lipstick.

Polymer Solutions does not normally do forensic investigations, but this was too good of an opportunity to show off the effectiveness of FT-IR. And in terms of stains, sometimes it’s best not to know, but for the truly curious, there’s always fourier transform infrared spectrometry.