Fourier transform infrared (FTIR) analysis is a popular analytical tool for material screening. The technique works because each different type of molecular bond in a molecule vibrates differently, and there is often a set of molecular vibrations that involve the entire molecule that form a characteristic “fingerprint.” When measuring a molecular substance, it is possible to identify an organic substance by comparing its FTIR spectrum to a library. In cases where a substance is a mixture of molecular components, FTIR would not be the best option because a separation technique (e.g. chromatography) would need to take place before identification could occur. However, there are many cases for which FTIR analysis is a fast and inexpensive option.
FTIR can be a useful first step as an aid in the identification of an unknown residue. Even if an exact identification is not possible (e.g. in the case of mixed residue), FTIR can often help identify the class of compound which can assist in unknown investigations or process monitoring. For example, if high residual manufacturing material was found on a device, FTIR could be used as an investigational tool to help identify the source. If FTIR was unable to find a conclusive library comparison match, additional testing would need to take place.
FTIR shines when comparing two or more materials in-hand. For example, when particulates are found on a device, it may be necessary to attempt particle identification so that the source can be found and eliminated. Other analytical chemistry techniques would fail at this task, because the amount of particulate material is too small to analyze. FTIR can be conducted through a microscope so that the “fingerprint” of a single particle can be collected. This spectrum can be compared against potential contaminants (e.g. paper, fibers from clothing, or materials unique to the device manufacture), or a library of materials which can greatly narrow the possibilities.
There are also cases where the question being asked is, “does a certain process change my material?” FTIR is a great resource for polymer characterization. Depending on the nature of the material and process, it may be possible to pinpoint what the change is. For example, oxidation of a polymer is clearly indicated by the introduction of carbon-carbon double bonds. If the details of the change are not derivable, it can at least detect if a change has occurred. Perhaps a manufacturer considers changing the radiation dose for their material; they can measure the original device and a device irradiated at the new level comparatively to see if anything is different.
Any FTIR study relevant to medical devices should be carefully thought through and designed with expertise. Simply ordering of a single test without a specific objective in mind will be marginally useful. With smart design – powerful conclusions can be made.