FTIR analysis works by passing a broad spectrum of infrared light through a sample (or, in some cases, by reflecting the light off of the sample), then measuring the intensity of the transmitted (or reflected) light to determine how much light the sample absorbed at each frequency. The absorption energies relate directly to the various molecular bonding energies that can be thought of as holding a given substance together. By viewing a complete spectrum of energy absorption across a wide band of frequencies, it is possible to create a relatively unique fingerprint of a given material.
Though this fingerprint contains a wealth of data about the composition of a material, an analyst must still do some further experimentation before successfully identifying the contaminant. Just like the actors on dramatic crime science shows must reference fingerprints or DNA against a database, a failure analyst must compare the FTIR spectrum of a given sample against a library of known materials (unlike the TV shows, this rarely involves giant touchscreen monitors or an intense soundtrack). By comparing the shape, intensity, and location of the peaks and valleys in a sample FTIR spectrum against the spectra in the library, it is finally possible to identify the material. Most FTIR library searches are mostly automated – however, an experienced analyst is still necessary to compare the sample spectrum against its closest matches to determine which of the results is most likely.
FTIR analysis is an exceptionally precise tool – with a comprehensive library, some materials (like plastics) can often be characterized so thoroughly that it is even possible to determine their manufacturer. This degree of precision makes FTIR an incredibly powerful tool for failure analysis since the proper identification of a contaminant will often provide clues as to how it was introduced into the system in the first place – allowing the analyst to readily identify the root cause of failure.
Derek Snider is a failure analyst at Insight Analytical Labs, where he has worked since 2004. He is currently an undergraduate student at the University of Colorado, Colorado Springs, where he is pursuing a Bachelor of Science degree in Electrical Engineering.