Beware of Spectral Misidentification

Library searching is a powerful tool for identification of material compounds using infrared spectroscopy (IR). However, there are a number of factors that can result in misidentification or incomplete identification. Most importantly, if the spectrum of the material of interest is not in the library, the unknown won’t be identified conclusively. Commercial libraries are typically purchased with IR instruments at the time of instrument purchase. Thus, compounds created more recently than the library creation date, as well as proprietary compounds, will not have published IR spectra. The searching software will therefore produce a list of the closest candidates among the spectra in the libraries. The analyst should review the spectra to look for similar bands that would indicate that similar functional groups are present. This may allow classification, if not conclusive identification, of an unknown. Critically, it is important to review the entire search result list, as often the ”best fit” is not the top material based on the “hit index.” Software results should always be reviewed by an experienced analyst prior to reporting results.

Compounds that contain similar functional groups will have some peaks in common, and have similar spectra. In these instances, minor peaks are used to conclusively characterize and identify a sample. For example, spectra of polyethylene and EPDM (ethylene:propylene:diene terpolymer) are dominated by peaks due to methylene groups, but EPDM also has additional peaks due to the methyl groups that are absent in polyethylene. These spectra are compared in Figure 1. The EPDM spectrum has a peak near 1377 cm-1 (marked with red arrow) due to the methyl groups. There are also some subtle differences between these spectra; the shoulder on the peak near 2924 cm-1 is more prominent in EPDM (due to the methyl groups) and the methylene rocking vibration peak near 722 cm-1 is less intense relative to the other peaks in the spectrum in the EPDM spectrum (see blue arrows in Figure 1).

The composition of the unknown material itself can complicate the identification process. If the unknown is a mixture, the search results may not be conclusive. When identifying an unknown, all of the peaks in the reference must be present in the sample spectrum. If there are additional peaks present in the sample spectrum, this may indicate that the sample is a mixture. It could also indicate that the unknown contains a component that is similar to the library spectrum, but is more complex. The spectra of polystyrene and poly(styrene:butadiene) are compared in Figure 2. These spectra are very similar to each other, but the poly(styrene:butadiene) spectrum contains an additional peak near 966 cm-1 which is due to the butadiene component.

Compounds with inorganic fillers (such as paint and coatings) or plastics and elastomers will yield peaks due to both the organic matrix and the inorganics. Calcium carbonate is a commonly used additive in many materials and it has a fairly complex infrared spectrum. The presence of these peaks can mask regions of the spectrum that would be needed for identification. Carbon black colorant can cause scattering of the infrared radiation and result in sloping or irregular baselines.

McCrone Associates‘ success at particulate identification is predicated by its project leaders practicing this analytical approach.

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