IR Identity

What it determines

Infrared light passing through or reflecting off the sample is absorbed at frequencies matching the vibrational modes of its chemical bonds. The instrument scans the infrared range and records absorption at each frequency, producing a spectrum with bands at positions characteristic of specific functional groups.

For peptides, the amide bands arising from the backbone carbonyl and N-H bonds dominate the spectrum, and the fingerprint region below them carries a pattern specific to the whole molecule. The measured spectrum is overlaid on a reference spectrum or on the pattern expected for the material class, and the correspondence of band positions is assessed.

Practical notes

• Moisture absorbs strongly in the infrared, and damp material carries broad water bands across the region of interest. Dry material gives a cleaner spectrum.
• Salt form is visible. Acetate and trifluoroacetate counter-ions produce their own bands, which is most useful when the expected salt form is stated with the sample.
• The analysis consumes the aliquot taken for it.