FTIR Identification on a Peptide COA

Many peptide certificates list “FTIR identification” next to an HPLC purity result. This guide explains what the technique measures, what a match actually tells you, and where it stops being informative. It is general analytical background for laboratory researchers.

What FTIR measures

Fourier-transform infrared (FTIR) spectroscopy passes infrared light through or onto a sample and records which wavelengths the sample absorbs. Chemical bonds absorb at characteristic frequencies, so the result is a spectrum, with absorbance on one axis and wavenumber (in cm⁻¹) on the other. The spectrum acts as a fingerprint of the material as a whole.

For freeze-dried powders the common setup is attenuated total reflectance (ATR). The powder is pressed against a crystal and measured as it is, with no dissolving step.

What a peptide spectrum looks like

Peptides share a recognisable pattern because they share a backbone:

• Amide I, roughly 1600 to 1700 cm⁻¹. Mostly the carbonyl stretch of the peptide bonds. Its shape shifts with secondary structure.
• Amide II, roughly 1480 to 1575 cm⁻¹. Mostly N–H bending combined with C–N stretching.
• A broad band near 3300 cm⁻¹ from N–H and O–H stretching, which is also affected by water in the sample.

Other components in the vial add their own bands. Residual trifluoroacetate from purification can add a strong band close to the amide I region, and sugars or sugar alcohols used as bulking agents absorb strongly around 1000 to 1100 cm⁻¹. See counter-ions on a COA for why these are present.

How identity is decided

The lab compares the sample spectrum with a reference spectrum of the same compound, taken from a reference standard or an in-house library. If the two overlay closely, the result is reported as consistent with that compound.

A match is only as meaningful as the reference. A report that says “confirmed” without naming the reference, or without showing the overlay, cannot be checked.

What a match does tell you

• The sample has the broad chemical character of the reference: a peptide, with features similar to the reference material.
• The bulk material is not obviously something else, such as a different compound class or a heavily contaminated sample.

What FTIR does not tell you

• Molecular weight or sequence. Peptides with similar composition, or a single residue difference, can give almost the same spectrum. This is why mass spectrometry is the usual identity test. See how to read a mass spectrometry report.
• Purity or amount. As normally run, FTIR is qualitative. A bulk spectrum is dominated by the main component, and impurities at a few percent are generally invisible. Purity comes from HPLC. See how to read an HPLC chromatogram.
• Each component of a blend. A blend gives one combined spectrum with overlapping bands. FTIR on its own cannot confirm that every named compound is present. See how to read a blend COA.
• Salt form or water content. These change the spectrum, but a single FTIR result does not quantify them.

What to look for on a COA

1. The method is named. FTIR, and ideally ATR, with a method or procedure reference.
2. The spectrum is shown. Axes labelled in wavenumber, not just a word such as “confirmed”.
3. The reference is stated. Either an overlay with the reference spectrum or a clear statement of what it was compared with.
4. The wording is honest. “Consistent with” describes a comparison. “Identical” is rarely justified for a bulk sample that includes excipients.
5. A second identity test is present. For peptides, FTIR alongside mass spectrometry is much stronger than FTIR alone.
6. The sample details match your vial. Lot, date and product name. See how to read a peptide COA.

How FTIR fits with the other tests

• HPLC separates components and gives purity as area percent. Retention time suggests identity only within one method.
• Mass spectrometry measures molecular weight, which is the strongest routine identity check for a peptide.
• FTIR gives a fast fingerprint of the bulk material and is a useful cross-check, not a substitute for the other two.

For the full overview, see how peptide purity is tested. Some certificates also cite numbered USP chapters beside the test; what those citations mean is covered separately.

Where to go next

The terms used above are defined in the peptide research glossary. Published certificates are on the COA page.

This page is general analytical background for laboratory researchers. It does not describe uses, doses or effects of any compound in humans or animals. For laboratory research use only.