How Peptide Purity Is Tested: HPLC, FTIR & Mass Spectrometry

Three analytical methods do most of the work in peptide quality control. This guide explains what each one measures, how a result is produced, and where the numbers stop being informative.

Why peptides need testing at all

Synthetic peptides are built one amino acid at a time. Each coupling step is very efficient but never perfect, so the crude product contains the target molecule plus related impurities: sequences with a missing amino acid (deletions), chains that stopped early (truncations), residues that kept a protecting group, and species that were modified by oxidation or other side reactions. The product is then purified, usually by preparative chromatography, and freeze-dried (lyophilized). Analytical testing shows how well those steps worked and whether the material is what the label says.

HPLC: how clean is it?

High-performance liquid chromatography (HPLC), in the reversed-phase form used for peptides, separates molecules by how strongly they stick to a column. A small amount of the dissolved sample is pushed through a column packed with hydrophobic particles (commonly C18) by a solvent mixture whose composition changes during the run, typically water and acetonitrile with a small amount of an acid such as trifluoroacetic acid. Molecules leave the column at different times, called retention times, and a UV detector records a peak for each one.

For a walk-through of the chart itself, see how to read a peptide HPLC chromatogram.

The result is a chromatogram. Purity is calculated as the area of the main peak divided by the total area of all peaks, expressed as a percentage (“area percent”).

Term	What it means
Retention time	When a peak elutes. The same compound should elute at a consistent time under the same method.
Peak area	The integrated signal for a peak. Area percent purity compares the main peak with all others.
Detection wavelength	Peptide bonds absorb at roughly 214–220 nm. At 280 nm mainly aromatic residues (tryptophan, tyrosine) are seen. Different wavelengths give different purity values for the same sample.
Gradient and column	Determine which impurities separate from the main peak. A method that cannot separate an impurity will not report it.

What HPLC purity does not tell you

• It is relative. It reports how pure the detected material is, not how much peptide the vial contains.
• It can miss things. Impurities that co-elute with the main peak, or that do not absorb at the detection wavelength, are invisible to it. Water, salts and many excipients are also not seen.
• It does not confirm identity. A single clean peak shows one species was detected, not which species it was.

Mass spectrometry: is it the right molecule?

Mass spectrometry (MS) measures the mass-to-charge ratio of ions. For peptides it is usually run with electrospray ionisation, often directly after an HPLC column (LC-MS), or with MALDI-TOF. Peptides typically pick up several charges, so the instrument reports a series of peaks for the same molecule; software converts them into a single molecular mass.

For how to check the numbers on a report, see how to read a peptide mass spectrometry report.

That measured mass is compared with the theoretical mass calculated from the amino acid sequence. A match within the instrument’s tolerance supports the identity of the compound. Mass spectrometry is also good at spotting deletion and truncation impurities, because their masses differ from the target by the mass of the missing residues, and at flagging common modifications such as oxidation (typically +16 Da).

Limits of mass spectrometry

• It is not normally a quantitative purity measurement on its own.
• Molecules with the same mass, such as some isomers, can look identical.
• It confirms mass, not the sequence order. Sequence confirmation needs tandem MS (MS/MS) or other methods.

FTIR: does the spectrum match?

Fourier-transform infrared spectroscopy (FTIR) shines infrared light through the sample and records which wavelengths it absorbs. The resulting spectrum acts as a fingerprint of the molecule’s chemical bonds. A laboratory compares it with a reference spectrum for the compound, and characteristic bands support the presence of that peptide. The certificates of analysis we publish from BT Labs use FTIR for identification, together with HPLC for purity and potency. For a closer look, see FTIR identification on a peptide COA; some certificates also cite USP chapters, explained in USP method citations on a peptide COA.

Limits of FTIR

• It is a qualitative identity check, not a purity measurement.
• It does not report an exact molecular mass or confirm the amino acid sequence the way mass spectrometry does.
• Excipients and fillers in the sample also absorb infrared light. A report may say the peptide was detected “with addition of excipients/fillers”.

Why labs use more than one method

Each method answers a different question, so a complete picture combines them:

Question	Typical method
Is it the right compound?	FTIR (infrared spectral fingerprint) or mass spectrometry (molecular mass); different laboratories use different identity methods
How clean is it?	HPLC area percent purity
How much peptide is in the vial?	HPLC assay against a reference standard, or amino acid analysis
How much water, salt or excipient?	Karl Fischer titration, ion analysis, composition analysis
Is it free of microbial contaminants?	Separate sterility and endotoxin testing (not part of a routine purity panel)

The reports we publish are structured this way: an identification test (FTIR on most of our BT Labs reports; one report from another laboratory uses LC-MS), HPLC purity, and a potency assay, with the laboratory stating what the panel does not cover. For a walk-through of a report, see How to Read a Peptide Certificate of Analysis.

What a “99% purity” statement should come with

• The method: column, gradient and detection wavelength.
• The chromatogram itself, with retention time and peak areas.
• An identity result (mass or spectrum) for the same lot.
• The laboratory, report number and date.

A purity figure without those details can’t be checked. Research-grade peptides are commonly specified at 95% or 98% and above; the purity results on our certificates are above 99%, and the reports behind them are published on the certificates of analysis page. How we test is described on Quality & Testing.

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