Testing methods

Peptide Testing Explained: HPLC, Mass Spec, and What Each Test Tells You

Published October 2026 · 8 min read

"Lab tested" on a product page can mean almost anything. A single peptide batch can be tested for identity, purity, quantity, water, counterion, sterility and endotoxin, and each of those is a different test that answers a different question. This guide lays out what each one measures so you can tell what a report covers and what it quietly leaves out.

The short version

Question	Test	Usually on a research COA?
Is it the right molecule?	Mass spectrometry (MS, LC-MS)	Usually
How clean is it?	HPLC / UPLC purity	Almost always
How much peptide is in the vial?	Amino acid analysis, nitrogen analysis	Often missing
How much water is in it?	Karl Fischer titration	Often missing
Which salt form is it?	Ion chromatography, 19F NMR	Rare
Is it free of microbes and endotoxin?	Sterility, LAL endotoxin	Rare, and only needed for some work

HPLC: how clean is the sample?

High-performance liquid chromatography separates a sample into its components as they pass through a column. A UV detector draws each component as a peak, and the purity figure is the main peak's share of the total peak area. It is the most common peptide purity test and the one behind nearly every "99%" claim. For what it can and can't tell you, read why purity isn't content. For the HPLC versus LC-MS distinction, see HPLC vs. LC-MS.

A purity result is only meaningful with its method: column, gradient, detection wavelength, and the integration of the chromatogram. The same sample can read 96% on one method and 99% on another. When you compare two reports, compare the methods too.

Mass spectrometry: is it the right molecule?

A mass spectrometer measures the mass-to-charge ratio of ions. For a peptide, the observed molecular weight should match the theoretical value calculated from its sequence, within the accuracy of the instrument. It is the standard check on identity. HPLC can't provide it: a peak at 99% area could be a different peptide entirely. The reports that matter show the observed mass next to the expected mass, not just a statement that identity was "confirmed".

Amino acid analysis: how much peptide is there?

The peptide is broken down into its individual amino acids, which are then counted. Comparing the result with the expected composition gives the actual peptide quantity, which is the number behind net peptide content. It also gives a sanity check on composition. It takes time and costs more, which is why it's often skipped. Elemental nitrogen analysis is a related approach for quantity.

Karl Fischer: how much water?

Lyophilized peptides hold residual moisture, and water is a meaningful part of a vial's weight. Karl Fischer titration measures it directly. It also tells you about how well the material was dried, which relates to stability. See storage and stability.

Counterion testing: which salt is it?

Ion chromatography or fluorine NMR can quantify residual trifluoroacetate and other counterions. This matters for cell-based and structural work. See TFA versus acetate.

Sterility and endotoxin: a different kind of question

These tests look for contamination rather than for the peptide itself. They matter for certain biological applications and are not routine for every research-grade lot. For the distinction between them, see sterility and endotoxin testing.

Reading a report as a whole

A thorough report names the lab, identifies the lot, shows the raw data and not only the summary number, and states what was not tested. The absence of a test isn't a defect in itself, since not every application needs every test, but you should know which tests you're relying on. The COA primer walks through a report line by line, and verifying the lab covers who ran it.

Related reading

• How to Read a Certificate of Analysis
• Peptide Purity Testing: Why “99% Pure” Isn't the Same as Content
• Peptide Testing Labs: How to Check the Lab on a COA
• Research peptide glossary

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