Peptide purity is one of the first numbers researchers look at when comparing products, but a figure like “99%” only means something if you know how it was measured. Two vials can carry the same purity number and still differ in identity, salt content or water content.
This guide explains what peptide purity actually measures, how high-performance liquid chromatography (HPLC) and mass spectrometry are used, what net peptide content is, and what to look for in a certificate of analysis.
What does peptide purity mean?
When a supplier quotes peptide purity, they almost always mean chromatographic purity: the proportion of the material in a sample that is the target peptide, compared with related impurities such as shortened sequences, sequences with a missing amino acid, or chemically modified versions. These impurities are by-products of peptide synthesis and purification.
A purity of 99% therefore means that, of the peptide-related material detected by the test, about 99% was the intended sequence. It does not mean that 99% of the powder’s weight is peptide; that is a different measurement covered below.
How HPLC measures peptide purity
High-performance liquid chromatography is the standard method. In reversed-phase HPLC, a dissolved sample is pumped through a column packed with a water-repelling material, and the solvent is gradually changed from mostly water to mostly acetonitrile. Different molecules leave the column at different times depending on how strongly they cling to it.
- A UV detector, usually set around 214–220nm where peptide bonds absorb light, records each compound as a peak.
- The target peptide should produce one large main peak.
- Purity is calculated as the area of the main peak divided by the total area of all peaks.
A clean chromatogram with one dominant peak and only small side peaks is what a high-purity sample looks like.
Why mass spectrometry matters too
HPLC tells you how much of one substance is present, but not what that substance is. Mass spectrometry fills that gap by measuring the molecular mass of the compound, typically using electrospray ionisation (ESI) or MALDI. If the measured mass matches the theoretical mass of the sequence, that confirms identity. In short, HPLC answers “how pure?” and mass spectrometry answers “is it the right molecule?”. Both are needed.
Peptide purity vs net peptide content
Synthetic peptides are usually isolated as salts, commonly with trifluoroacetate (TFA) or acetate counter-ions, and the dry powder also holds some residual water. As a result, the actual weight of peptide in a vial (the net peptide content) is often noticeably lower than the total weight of powder, even when chromatographic purity is very high. Net peptide content is measured separately, for example by amino acid analysis or nitrogen analysis.
3 key checks on a certificate of analysis
- Batch match: the certificate should name the compound and a batch or lot number that matches the vial you received, plus a test date.
- HPLC data: look for the purity figure together with the chromatogram itself and the method used (column, wavelength and gradient).
- Identity data: a mass spectrometry result showing the observed mass alongside the expected mass for the sequence.
Useful extras include the appearance, solubility, the CAS number where one exists and the name of the laboratory that performed the tests. CAS numbers can be cross-checked on PubChem.
How we present product information
Every product page in our research peptides range lists the stated purity, the CAS number where available, the appearance, the water solubility and a reconstitution reference. If you need batch-specific documentation for your records, contact us before ordering to ask what is available for the batch you will receive.
Summary
Peptide purity is a useful number, but only alongside identity and context. HPLC measures how much of the sample is the target sequence, mass spectrometry confirms that it is the right molecule, and net peptide content tells you how much of the powder is peptide at all. Good storage then protects that purity: see our guide on how to store peptides.