Peptide purity: what “≥99% by HPLC” means

What the purity figure measures

A purity figure such as “≥99%” on a synthetic peptide almost always comes from reversed-phase high-performance liquid chromatography (RP-HPLC) with a UV detector. The sample is separated on a column, the detector records each component as it comes off, and purity is the area of the main peak as a share of all the peaks detected. [5]

The figure is therefore relative: it describes how much of what the detector sees is the main peak. Method development for peptides is demanding, because many variables change the separation, and an impurity that comes off together with the main peak is not counted as an impurity. [2] [5]

Where impurities come from

Most peptides are made by solid-phase peptide synthesis (SPPS), building the chain one amino acid at a time. Incomplete steps leave deletion sequences with an amino acid missing, excess reagent can insert an extra one, and deprotection steps can turn an amino acid into its mirror form (racemisation) or leave protecting groups attached. [1]

Other impurities form as the peptide degrades: oxidised side chains, dimers and larger aggregates, and rearrangements such as pyroglutamate, diketopiperazine and succinimide formation. Counter-ions such as trifluoroacetate (TFA), left over from synthesis and purification, can also remain in the final powder. [1]

What the purity figure does not tell you

Identity. HPLC shows that one component dominates, not that it is the right peptide. Mass spectrometry, usually coupled to liquid chromatography (LC-MS), measures the molecular mass; it is the standard way to confirm identity and to characterise impurities, including isomers that are hard to separate from the main peak. [2]

Counter-ions. Synthetic peptides are usually obtained as TFA salts. A purity figure describes the peptide-related peaks, not how much of the powder is peptide: the counter-ion needs its own test, and residual TFA has been shown to affect biological assays, which is why its content matters in experiments with synthetic peptides. [3]

Why certificates should be checked

A group at Ghent University tested a set of research peptides ordered with a required purity of at least 95%. In the laboratory's own analysis only 44% met that purity, there was a large gap between the suppliers' certificates and the measured values, and the main compound of one sample had a different structure from the peptide ordered. Most impurities were sequences missing one or more amino acids. [4]

Impurities can change the outcome of early functional studies and lead to wrong conclusions. The Ghent authors concluded that relying only on a supplier's certificate can have serious consequences for research, and that laboratories should keep their own quality control. [1] [4]

What a complete certificate shows

A certificate of analysis is most useful when it shows the evidence and not only the result: the HPLC chromatogram with its integrated peaks, the method used (column, mobile phase, detection wavelength), and a mass spectrum with the measured mass next to the expected one. [2] [5]

It should also name the batch it describes, so the result can be matched to the vial, and say which laboratory ran the analysis and when. For in vitro laboratory research, an independent test of a retained sample remains the strongest check. [4]

Common questions

Is a 99% pure peptide 99% peptide by weight?

No. HPLC purity compares the peptide-related peaks the detector sees. Counter-ions such as TFA are measured with separate tests, so the share of the powder's weight that is peptide is a different number. [3]

Does an HPLC result prove the peptide is the right one?

No. It shows that one component dominates. Identity is confirmed by mass spectrometry, which measures the molecular mass and can also identify impurities. [2]

Why can two analyses of the same batch give different purity?

Purity depends on the method: the column, the conditions and which components separate from the main peak. Independent testing of research peptides has found large gaps between certificates and measured values. [4] [5]

What are the most common impurities in synthetic peptides?

Sequences with a missing or extra amino acid, amino acids in their mirror form, leftover protecting groups, oxidised forms, dimers and degradation products. [1]