“Purity” is one of the most-quoted numbers on a research peptide’s paperwork, and one of the most misread. A single figure — 98%, say — actually summarises the output of specific analytical methods, each answering a different question. This guide explains how that number is generated, what the main techniques (HPLC and mass spectrometry) really measure, and why chromatographic purity and net peptide content are not the same thing.
The aim here is orientation for laboratory buyers and researchers reading analytical documentation. It is a methodology explainer only: it does not describe any use of these materials, which are supplied for laboratory research use only and are not for human or veterinary use.
- Purity is measured, not declared — mainly by reversed-phase HPLC (how much of the sample is the target peptide) and mass spectrometry (whether the target peptide is actually present).
- HPLC purity is usually reported as “% area”: the target peak’s area as a fraction of all detected peaks.
- Mass spectrometry (commonly ESI-MS) confirms identity by measuring molecular weight, not quantity.
- Chromatographic purity is not the same as net peptide content — a vial also contains water, salts and counter-ions such as acetate.
- These figures describe the quality of a research material; they say nothing about suitability for any use.
What purity means for a peptide
For a synthetic peptide, “purity” is shorthand for how much of the material in the vial is the intended molecule and how much is something else. That “something else” has several origins: truncated or deletion sequences left over from synthesis (chains missing one or more residues), peptides where a protecting group was not fully removed, oxidised or otherwise modified variants, plus non-peptide material such as residual solvents, water and salts. No single measurement captures all of this, which is why a credible specification rests on more than one technique.
It also helps to separate two different questions. One is “how much of the peptide material is the correct sequence?” — a question of relative composition, answered by chromatography. The other is “how much actual peptide is in this vial at all?” — a question of absolute content, answered by peptide-content analysis. Both matter, and conflating them is the most common misreading of a certificate.
HPLC: measuring chromatographic purity
High-performance liquid chromatography (HPLC) is the workhorse for peptide purity. In reversed-phase HPLC (RP-HPLC), the dissolved sample is pushed through a column packed with a hydrophobic stationary phase while a solvent gradient runs through it. Different molecules stick to the column to different degrees and therefore emerge (elute) at different times. A detector — typically measuring ultraviolet absorbance around 214 nm, where the peptide bond absorbs — records a signal as each component leaves the column.
