Ask what a peptide supplier means by purity and you will usually get a number without a method. The number is nearly always the same measurement, and it is worth knowing precisely what it does.
The measurement
A sample is injected onto a reversed-phase column. Molecules partition between a hydrophobic stationary phase and a mobile phase whose organic content increases over the run; more hydrophobic species elute later. This is the standard analytical and preparative method for peptides1.
A detector at the column outlet records absorbance over time, producing peaks. Software integrates the area under each peak. Purity is the area of the main peak divided by the total area of all peaks, expressed as a percentage.
Two words in that sentence carry the whole caveat: all peaks. Not "everything in the vial", everything the detector saw.
What the detector sees
Peptide analysis almost always uses UV detection at 214-220 nm, where the peptide bond itself absorbs. This is why the method works for peptides lacking aromatic residues, which would be nearly invisible at the more common 280 nm2.
It also defines the blind spots. Water does not absorb meaningfully there. Neither do simple inorganic salts. Acetate is weak; trifluoroacetate absorbs somewhat but is usually not integrated as a peptide-related peak.
The wavelength targets the peptide bond, which sets both the strength and the blind spots
Counted in the percentage
- The target peptide
- Truncated and deletion chains
- Oxidised and deamidated variants
- Chains still carrying protecting groups
Not counted
- Residual water
- Inorganic salts
- Acetate counter-ion (weak absorber)
- Anything the injection never reached
So a vial containing 65% peptide, 25% counter-ion salt and 10% residual water can legitimately report >99% purity, because among the species the detector counted, the target dominated. Nothing dishonest has happened, the number simply answers a narrower question than most readers assume.
Share of vial mass, against what UV detection at 214 nm counts
Illustrative composition used in the text, not a measurement of any particular batch. Chart by TrueLab Care.
What a high number does tell you
It is not a meaningless figure. A purity of 99% means the purification step removed the bulk of the synthesis-related impurities described in the piece on how peptides are made: truncated chains, chains carrying leftover protecting groups, oxidised or deamidated variants.
Those are exactly the species that would confound an experiment, because they are peptide-like and may retain partial activity. Removing them matters.
Where it can still mislead
Co-elution. A chain missing one residue differs from the target by a small hydrophobicity increment. On a shallow gradient the two may not resolve, and the integrator will count both as one peak. This is the single most important limitation, and it is why identity confirmation by mass spectrometry is a separate requirement rather than a redundancy.
Gradient and column dependence. The same sample analysed with a steeper gradient, a shorter column or a different pore size can yield a different number. A purity figure without the chromatographic conditions is not fully reproducible.
Wavelength choice. Reporting at 280 nm rather than 214 nm changes which impurities are visible, favouring anything with aromatic residues.
Injection amount. Trace impurities are easier to see when more material is loaded. A very small injection can flatter a sample.
Reading a certificate properly
Three things make a purity statement checkable:
- The method line, column type, gradient, detection wavelength. Its absence is not proof of anything, but its presence means the claim can be reproduced.
- The chromatogram itself, not just the number. A trace with a clean baseline and one dominant peak communicates more than "99.1%".
- A separate identity result, mass spectrometry confirming the expected mass. Purity says the sample is one thing; mass says which thing.
The number that usually is not there
Peptide content, the fraction of the vial's mass that is peptide rather than counter-ion and water, is a different determination, typically by amino acid analysis or nitrogen determination. Most research-grade certificates omit it.
That omission is the reason a protocol requiring an exact molar amount cannot be built on net mass alone. We take that apart in the next piece.
