"99% pure" is an area measurement, not a weight measurement. It means that when the sample was run down a reversed-phase column and the detector output was integrated, the main peak accounted for 99% of the total area. It is a real and useful number. It is also routinely read as answering a question it does not touch — how much peptide is in the vial — and the gap between those two readings is where most misunderstandings about peptide quality live.
How the number is produced
The sample is dissolved and injected onto a C18 column. A gradient of increasing acetonitrile against water, both with a small amount of trifluoroacetic acid, washes components off in order of hydrophobicity. A UV detector watches the eluent, and the resulting trace is integrated: every peak gets an area, the areas are summed, and the main peak is expressed as a percentage of that sum.
Three consequences follow directly, and all three matter.
It is relative. The denominator is what the detector saw, not what was in the vial. Anything that does not absorb, does not elute within the run, or sticks to the column is absent from the arithmetic entirely.
It is method-dependent. A shallower gradient resolves peaks a steeper one merges. Purity figures from different methods are not strictly comparable, which is why the method belongs on the certificate.
It weights by absorbance, not by mass. Peaks are compared by how strongly they absorb, and different molecules absorb differently.
Why 214 nm and not 280 nm?
Because 214 nm sees the peptide bond itself. The amide backbone absorbs strongly in the far UV, so every peptide is detected in proportion to how many bonds it has, regardless of composition. 280 nm sees aromatic side chains — tryptophan, tyrosine, and weakly phenylalanine — which is selective and sensitive when those residues are present and blind when they are not. A peptide with no aromatic residues is close to invisible at 280 nm. For purity, where the point is to see everything, the backbone wavelength is the correct choice.
Does area percent equal mass percent?
Only approximately, and the approximation fails exactly where it matters. Response factor — absorbance per unit mass — varies between molecules. A truncated impurity missing three residues has fewer peptide bonds than the target, absorbs less per microgram, and therefore reports as a smaller area than its true mass share. A deletion sequence at a genuine 1.5% by mass may integrate at 1.2%. In the other direction, an impurity richer in aromatic residues over-reports. The figure is a good estimate and it is not a weighing.
What purity does not tell you
This is the substantive point. Purity is a proportion of the peptide present, and a lyophilised vial contains material that is not peptide at all.
| Component | Typical share by mass | Does purity see it? |
|---|---|---|
| Target peptide | 70–85% | Yes — it is the main peak |
| Counter-ion (usually TFA) | 5–20% | No |
| Bound and residual water | 3–10% | No |
| Peptide-related impurities | 1–3% | Yes |
| Residual salts | Trace | No |
Can a 99% pure peptide be only 80% peptide by weight?
Yes, and this is the ordinary case rather than an edge case. Purity describes the peptide fraction internally: of the peptide in the vial, 99% is the right sequence. Net peptide content describes what share of the powder is peptide at all. A vial can be 99% pure and 78% peptide by mass at the same time, with both figures entirely honest, because they are measuring different denominators. The synthesis leaves a counter-ion paired to every basic site, and lyophilised peptide is hygroscopic enough to hold water no drying step fully removes. The consequences for the concentration you calculate are worked through in what a 5 mg vial actually contains.
What mass spectrometry adds
HPLC separates and quantifies; it does not identify. Two different molecules eluting at the same retention time are one peak. Mass spectrometry supplies the identity: the observed mass is compared against the theoretical mass of the sequence, and for a peptide of a few thousand daltons the agreement should be within a fraction of a dalton on a modern instrument — once both figures are on the same basis, monoisotopic or average.
The two techniques answer complementary questions, and a certificate carrying only one is carrying half the evidence. Purity without identity tells you the sample is homogeneous without saying what it is homogeneous in. Identity without purity confirms the right molecule is present without saying what else is.
What is the difference between HPLC and mass spec on a certificate?
HPLC answers "how much of what is here is the main component". Mass spectrometry answers "what is the main component". Neither substitutes for the other. In practice the two are often run in sequence on the same injection — LC-MS — which gives a mass for each resolved peak and turns anonymous impurities into identifiable ones. An impurity identified as the des-Gly deletion sequence is a synthesis artefact with known behaviour; the same peak unlabelled is just a number.
What is in the missing 1%?
Mostly close relatives of the target, because they are the hardest to remove. Deletion sequences, where a coupling step failed and a residue is missing. Truncated sequences from incomplete chain assembly. Incompletely deprotected material carrying a side-chain protecting group. Oxidised methionine or tryptophan. Diastereomers from partial racemisation during coupling. For peptides with multiple cysteines, the disulphide isomers — the right atoms connected the wrong way, with an identical mass.
These all sit close to the target in hydrophobicity, which is why they co-purify. The distant impurities come out in preparative chromatography; the near ones are what remains.
Is a single sharp peak proof of purity?
It is good evidence and it is not proof. A single peak means nothing else resolved under that method — which leaves open co-elution of something with nearly identical hydrophobicity, non-absorbing material, and anything retained on the column past the end of the run. A peak's shape carries information too: asymmetry or a shoulder suggests an unresolved neighbour, and a broad peak where a sharp one is expected can indicate aggregation or a slowly interconverting conformation. This is why a chromatogram is worth more than the figure derived from it, and why a certificate you can retrieve by lot number is worth more than a figure quoted on a page.
Does a higher purity figure always mean better material?
Not by itself. 99.2% against 98.4% is a real difference, but it is smaller than the difference between a batch with 80% net peptide content and one with 72%, and smaller still than the difference between material with a documented endotoxin result and material with none. Purity is one of four panels, and choosing on it alone optimises the number that is easiest to print.
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