A certificate of analysis for a synthetic peptide carries at least two numbers that are easy to conflate. One describes what fraction of the material a chromatographic detector sees as the intended molecule. The other describes what fraction of the vial's mass is peptide at all. They answer different questions, they are measured by different techniques, and a sample can score well on one while scoring poorly on the other.
Chromatographic purity
Purity, in the sense used on almost every peptide certificate, is a chromatographic quantity. A sample is dissolved, injected onto a reversed-phase column, and eluted with a gradient; a detector records absorbance against time; the area under the main peak is expressed as a percentage of the total area of all detected peaks. Reference-standard work describes this as the sum of impurities reported as a share of total detected area, with the main peak determined from replicate injections taken from a single container 1.
Three consequences follow from that definition, and all three matter when a figure is read off a certificate.
The first is that purity is relative, not absolute. It compares the target peak with other peaks in the same run. It does not compare peptide with anything the detector cannot see.
The second is that the detector's blindness is part of the measurement. Ultraviolet detection at 214 or 220 nm responds to the amide bond, and at 280 nm to aromatic side chains. Water, acetate, trifluoroacetate and inorganic salts produce little or no signal at these wavelengths. A vial that is a third salt by mass can still return a high purity figure, because the salt never appears in the denominator.
The third is that the separation determines what counts as an impurity. Species that co-elute with the target peak are counted as target. This is why identity is confirmed by mass spectrometry rather than by chromatography alone, and why a diastereomer arising from racemisation, which has the same mass as the intended peptide, has to be resolved chromatographically to be seen at all.
Net peptide content
Net peptide content is the mass question. It is the proportion of a gravimetrically weighed sample that is peptide, explicitly excluding the water and counter-ions present in every lyophilised preparation 2. In a reference-standard context the non-peptide balance is enumerated as water content, counter-ions such as acetic or trifluoroacetic acid, residual solvents and inorganic impurities 1.
The practical effect is that weighing a lyophilised solid does not tell a laboratory how much peptide has been weighed. For any work where the quantity of peptide is the independent variable — a binding assay, a standard curve, a potency comparison — the content figure, not the label mass, is the number that governs the calculation.
How content is measured
Three approaches appear in the literature, and they are not interchangeable.
Amino acid analysis hydrolyses the peptide bonds and quantifies the amino acids released. Published conditions use 6 M hydrochloric acid at 110 °C for 18 to 24 hours, after which the released residues are separated and measured 3. The hydrolysis is destructive in a way that constrains which residues can be used: tryptophan is destroyed, and methionine and cysteine may be oxidised, so quantitation is based on stable residues, or on deliberate measurement of the oxidised forms 32. The recommendations for mass-spectrometry assay peptides list alanine, arginine, glycine, histidine, isoleucine, leucine, lysine, phenylalanine, proline and valine as the residues suited to the purpose 2.
Quantitative NMR determines content against an internal standard of known purity, and is one of the orthogonal routes compared in reference-standard programmes 1.
An HPLC assay against a characterised reference material works in two stages: the purity of a bulk material is established first, and that bulk material then serves as the physical standard from which the peptide mass content of individual lyophilised vials is assigned by a compendial method 1.
Because the three routes carry different assumptions, results are compared rather than pooled. Reference-standard value assignment across multiple laboratories uses inverse-variance weighting and statistical identification of outliers, precisely because within-laboratory variability itself differs between laboratories 1. A single content figure quoted without its method and its reference material is therefore difficult to interpret.
Reading the two numbers together
The two figures constrain different sources of error, and neither substitutes for the other.
A high purity figure with no content figure leaves the mass of peptide in a vial unknown. A content figure with no purity figure leaves the composition of that peptide mass unknown: material can be predominantly peptide by mass and still contain a substantial deletion or oxidation impurity within it.
A certificate that reports both, alongside the method and the detection conditions for each, allows a laboratory to do two things that one figure alone does not support: calculate a concentration from a weighed mass, and judge what else is present in the sample when an assay behaves unexpectedly.
What this site reports
Every released lot is characterised by reversed-phase HPLC with the wavelength and gradient stated, and identity is confirmed by mass spectrometry against the calculated mass for the intended structure. Where a content figure is determined, the method and the reference material are named on the certificate alongside it. Figures without a stated method are not reported, because as the reference-standard literature shows, the method is part of the result 1.