Peptide Purity, Identity & Content Explained

What is the difference between identity, purity, and content?

Identity asks whether the expected compound is present. Purity assesses a stated measure of the main component relative to the impurities alongside it. Content, sometimes labelled assay, measures how much of the analyte is actually there. ICH Q2(R2) distinguishes these analytical purposes and ties validation to a procedure’s intended use, which is another way of saying that a method is only validated for the question it was designed to answer.

The practical consequence is that these three values cannot substitute for one another. A correct identity result says nothing about how much material you have. A high purity percentage says nothing about whether the sequence is the one you ordered. For a laboratory purchasing record, keep the fields separate and record the method and unit next to each value. Never copy a purity percentage into an amount or concentration field: the two describe different quantities, and the substitution is difficult to detect once it is in your records.

Sources: ICH Q2(R2): Validation of Analytical Procedures (2023)

Why do chromatography and mass spectrometry appear together?

The two techniques answer complementary questions, which is why reports frequently pair them. The NIST report for peptide reference material RM 8321 describes characterising synthetic peptides with LC–MS and LC–MS/MS, assessing their identity, purity, and both chromatographic and mass spectrometric behaviour. It then used additional LC–MS/MS analyses and spectral comparisons to assess the mixture as a whole.

That example illustrates how multiple independent observations can support a characterisation. It does not mean that any single mass match excludes all impurities, and it does not mean a matching mass proves an entire sequence. Different peptides can share a nominal mass, and a component that does not ionise well may be under-represented regardless of how much of it is present. Ask what identification evidence was actually collected for the material you are reviewing, rather than inferring it from the technique’s reputation. The historical NIST report is an analytical example, not a specification for Savage Labs products.

Sources: NIST RM 8321: Peptide Mixture for Proteomics — historical report of investigation

Does a high purity percentage tell you how much peptide you have?

Not on its own. NIST’s angiotensin I investigation evaluated several approaches to assigning peptide purity as a mass fraction, which is a broader measurement problem than reading the relative size of a chromatographic peak. A lyophilised vial also contains water, residual solvent and the counter-ion the peptide was isolated as. Those components are part of the mass in the vial even when they do not appear as impurity peaks in a chromatogram.

The IUPAC technical report on organic purity describes mass balance and quantitative approaches, and considers related impurities, water, volatile compounds, traceability, and measurement uncertainty. Each of those is a reason two percentages may not be equivalent even when both are honestly reported. Before treating one figure as better than another, check the stated measurement basis. If one is a chromatographic area percentage and the other is a mass fraction determined by quantitative NMR, the larger number is not necessarily the better material.

Sources: NIST: SI traceable purity assignment using angiotensin I; NIST / IUPAC: Methods for the SI value assignment of organic compound purity; Erckes et al., Pharmaceuticals (2025): Analysis and exchange of TFA as a counterion in synthetic peptides

How should you record a comparison between two products?

When comparing candidate compounds, create separate entries for the listed catalogue specification and for the batch result, because they are different kinds of claim. A specification describes what a product line is intended to meet; a batch result describes what was measured in one lot. Add the method, units, report date, and report identifier to each entry. Where one report measures content and another reports a chromatographic percentage, flag the pair for clarification rather than ranking them by the larger number.

Ask support or the issuing laboratory which additional evidence is available for your protocol, and record the answer including the cases where nothing further exists. A comparison built from documents that answer different questions will look decisive while resting on nothing. These resources support document literacy for in vitro research. They do not establish suitability for human or veterinary use, and nothing in a Certificate of Analysis speaks to that question.