Almost every data sheet for a research peptide carries a percentage figure. It is usually read as the quality statement. In fact it answers only one of three questions that can be asked of a research sample. This page separates the three dimensions of purity, identity and content from one another.
Three dimensions, three methods
- Purity – how large is the proportion of the main component compared with all detected components? Method: RP-HPLC with UV detection. Statement: area percent with the detection wavelength named.
- Identity – is it the stated compound at all? Method: LC-MS or mass spectrometry. Statement: measured versus calculated molecular mass.
- Content – how much peptide is contained in the weighed amount of substance? Method: amino acid analysis, quantitative nitrogen determination or UV quantification. Statement: net peptide content as a percentage.
Purity is a ratio, identity is an assignment, content is a quantity. None of the figures replaces another.
Purity: a relative value
A chromatographic purity figure is an area ratio: the area of the main peak divided by the sum of all integrated peak areas. The value therefore describes the proportion of the main component in what the method made visible under the chosen conditions.
Two limitations are associated with this. First, the result depends on the method: gradient, column, detection wavelength and run time all help determine which minor components are separated and recorded at all. Second, everything that does not absorb at the chosen wavelength remains invisible. Water and inorganic salts typically do not appear in a UV chromatogram. In detail in HPLC in peptide analysis.
Identity: is it the expected molecule?
A high purity proves only that a substance is uniform, not which substance it is. A cleanly synthesised but incorrect product can look impeccable chromatographically. The question of identity is therefore answered with an independent method, in practice usually by mass spectrometry. For questions of sequence the mass alone is not sufficient, because sequence variants with the same composition can have the same mass. Details in LC-MS in peptide analysis.
Content: what the weighed portion really consists of
A lyophilised peptide is rarely pure peptide substance. The mass is typically distributed across four constituents, only one of which is the peptide:
- Peptide – the synthesis product, visible in the chromatogram as the main peak.
- Counter-ions – frequently trifluoroacetate from the acidic mobile phase of the preparative purification. As a rule not visible at 214 nm.
- Residual moisture – from the hygroscopicity of the lyophilisate and from drying. Not visible in the UV chromatogram.
- Inorganic salts and buffers – from the work-up. Likewise not visible.
Precisely those constituents that dilute the peptide fraction are therefore the ones a UV-based chromatography does not record. The net peptide content describes the mass fraction that actually falls to the peptide substance: peptide mass = weighed portion multiplied by the net peptide content.
Peptides with many basic side chains, for example arginine or lysine, bind a correspondingly large number of counter-ions. The counter-ion content is therefore not a question of diligence but a consequence of the sequence and the purification system. A low net peptide content is therefore not automatically a quality defect; what matters is that the value is known and documented.
Why the values cannot be converted into one another
Purity and content vary independently of one another. Two batches with identical purity figures can contain different amounts of peptide per milligram if their proportion of counter-ions and residual moisture differs. Conversely, a batch with more minor components in the chromatogram can show the same peptide fraction as a chromatographically more uniform one. Both constellations are internally consistent. A comparison between batches is only meaningful if both figures are available and use the same reference basis.
Pay attention to the reference basis: a figure given as is or as supplied refers to the substance in the condition delivered, including counter-ions and residual moisture. A figure given as peptide or net peptide refers to the pure peptide mass. The two must not be mixed.
Common misinterpretations
- Reading a higher purity figure as more peptide per milligram.
- Deriving the content from the purity. The diluting constituents are not recorded by the purity determination at all.
- Comparing purity values from two documents without looking at gradient, column and wavelength.
- Treating the molecular mass as complete proof of sequence.
- Reading a missing figure as a statement about the sample. It is a statement about the scope of testing.
Documentation context
A certificate of analysis is the place where the three dimensions are documented on a batch-specific basis. What matters is that every figure carries a method and a batch identifier; without this assignment a number is not verifiable. How such a document is read is described in Reading a COA correctly; the link between document and sample is covered by Batch testing and traceability. Batch-related documents are published under Analytics & Certificates insofar as corresponding real documents are available.
For laboratory research only. This text serves as technical information and contains no medical, diagnostic or therapeutic statements, no dosages and no application instructions for humans.
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