Reading Purity Data

Start with the governing document

Quality figures are batch-specific. The certificate of analysis issued for the particular lot you received is the document that governs that lot — not a catalogue page, not a previous batch, and not a general product description. When a figure is needed for a record or a publication, cite the batch COA and its date.

What the purity number typically means

In peptide documentation, purity most often refers to an HPLC result expressed as area percent at a stated detection wavelength. In this approach each peak's area is compared with the total area of the peaks detected. It answers "of what the method detected, how much was the target peak?".

What it does not mean

Area percent is not the same as weight percent, and it is not a statement about everything that might be present. Material that does not elute under the stated method, does not absorb at the chosen wavelength, or is retained on the column is not represented. Water content, residual salts, and counter-ions associated with the peptide are generally not captured by the same figure.

For this reason, two numbers produced by different methods, columns, gradients, or wavelengths should not be compared as if they were equivalent. Meaningful comparison requires the method to be stated.

Identity is confirmed separately

Purity describes composition; it does not by itself prove identity. Identity is normally confirmed by mass spectrometry, which measures the mass-to-charge ratio and is compared with the theoretical molecular weight of the sequence, frequently observed as protonated or multiply charged species. Reading purity together with mass confirmation gives a much more complete picture than either alone.

Common impurity categories

Impurities generally reflect process history:

• Deletion sequences — chains missing one residue from a failed coupling.
• Truncated sequences — chains that stopped assembling early.
• Residual protecting groups — incomplete final deprotection.
• Adducts and oxidation products — additions to sensitive residues such as methionine or tryptophan.

Quantitation is a separate question

Preparing a solution of known concentration is different from knowing purity. Lyophilised peptide can contain water and counter-ions, so weighing a quantity of powder does not necessarily deliver that mass of peptide. Where a stated net peptide content or a lot-specific figure is provided on the COA, use it; otherwise consider independent quantitation appropriate to the analytical work.

If you need something different

Different applications legitimately need different things — a different purity specification, a particular salt form, low endotoxin handling, a specific solvent system, or additional testing. These are worth specifying before an order is quoted, since they influence both feasibility and what the COA will report.