Glossary

The area of one chromatographic peak expressed as a percentage of the total area of all detected peaks. The standard way purity is reported. It is a proportion of what the detector registered — components invisible at the detection wavelength are absent from both numerator and denominator.

How long a component takes to travel through the column and reach the detector. Characteristic of a compound under fixed conditions, but not unique to it: two different compounds can elute at the same time, which is why retention time alone never establishes identity.

Reversed-phase high-performance liquid chromatography. A non-polar stationary phase (commonly C18) with a polar mobile phase. The dominant separation technique for peptides and small molecules.

The programmed change in mobile-phase composition over the run, e.g. 5→65% acetonitrile over 30 minutes. Shallower gradients resolve closely-eluting impurities that steeper ones merge into the main peak — which is why the gradient belongs on the method line.

Ionises the sample and sorts ions by mass-to-charge ratio (m/z), giving the molecular weight. Answers identity, which chromatography cannot. ESI+ (electrospray, positive mode) is the usual configuration for peptides.

Liquid chromatography feeding directly into a mass spectrometer: components are separated, then identified as they elute. The combination that supports both a purity figure and an identity confirmation on the same run.

The proportion of the fill mass that is actually the target peptide, the remainder being water and counterion. Distinct from purity and typically 15–25 points lower. Measured by amino acid analysis or nitrogen determination — never by chromatography. A vial reported at 98% purity and 81% content is not contradicting itself.

The salt paired with the peptide after purification, most often trifluoroacetate (TFA) or acetate. Can represent 10–20% of the mass of a lyophilised peptide. Not an impurity in the chromatographic sense, but it does occupy weight in the vial.

The reference method for water content. Lyophilised material is hygroscopic and routinely carries several percent water by mass — part of the gap between purity and content.

Bacterial endotoxins are lipopolysaccharide fragments from Gram-negative cell walls. Measured by Limulus amebocyte lysate assay and reported in endotoxin units per milligram (EU/mg). A separate assay entirely — chromatography cannot detect endotoxin, so its absence from a certificate means it was not tested.

A compendial test for viable micro-organisms, by membrane filtration or direct inoculation, with a defined incubation period. Rarely performed on research-grade material. "Sterile-filtered" describes a process step and is not a sterility result.

Freeze-dried solid. The physical form most peptides ship in. Its appearance — cake, powder, film — carries no information about purity and should not be read as if it did.

The international standard for the competence of testing and calibration laboratories. Covers method validation, equipment calibration, traceability, personnel and documentation. Verifiable against the accreditation body’s public register. Says nothing about who commissioned the test or who chose the sample.

The specific methods and matrices an accreditation covers. Never blanket. A laboratory accredited for one technique may report another outside its scope without misrepresenting anything — which is why the scope certificate is worth opening.

A document reporting the results of specified tests on a specified sample. Its authority extends exactly as far as the sample analysed and the assays listed, and no further. It is not a quality rating, a safety assessment or a general statement about a batch.

Limit of detection: the smallest amount distinguishable from background. Limit of quantitation: the smallest amount measurable with acceptable precision. "None detected" means below LOD under that method — not absent.