Analytical glossary

A document issued by the laboratory that analysed a specific lot. It states the material, the lot number, the determinations performed, the method and conditions for each, the results, the date, and who signed for it.

On this site every listed size carries the certificate for the lot in stock, reproduced as received from the laboratory. The figures beside it are transcribed from the document; the document is the record.

One batch of material made in one run carries one lot number, printed on the vial and on the certificate. The two must match for the certificate to describe the material in hand: a certificate for another lot describes different material, however similar.

Here, three: identity, purity and net content — what the material is, how much of it is the stated compound, and how much is in the vial. Endotoxin, microbial contamination and elemental impurities are separate determinations that are not purchased, and no figure on this site should be read as one.

Established by comparing a measured property with the one expected for the named substance. For a peptide the usual measurement is molecular mass by mass spectrometry: the observed mass is compared with the mass calculated from the sequence or formula, and agreement within the instrument’s stated tolerance confirms identity. A retention time that matches a reference standard under the same chromatographic conditions is supporting evidence.

On a chromatographic certificate purity is an area percentage: the area of the main peak as a percentage of the total peak area detected, at the stated wavelength. It measures how much of what the detector saw was the main component; it does not by itself say what the remainder was.

A purity figure belongs to its method. Two figures from different methods, wavelengths or columns are not directly comparable, which is why a certificate states the conditions beside the number.

The way chromatographic purity is usually expressed. Each component that elutes produces a peak whose area is proportional to the amount detected; the main peak’s share of the total is reported as purity. It assumes every component responds to the detector, which is why the wavelength is stated.

The sample, dissolved, is pumped through a column packed with a non-polar stationary phase while the mobile phase is made progressively less polar. Components separate by hydrophobicity and leave the column at different retention times. It is the standard separation for peptide purity.

As the eluent leaves the column it passes a detector that measures absorbance at a fixed wavelength. For peptides the amide bond absorbs near 214 to 220 nm, so those wavelengths are usual; aromatic residues also absorb near 280 nm. The resulting chromatogram is what an area-percent purity is calculated from.

The chromatograph separates the sample and the mass spectrometer ionises each eluting component and measures its mass-to-charge ratio. The observed molecular mass of the main peak is what identity is confirmed against. A peptide often appears at several charge states; the reported mass is deconvoluted from them.

Measured from injection to the peak’s apex. It is characteristic of a compound only under the exact column, mobile phase, gradient and temperature used, and is not transferable between methods or laboratories.

The theoretical value is calculated from the molecular formula or sequence. The observed value is measured by mass spectrometry. A certificate that states both lets the reader check identity for themselves: the two should agree within the instrument’s stated tolerance, and a discrepancy beyond it is the first thing to query.

A lyophilised peptide is supplied as a salt, and the solid also holds residual water. Net peptide content is the peptide’s share of the total mass once counter-ions and water are excluded; it is determined by amino-acid analysis or nitrogen determination and is distinct from chromatographic purity, which describes the peptide fraction alone. A vial’s gross mass overstates peptide mass by the counter-ion and water fractions.

Peptides purified by RP-HPLC in trifluoroacetic acid are usually isolated as trifluoroacetate salts; acetate or hydrochloride salts result from a later ion exchange. The counter-ion contributes to the solid’s mass but is not peptide, which is why net peptide content is stated separately.

Determined by the Karl Fischer method, which reacts water with iodine and measures the iodine consumed, or by loss on drying, which weighs the sample, heats it, and weighs it again. Stated so that a mass figure can be corrected for the water it includes.

The solution is frozen and the ice removed by sublimation under vacuum, leaving the solute as a porous cake or powder. The form in which most peptides here are supplied; the certificate’s water content figure describes what the process left behind.

Reported in milligrams as measured, as distinct from the nominal size printed on the label. Where a certificate reports it, the product page shows it beside the nominal size.

Assigned by the Chemical Abstracts Service to a single substance and to nothing else. Names and abbreviations vary between suppliers and papers; the CAS number does not, which is why the reference pages lead with it and why a certificate should carry it.

Some certificates state a date after which the reported figures should not be relied on without a new analysis. Read it alongside the date of analysis; a certificate does not describe the material indefinitely.