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Glossary

Terms beginning with A

17 of 238 glossary terms.

Area percent: Purity expressed as the main peak's integrated area divided by the total integrated area of all peaks, times 100. A relative quantity. It is not a concentration and not a content.
Asymmetry factor (As): Peak shape measure taken at 10 % of peak height: the width of the rear half divided by the width of the front half. Symmetric = 1.00. Above 1.50 the index marks integration unreliable.
Adduct: An ion formed by association with a cation other than a proton — sodium at +21.982, potassium at +37.956. Not an impurity; the same molecule with a different cation.
Average mass: The abundance-weighted mean mass over the whole natural isotope distribution. What a low-resolution instrument reports. About 600 ppm above the monoisotopic mass for a peptide.
Amino-acid analysis (AAA): Acid hydrolysis followed by chromatographic quantification of the liberated residues. The reference method for peptide content where no standard of the intact peptide exists.
Anhydrous basis: A content figure recalculated as if the sample contained no water. Must be stated, or a mass balance will not close.
Assay: Quantification of an analyte against a material of known content. Distinct from area percent, which compares peaks within one chromatogram.
Acetylation: Addition of an acetyl group, +42.011 Da. Either a deliberate synthesis capping step or an unwanted side reaction.
Aggregate: A non-covalent assembly of two or more molecules. Dissociated by the acidic organic mobile phase of reverse-phase HPLC, so invisible to it. Seen by size-exclusion chromatography.
Aspartimide: A cyclic imide formed at an Asp-X motif, −18.011 Da. Hydrolyses to a mixture of α- and β-aspartyl peptides, the latter mass-identical to the parent.
Asp-Gly motif: An aspartate followed by a glycine. The most aspartimide-prone sequence in peptide chemistry. Epitalon's defining analytical problem.
Aib: α-Aminoisobutyric acid. An achiral, sterically hindered residue used to resist enzymatic cleavage. Present in every incretin co-agonist in this index; its hindered couplings drive epimerisation nearby.
Acylation: Attachment of a fatty-acid chain to a side chain, conferring albumin binding and a long half-life. The defining modification of the long-acting GLP-1 analogues.
AEEA linker: 2-(2-(2-aminoethoxy)ethoxy)acetic acid. The spacer between the peptide backbone and the fatty-acid chain in semaglutide. Its loss gives the diagnostic −145 Da impurity.
Accuracy: Closeness of a measured value to the true value. Demonstrated by recovery experiments during validation.
Assay report: The primary object in this index: one analysis of one sample of one batch by one laboratory. Everything else on the site is a rollup of these.
Annotation: A short signed technical comment attached to a record. The index's only community surface — there is no forum and no voting.