Peptide Testing Glossary

Published October 4, 2026 · Last updated October 4, 2026 · Publisher: ENOS Labs USA

Area percent

The share of total integrated peak area in a chromatogram that belongs to one peak. HPLC purity is usually reported this way. Read more.

Certificate of analysis (COA)

A laboratory document reporting test results for one specific lot, including the lot number, laboratory, analysis date and methods. Read more.

Chromatogram

The detector trace from a chromatography run, showing signal over time with one peak per resolved component.

Co elution

Two or more substances leaving a chromatography column at the same time, so they appear as a single peak.

Counterion

An ion that balances the charge of a peptide salt, commonly acetate or trifluoroacetate. It adds weight but is not peptide.

Deletion sequence

A synthesis impurity in which one amino acid is missing from the intended chain.

Electrospray ionization (ESI)

A way of creating gas phase ions from a solution for mass spectrometry, typically producing multiply charged peptide ions. Read more.

Expected mass

The mass calculated from a peptide's sequence and modifications, used as the comparison value for an identity test.

Gradient

The programmed change in mobile phase composition during an HPLC run, usually increasing organic solvent over time.

HPLC

High performance liquid chromatography: a method that separates sample components on a packed column. Reverse phase HPLC is standard for peptide purity. Read more.

Identity

Whether the molecule present is the one claimed, usually established by mass spectrometry. Read more.

LC-MS

Liquid chromatography coupled directly to mass spectrometry, so each chromatographic peak can be assigned a mass.

Lot number

The identifier that ties a vial to a production batch and to that batch's test record. Read more.

Lyophilization

Freeze drying. Water is removed from a frozen solution under vacuum, leaving a dry powder or cake. Read more.

MALDI

Matrix assisted laser desorption ionization: a mass spectrometry ionization method that mostly produces singly charged ions.

Mass to charge ratio (m/z)

The quantity a mass spectrometer measures: ion mass divided by its charge number.

Monoisotopic mass

The mass of a molecule calculated with the most abundant isotope of each element, as opposed to the average mass.

Net peptide content

The percentage of a powder's weight that is peptide, after water and counterions are accounted for.

Oxidation

A common degradation, especially of methionine and tryptophan, that adds about 16 Da per oxygen atom.

Purity

How much of the material is the target molecule rather than related substances, usually HPLC area percent. Read more.

Related impurity

A substance structurally close to the target peptide, such as a deletion, truncation or modified form, formed during synthesis or storage. Read more.

Retention time

The time a component takes to pass through the chromatography column under a given method.

Reverse phase

A chromatography mode using a hydrophobic stationary phase and a polar mobile phase.

Tandem mass spectrometry (MS/MS)

Fragmenting selected ions and measuring the fragments to obtain sequence level information.

Traceability

The documented chain that links a vial, through its lot number, to its source batch and test records. Read more.

Water content

The fraction of a sample's weight that is water, often measured by Karl Fischer titration.

To continue, see what HPLC actually measures, how mass spectrometry confirms identity, how to read a certificate of analysis and the compound directory.