Reference

Research Peptide Glossary

Published October 2026 · 4 min read

Plain-language definitions of the terms that show up on peptide documentation and in our articles. Short on purpose; each entry links to a longer note where one exists.

Acetate salt

A peptide form in which the charged groups are paired with acetate. Often preferred for biological work. See counterions.

Amino acid analysis (AAA)

A method that breaks a peptide into its amino acids and counts them. Used to determine net peptide content.

Bioburden

The number of viable microorganisms in a material that is not claimed to be sterile.

CAS Registry Number

A unique numeric identifier assigned to a chemical substance by the Chemical Abstracts Service. See CAS numbers explained.

Certificate of Analysis (COA)

A document reporting the test results for a specific lot of material. See how to read one.

Chromatogram

The graph produced by chromatography, with time on one axis and detector signal on the other. Each peak is a component of the sample.

Counterion

The ion that balances the charge on a peptide in solid form, most often trifluoroacetate (TFA), acetate or chloride.

Deamidation

A chemical change in which an asparagine or glutamine side chain loses an amide group. A common way peptides degrade.

Deletion sequence

A synthesis impurity in which one amino acid failed to couple, leaving a peptide missing a residue.

Endotoxin

Lipopolysaccharide fragments from Gram-negative bacteria. Heat-stable, measured by the LAL assay in endotoxin units (EU). See sterility and endotoxin.

HPLC

High-performance liquid chromatography. Separates a sample's components and quantifies them, giving a purity percentage. See HPLC vs. LC-MS.

Hygroscopic

Tending to absorb moisture from the air. Most lyophilized peptides are.

Karl Fischer titration

A method for measuring the water content of a solid sample.

LAL assay

Limulus amebocyte lysate assay, the standard test for bacterial endotoxin.

LC-MS

Liquid chromatography coupled to mass spectrometry. Separates components and measures their mass, confirming identity.

Lot (batch)

A defined quantity of material produced in one run and given one identifier. See lot numbers and traceability.

Lyophilization

Freeze-drying. Water is removed from a frozen sample under vacuum, leaving a stable, dry cake.

Molecular weight

The mass of a molecule. Average molecular weight uses natural isotope abundance; monoisotopic mass uses the most abundant isotope of each element, and is what high-resolution mass spectrometers typically report.

Net peptide content

The fraction of a powder, by weight, that is actually peptide. The remainder is counterions, water and salts. See purity vs. content.

Oxidation

A chemical change, commonly at methionine, cysteine or tryptophan, that alters a peptide and appears as a related impurity.

Purity (HPLC)

The percentage of detected peak area attributed to the main compound. A relative measure, not an amount.

Retention time

How long a compound takes to travel through the chromatography column. Used to identify peaks and compare runs.

Retest date

The date after which material should be re-analyzed before further use, set by the manufacturer for a given lot and storage condition.

RUO (research use only)

A label meaning the material is intended for laboratory research and not for human or veterinary use.

TFA salt

A peptide paired with trifluoroacetate, the default result of standard synthesis and purification. See TFA vs. acetate.

Truncated sequence

A synthesis impurity in which the chain stopped growing early, leaving a shorter peptide.

UPLC

Ultra-performance liquid chromatography. A higher-pressure form of HPLC that gives faster, sharper separations.

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