Home / Peptide GlossaryUpdated October 12, 2026 · Astramide Research Team

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Peptide glossary

47 terms you will meet on a peptide lab report, in plain words.

This glossary defines the terms used on research peptide certificates of analysis: what a peptide is, what HPLC purity and mass spec identity mean, how net peptide content differs from purity and from the label claim, what endotoxin screening is, and how storage affects a result.

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Peptide basics

Amino acid: An amino acid is the building block of a peptide. Twenty standard amino acids, each with its own side chain, link end to end to form peptide chains.

Related: Peptide, Peptide bond

Peptide: A peptide is a short chain of amino acids joined by peptide bonds, usually from 2 to about 50 amino acids long. Longer chains are generally called proteins.

Peptide bond: A peptide bond is the chemical bond that links one amino acid to the next in a chain.

Related: Amino acid, Peptide

Sequence: A sequence is the exact order of amino acids in a peptide. Change one position and it is a different molecule with a different molecular weight.

Molecular weight (Da): Molecular weight is the mass of one molecule, measured in daltons (Da). Every sequence has a theoretical molecular weight that mass spectrometry can check against the sample.

Solid-phase peptide synthesis (SPPS): Solid-phase peptide synthesis is the standard way research peptides are made: amino acids are added one at a time to a chain anchored on a resin bead, then the finished chain is cut free and purified.

Related: Deletion sequence, Counter-ion

Lyophilized: Lyophilized means freeze-dried. The water in a peptide solution is frozen and then removed under vacuum, leaving a dry powder or cake. A lyophilized peptide is far more stable in storage than the same peptide in liquid, as long as it stays cold, dark and sealed.

On a real report: appearance "white lyophilized powder".

Research use only (RUO): Research use only is a label meaning a material is supplied for laboratory research and identification only. It is not for human or animal use and has not been evaluated by the FDA.

Related: Certificate of analysis (COA)

On the lab report

Certificate of analysis (COA): A certificate of analysis is the lab report for one specific batch of a peptide. It lists each test that was run, the method used, the specification the batch had to meet and the result, along with the lot number and test dates. A COA describes the lot it was written for and no other.

On a real report: lot SK10, received June 8 and reported June 9, 2026, with purity, identity, net content, endotoxin and appearance results.

Lot number (batch number): A lot number is the code that ties a vial to its batch and its report. The number on the vial should match the number on the COA.

On a real report: Lot SK10.

Lot vs batch: A batch is the quantity made in one production run; a lot is a quantity identified by one number for testing and release. Many suppliers use the two words interchangeably.

Related: Lot number (batch number)

Lab reference (accession number): A lab reference is the testing lab's own ID for the sample it received. It is useful when asking a lab or supplier to confirm a result.

Related: Certificate of analysis (COA)

Specification (spec): A specification is the bar a result had to clear, such as "at or above 98.0%". A result only means something next to its specification.

Acceptance criteria (pass/fail): Acceptance criteria are the set of specifications a lot must meet, test by test, to pass. Each result on a COA is marked against its criterion.

Assay: An assay is a test that measures how much of a substance is present. On a peptide COA it usually refers to the quantitative test behind purity or net content.

Label claim: A label claim is the amount printed on the vial, such as 10 mg.

Related: Net peptide content

Net peptide content: Net peptide content is the actual mass of peptide in a vial, not counting water, counter-ions such as acetate or TFA, and other non-peptide material. It is measured on its own, apart from purity, and it is not purity multiplied by the label weight. It is rarely identical to the label claim and can sit slightly above or below it.

On a real report: 11.08 mg measured against a 10 mg label claim (lot SK10); 10.53 mg against 10 mg (lot TSM10).

Read the full net peptide content guide

Counter-ion: A counter-ion is a small charged molecule that pairs with a peptide as a salt. It adds weight to the powder but is not peptide, which is one reason net peptide content is lower than the weight of the powder.

TFA (trifluoroacetate): TFA is a counter-ion left over from peptide synthesis and purification, where trifluoroacetic acid is used to cut and wash the chain. A peptide supplied as a TFA salt carries it as part of its weight.

Acetate: Acetate is a counter-ion often used in place of TFA after an exchange step. A peptide supplied as an acetate salt carries acetate as part of its weight.

Appearance: Appearance is a visual check of the vial contents, such as color and form.

Related: Lyophilized

Released / not released: Released means a lot met its release criteria and went on sale; not released means it did not. A lot can score well on one test and still not be released because of another.

On a real report: lot CP10, 99.85% purity, identity failed, not released.

Testing methods

HPLC (high-performance liquid chromatography): HPLC is a method that separates a sample into its components and measures each one. It is the standard test for peptide purity.

HPLC-UV: HPLC-UV is HPLC with an ultraviolet detector, which records each component as it absorbs UV light. It only sees material that absorbs at the chosen wavelength.

On a real report: Chromatographic purity, HPLC-UV, 99.82%.

Chromatogram: A chromatogram is the graph HPLC produces: peaks plotted against time. One tall peak is the target peptide; small peaks are impurities.

Retention time: Retention time is how long a component takes to pass through the HPLC column. A given peptide comes out at a characteristic time for a given method.

Chromatographic purity: Chromatographic purity is the area of the main peak as a share of all detected peaks on an HPLC trace, reported as a percentage. It shows how clean a sample is. It does not show which compound the main peak is, and it does not measure how much peptide is in the vial.

On a real report: HPLC-UV purity 99.82%, spec at or above 98.0%.

Impurity: An impurity is anything in the sample other than the target peptide, such as deletion sequences, oxidized forms or closely related chains left over from synthesis.

Related: Deletion sequence, Oxidation

Deletion sequence: A deletion sequence is a peptide chain missing one or more amino acids, usually from an incomplete step during synthesis. It is one of the most common impurities and shows up as a separate peak.

Mass spectrometry (MS): Mass spectrometry is a method that measures molecular weight. Comparing observed weight with theoretical weight confirms a peptide's identity.

m/z (mass-to-charge ratio): m/z is the mass-to-charge ratio a mass spectrometer actually measures. A peptide can carry more than one charge, so the instrument works back from m/z to the molecular weight.

LC-MS: LC-MS is liquid chromatography coupled to mass spectrometry. The sample is first separated, then each part is weighed, so the instrument can confirm that the main component has the molecular weight of the labelled peptide. It is the usual method behind an identity result on a COA.

On a real report: Identity, LC-MS, Confirmed.

Identity: Identity is confirmation that the compound in the vial is the one on the label, usually by mass spectrometry or by matching a certified reference standard.

Reference standard: A reference standard is a certified sample of a known compound. A lab can confirm identity by showing that a test sample behaves the same way, for example the same HPLC retention time.

Related: Identity, Retention time

Limit of detection: The limit of detection is the smallest amount a test can reliably detect. A result reported as "below" a value means nothing was found above that limit.

On a real report: endotoxin below the 0.05 EU/mL sensitivity of the assay.

Related: EU/mL, Endotoxin

Contaminant screens

Endotoxin: Endotoxin is a molecule from the outer wall of certain bacteria. It does not show up on HPLC, so labs screen for it with a separate test, usually the LAL test, and report the result in endotoxin units per milliliter (EU/mL).

On a real report: pass, below the 0.05 EU/mL sensitivity of the assay.

LAL (limulus amebocyte lysate): The LAL test is the standard lab test for endotoxin. It uses a reagent made from horseshoe crab blood cells that reacts to endotoxin, and the result is read against a set limit. The method is described in USP chapter 85.

Related: Endotoxin, EU/mL, USP <85>

EU/mL: EU/mL means endotoxin units per milliliter, the unit an endotoxin result is reported in.

Related: Endotoxin, Limit of detection

USP <85>: USP <85> is the United States Pharmacopeia chapter that describes the bacterial endotoxins test. A COA may cite it next to an endotoxin result.

Heavy metals (ICP-MS): A heavy metals screen tests for trace metals such as lead, arsenic, cadmium and mercury, usually by inductively coupled plasma mass spectrometry.

Related: Residual solvents

Residual solvents: Residual solvents are traces of solvents left over from synthesis and purification, measured against set limits.

Water content (Karl Fischer): Water content is how much water is in the powder, usually measured by Karl Fischer titration. Freeze-dried peptides still hold a small amount.

Storage and stability

Hygroscopic: Hygroscopic means tending to pull moisture from the air. Most freeze-dried peptides are, which is why vials stay sealed until use.

Freeze-thaw cycle: A freeze-thaw cycle is one round of freezing and thawing. Repeated cycles can break down a peptide, so samples are often split into portions before freezing.

Related: Degradation, Lyophilized

Degradation: Degradation is chemical breakdown over time, sped up by heat, light, moisture and oxygen. It lowers purity after the COA was issued.

Oxidation: Oxidation is a reaction with oxygen that changes some amino acids, especially methionine and tryptophan. It shows up as an extra peak on HPLC.

Related: Degradation, Impurity

Desiccant: A desiccant is a drying agent stored with vials to keep moisture down.

Related: Hygroscopic

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See the terms on a real report

The fastest way to learn the words is to read a real certificate. Start with a guide, then open any lot in the library.

Read a COA in 5 Checks HPLC vs Mass Spec Browse the COA Library

Common questions

What does net peptide content mean on a COA?

Net peptide content is the amount of peptide the lab actually measured in the vial, not counting water and counter-ions. The label claim is what the vial says. The two are rarely identical, and a COA should show the measured figure next to the label claim.

Is net peptide content the same as purity?

No. Purity, measured by HPLC, is the share of detected material that is the target peptide. Net peptide content is the mass of peptide in the vial. A powder can be 99% pure by HPLC and still carry part of its weight as water and counter-ions, so the two numbers answer different questions.

What is the difference between purity and identity?

Purity, usually measured by HPLC, is the share of detected material that is one main compound. Identity, usually confirmed by mass spectrometry, shows whether that compound is the one on the label. A sample can be very pure and still be the wrong compound.

What is endotoxin testing on a peptide COA?

Endotoxin is a molecule from the cell wall of certain bacteria that HPLC cannot detect. Labs screen for it with the LAL test, described in USP chapter 85, and report the result in endotoxin units per milliliter (EU/mL).

What does research use only mean?

Research use only (RUO) means a material is supplied for laboratory research and identification only. It is not for human or animal use and has not been evaluated by the FDA.

Research use only. All products are intended for research and identification purposes only. These products are not intended for human dosing, injection, or ingestion. The statements on this website have not been reviewed by the FDA. These research peptides are not intended to diagnose, treat, cure, or prevent any disease.