Editorial Reference / Glossary

Glossary

A plain-language reference for the terms that recur across our peptide lab-testing articles: analytical methods, quality attributes, and the standards a Certificate of Analysis is measured against.

A

Accreditation scope: The specific list of test methods, analytes, sample matrices, and measurement ranges a laboratory has been assessed as competent to perform under ISO/IEC 17025. Accreditation is never a blanket endorsement: a result only carries accredited status when the exact test sits inside the published scope schedule.See: ISO 17025 Explained: Why Lab Accreditation Matters
Area percent purity: The proportion of total detected peak area on an HPLC chromatogram that belongs to the target peptide, expressed as a percentage. It is a relative measure of how uniform a sample is, not an absolute measure of how much peptide is present, and it counts only what the detector registers.See: Peptide Purity Testing: What the Percentage Means

B

Bacteriostatic water: Sterile water for injection preserved with 0.9% benzyl alcohol, supplied in a multiple-dose container so it can be entered repeatedly to reconstitute lyophilised material. The preservative inhibits microbial growth but does not sterilise the water or protect the dissolved peptide from chemical degradation.See: Bacteriostatic Water: Quality and Sterility
Batch/lot number: A unique alphanumeric identifier tying a Certificate of Analysis to one specific synthesis run. It exists for traceability: a certificate means something only when the batch number on the document matches the batch number on the vial in hand.See: How to Read a Peptide Certificate of Analysis (COA)

C

Certificate of Analysis (COA): The document a manufacturer or testing laboratory issues to attest that a specific batch meets a defined specification, reporting identity, purity, and other measured results against acceptance criteria. A credible COA is a technical record traceable to its source, not a branding certificate.See: How to Read a Peptide Certificate of Analysis (COA)
Chain of custody: Documentation of where a sample came from, who submitted it, when it arrived, and how it was handled. Because a result speaks only to the sample actually tested, chain-of-custody records are what tie a certificate to a specific physical product rather than to an anonymous submission.See: The Third-Party Peptide Testing Landscape Explained
Coelution: When an impurity is chromatographically similar enough to the target peptide that it elutes inside the main HPLC peak. It is the most common cause of overstated purity, because the hidden impurity is counted within the main peak area; orthogonal methods such as mass spectrometry can separate coeluting compounds by mass.See: HPLC for Peptides: What Every Buyer Should Look For
Counterion: The salt ion paired with a peptide after solid-phase synthesis, most commonly trifluoroacetate (TFA) introduced during cleavage and purification. Counterion mass is non-peptide material, so a vial labelled by total mass can hold less actual peptide; some manufacturers exchange TFA for acetate before use.See: Peptide Purity Testing: What the Percentage Means

D

Depyrogenation: The removal or inactivation of pyrogens, chiefly bacterial endotoxin, from a material or surface. Because endotoxin is heat-stable it survives ordinary sterilisation; dry heat at 250 °C for 30 minutes achieves at least a 3-log (1000-fold) reduction, a far harsher treatment than killing organisms requires.See: Sterility Testing (USP 71): What It Actually Proves

E

Elemental impurities (heavy metals): Trace metal contaminants controlled under ICH Q3D, which sets permitted daily exposures for 24 elements across oral, parenteral, and inhalation routes. The four elements usually meant by heavy metals, arsenic, cadmium, mercury, and lead, are Class 1 and are actively screened for.See: Heavy Metals Testing for Peptides: ICP-MS & ICH Q3D
Endotoxin: Lipopolysaccharide shed from the outer membrane of gram-negative bacteria, and a potent pyrogen that can cause fever or, at high dose, septic shock. It is a distinct contaminant from viable microbes and has its own assay, so a sterile or high-purity result says nothing about endotoxin load.See: Endotoxin Testing Explained: LAL Method and USP 85

H

HPLC (high-performance liquid chromatography): The separation method that produces the purity figure on a peptide COA. Peptides are almost always run in reversed-phase mode, where they separate by how strongly each interacts with a non-polar C18 column, emerging as peaks whose relative areas give the purity percentage.See: HPLC for Peptides: What Every Buyer Should Look For

I

ICH guideline: A harmonised quality guideline from the International Council for Harmonisation, adopted by regulators internationally. Several govern peptide testing directly: Q3C (residual solvents), Q3D (elemental impurities), Q1A (stability), Q2 (analytical method validation), and Q6B (specifications for biological products).See: Residual Solvents Testing for Peptides: ICH Q3C Guide
ICP-MS (inductively coupled plasma mass spectrometry): The atomic-spectroscopy method used to quantify individual elemental impurities at trace levels, defined as a procedure under USP <233>. It measures each element separately against its ICH Q3D limit, unlike the non-specific sulfide screen it replaced.See: Heavy Metals Testing for Peptides: ICP-MS & ICH Q3D
In-house QC: Quality-control testing performed by the same company that made the product. It is legitimate and necessary first-party work, but under conformity-assessment vocabulary it is not third-party testing, because the party reporting the result has an interest in the outcome.See: Third-Party Tested vs COA on Request: What's Verified
ISO/IEC 17025: The international standard for the competence of testing and calibration laboratories. It is competence-based, asking whether measurements are technically correct rather than merely documented, and scope-specific, requiring validated methods and reported measurement uncertainty.See: ISO 17025 Explained: Why Lab Accreditation Matters

L

LAL assay: The Limulus Amebocyte Lysate test for bacterial endotoxin, the reference method under USP <85>, read by one of three techniques: gel-clot, turbidimetric, or chromogenic. Where results are disputed, the gel-clot technique is the deciding method.See: Endotoxin Testing Explained: LAL Method and USP 85
Lyophilisation: Freeze-drying that removes water from a peptide to leave a dry, porous cake. Because most degradation pathways need water, lyophilisation effectively pauses them, extending typical stability from days or weeks in solution to months or years in the dry state.See: Peptide Stability: Storage and Shelf Life

M

Mass spectrometry: The method that confirms peptide identity by measuring the mass-to-charge ratio of ions and recovering the molecular mass, then comparing it against the value expected from the sequence. It answers whether a sample is the right molecule, a question HPLC purity cannot.See: Mass Spectrometry: Confirming Peptide Identity
Monoisotopic mass: The mass of a molecule calculated using only the lightest, most abundant isotope of each element. It differs from average molecular weight, which reflects the natural isotope distribution, so an observed monoisotopic mass must be compared to the theoretical monoisotopic value or a correct molecule can look mismatched.See: Mass Spectrometry: Confirming Peptide Identity

N

Net peptide content: The percentage of a sample that is actual peptide rather than non-peptide material such as counterions, residual salts, and water. It is a different measurement from purity, determined typically by amino acid analysis or UV spectrophotometry, so a high-purity sample can still show a lower net peptide content.See: Peptide Purity Testing: What the Percentage Means

P

Permitted daily exposure (PDE): A regulatory daily exposure limit for an impurity, used by ICH Q3C for residual solvents and ICH Q3D for elemental impurities. The same substance carries a different PDE by route of administration, and parenteral limits are often stricter than oral because injection bypasses the gut.See: Heavy Metals Testing for Peptides: ICP-MS & ICH Q3D
Potency: How much active compound is present, or how much biological effect it produces. Potency is related to but distinct from purity and content, which describe composition rather than activity, so a purity percentage is not a direct readout of potency.See: Are Research Peptides Safe? A Testing-Based Answer

R

Reconstitution: Dissolving a lyophilised peptide in a diluent such as bacteriostatic water before use. Technique matters: the diluent is added slowly against the vial wall and the vial is swirled gently rather than shaken, because foaming and shear at the air-water interface can nucleate aggregation.See: Peptide Stability: Storage and Shelf Life
Residual solvents: Organic solvents such as acetonitrile, methanol, and dichloromethane left in a peptide after synthesis and purification, quantified against the ICH Q3C class limits by static headspace gas chromatography. A routine purity-plus-identity panel does not measure them.See: Residual Solvents Testing for Peptides: ICH Q3C Guide
Retest date: The date by which a lot should be re-evaluated to confirm it still meets specification, as distinct from an expiry date. A retest date implies testing can extend usability, whereas an expiry date implies the lot is to be retired.See: Peptide Stability: Storage and Shelf Life

S

Sterility: The absence of viable microorganisms, tested under USP <71> by membrane filtration or direct inoculation followed by at least 14 days of incubation. A sterile result proves only that nothing grew; it is silent on identity, purity, and endotoxin, and sterile is not the same as endotoxin-free.See: Sterility Testing (USP 71): What It Actually Proves

T

TFA (trifluoroacetic acid): The acid used both to cleave peptides from the synthesis resin and as the ion-pairing modifier in reversed-phase HPLC, which also leaves peptides as trifluoroacetate salts. ICH Q3C lists it among solvents with no adequate toxicological data, so it carries no standard ppm limit and the manufacturer must justify residual levels.See: Residual Solvents Testing for Peptides: ICH Q3C Guide
Third-party testing: Analysis performed by a laboratory independent of the seller and with no interest in the outcome, matching the ISO/IEC 17000 definition of a third-party activity. Independence is separate from accreditation: a lab can be independent without being accredited, and accredited without being the third party in a given transaction.See: Third-Party Tested vs COA on Request: What's Verified

U

USP general chapter: A numbered testing standard in the United States Pharmacopeia, cited on a certificate as the method behind a result. Chapters relevant to peptides include <621> (chromatography), <71> (sterility), <85> (bacterial endotoxin), <233> (elemental impurities), and <467> (residual solvents).See: How to Read a Peptide Certificate of Analysis (COA)

W

Water content (Karl Fischer): The residual moisture in a lyophilised peptide, measured by Karl Fischer titration. Freshly lyophilised material is typically 2–8% water by mass, and below 3% for a well-dried cake; higher values suggest poor lyophilisation or material that was reconstituted and re-dried.See: Peptide Stability: Storage and Shelf Life