Research Peptide Testing Glossary
Research Peptide Testing Glossary
Definitions for the analytical and process terminology that appears across our research-peptide product pages, Certificates of Analysis, and methodology articles. Each entry is written for researchers who want a clean, self-contained answer before going deeper.
Terms covered
Peptide
A peptide is a short chain of amino acids joined by peptide bonds, generally shorter than a full protein.
Peptides typically range from 2 amino acids (a dipeptide) to roughly 50 amino acids; beyond that length the molecule is usually classified as a protein. Research peptides used in laboratory work are most often synthesized chemically (solid-phase peptide synthesis), then purified, characterized, and supplied as a dry powder. The defined length and sequence are what make a peptide tractable as a research tool — its identity and purity can be confirmed analytically in a way that larger biologics cannot.
Lyophilized
Lyophilized means freeze-dried under vacuum: the peptide solution is frozen, then water is removed by sublimation, leaving a dry powder or cake.
Lyophilization is the standard format for research peptides because the dry form is far more stable than a solution. A well-lyophilized peptide forms a uniform cake at the bottom of the vial with no melt-back, no liquid residue, and minimal residual moisture (typically < 5% w/w). Cake appearance is one of the first visual quality indicators when inspecting a received vial.
HPLC
HPLC (High-Performance Liquid Chromatography) is the standard analytical technique for measuring peptide purity by separating a sample into its component compounds.
In a typical peptide HPLC run, the sample is dissolved in a solvent and pumped through a column packed with stationary phase under high pressure. Different compounds in the mixture travel through the column at different speeds and exit at different retention times. A detector (most commonly UV at 214 nm or 220 nm) measures the abundance of each component as it leaves the column. The resulting chromatogram is a series of peaks; the main peak represents the target peptide, smaller peaks represent impurities. Purity is calculated as the area of the main peak divided by the total area of all peaks.
For peptides, the most common variant is RP-HPLC (reversed-phase HPLC), in which the stationary phase is hydrophobic and peptides are separated by their hydrophobicity.
Mass spectrometry
Mass spectrometry (MS) measures the mass-to-charge ratio of ionized molecules and is the standard technique for confirming peptide identity by matching its observed mass to the theoretical mass of its sequence.
An HPLC run tells you how pure a sample is; a mass spectrometry run tells you what’s actually in it. The sample is ionized — commonly by electrospray ionization (ESI) for peptides — and the resulting ions are sorted by mass. The output is a spectrum showing peaks at specific m/z values. The peptide’s calculated monoisotopic mass should match the largest observed peak within instrument tolerance. Mass spec also detects truncated sequences, oxidized residues, and deletion impurities that HPLC alone might not resolve.
The two techniques are complementary, not interchangeable. A complete characterization uses both — HPLC for purity, MS for identity.
Peptide purity
Peptide purity is the fraction of a sample that consists of the target peptide, expressed as a percentage. The industry standard for research-grade peptides is ≥ 95% purity by HPLC; high-quality material typically exceeds 99%.
The remaining percentage represents synthesis-related impurities: truncated sequences (peptides missing one or more amino acids), deletion sequences, oxidation products, deamidation products, and residual solvents or salts. The reported purity number is meaningful only when the measurement method, wavelength, and column conditions are documented — a “99% purity” claim without a chromatogram is not a verifiable specification.
Identity verification
Identity verification is the analytical confirmation that the peptide in the vial is the peptide the label claims. The standard method is mass spectrometry: the observed mass must match the theoretical mass of the stated sequence within instrument tolerance.
Identity and purity are independent specifications. A sample can be 99% pure but the wrong compound. A sample can be the correct compound but only 90% pure. A complete Certificate of Analysis reports both, with separate methods for each. Identity is typically reported as a mass-spectrum peak at the expected m/z; a confirmed sequence by MS/MS fragmentation is the highest level of confirmation.
Certificate of Analysis
A Certificate of Analysis (COA) is a laboratory-issued document that records the analytical tests performed on a specific batch of material and the results of each test.
For a research peptide, a complete COA reports — at minimum — the compound name, the batch (lot) number, the date of analysis, the testing methodology (typically HPLC for purity and mass spectrometry for identity), and the numerical results. It should also identify the laboratory that performed the analysis. A COA tied to a specific batch is the fundamental document of trust between supplier and researcher: it ties the powder in the vial in front of you to a specific, verifiable test result.
An “in-house” COA generated by the same vendor selling the product is less rigorous than an independent third-party COA from an external testing laboratory.
Batch / lot number
A batch number (also called a lot number) is a unique identifier assigned to a specific manufacturing run of a peptide. Every vial from that run shares the same batch number; vials from different runs have different batch numbers.
The batch number is the linchpin of accountability. It allows the vial in a researcher’s hand to be tied back to the exact analytical results on the corresponding Certificate of Analysis. If a problem emerges later (instability, atypical result, contamination), the batch number lets the supplier and the researcher trace the issue to a specific production run rather than the entire product line.
A batch number that does not match the COA, or a COA with no batch number at all, is a meaningful quality-control failure.
Research Use Only (RUO)
Research Use Only (RUO) is the regulatory classification for materials that may be sold and used for laboratory research but are not approved for human or veterinary diagnostic, therapeutic, or consumption use.
An RUO product is not a drug. It has not gone through clinical trials for safety or efficacy in humans, it is not regulated as a medical device or pharmaceutical, and it cannot be lawfully marketed with medical claims. The classification puts the burden of intended use on the researcher, who must ensure the material is handled in an appropriate laboratory setting for non-clinical research purposes.
Research peptides are a standard RUO product category. Every research-peptide product, methodology page, and Certificate of Analysis on this site is presented under the RUO framework.
Aliquot
An aliquot is a small portion of a larger sample, separated for individual use or analysis.
In laboratory practice, aliquoting a reconstituted peptide solution into multiple small-volume tubes — and freezing each — minimizes the number of freeze-thaw cycles the bulk material undergoes. Repeated freeze-thaw is one of the most common causes of peptide degradation in solution; aliquoting protects sample integrity by limiting how often any given sub-sample is thawed.
Peptide stability
Peptide stability is the degree to which a peptide retains its intact structure and concentration over time under defined storage conditions.
Stability depends on temperature, pH of the solvent, exposure to light, the peptide’s amino-acid composition (sequences containing methionine, tryptophan, or cysteine are more prone to oxidation), and the number of freeze-thaw cycles the sample undergoes. A lyophilized peptide stored frozen and protected from moisture is generally far more stable than the same peptide in aqueous solution. Most research-grade peptides are specified for multi-year stability in lyophilized form at -20 °C or below.
Reconstitution
Reconstitution, in laboratory chemistry, is the process of dissolving a dry (typically lyophilized) substance back into a liquid solvent to return it to a workable solution.
For peptides the choice of reconstitution solvent depends on the peptide’s solubility characteristics — water for highly soluble sequences, dilute acetic acid or DMSO for poorly soluble sequences. Once reconstituted, peptide solutions are significantly less stable than the lyophilized powder and should be handled to minimize freeze-thaw exposure (see aliquot and peptide stability).
Endotoxin
Endotoxins are lipopolysaccharide components of the outer membrane of Gram-negative bacteria. Endotoxin testing measures the level of these contaminants in a sample, typically using a Limulus Amebocyte Lysate (LAL) assay.
For peptides produced by chemical synthesis (solid-phase peptide synthesis), endotoxin contamination is typically very low because the synthesis route is not bacterial. Endotoxin testing becomes more relevant for peptides produced by recombinant expression in bacteria. Where reported on a Certificate of Analysis, endotoxin levels are usually expressed in endotoxin units per milligram (EU/mg).