Reference
Plain definitions for the terms that appear on a certificate of analysis, and what each one does and does not tell you about a batch.
The detection wavelength used on the certificates published here.
Peptide bonds absorb strongly in the low ultraviolet, roughly 205 to 215 nm, so wavelengths in that band detect the peptide backbone itself and are a general-purpose choice for peptide purity. Higher wavelengths such as 280 nm respond mainly to aromatic residues, so a peptide lacking tryptophan, tyrosine or phenylalanine can be badly under-detected there.
A protein that binds free actin monomers.
A protein binding monomeric G-actin and holding it out of the polymerised pool, thereby influencing cytoskeletal assembly. Thymosin beta-4 is the principal example in this context, and fragments corresponding to its binding region are studied for the same interaction.
A charged form of a molecule, which is what a mass spectrometer actually detects.
Mass spectrometers measure ions, not neutral molecules, so a sample is ionised first. [M+H] means one proton was added, giving a detected mass about one unit above the true molecular weight. [M+2H] carries two protons across two charges, appearing at roughly half the mass. This is why a detected value never exactly equals the molecular weight, and why a single compound can produce several peaks.
Loss of peptide onto container surfaces.
Peptide binding to the walls of a vial or tube, which removes material from solution without any visible change. The effect is most significant at low concentration and is reduced by low-binding polypropylene rather than glass. Losses are real and are not reflected in the stated concentration.
A single-use portion divided from a stock solution.
A measured portion separated from a larger solution so the remainder is not repeatedly thawed. Dividing stock into aliquots at the point of first reconstitution avoids the freeze-thaw cycling that drives aggregation, and the decision cannot be made retroactively.
The building block of peptides and proteins.
A molecule carrying both an amino group and a carboxyl group, joined into chains through peptide bonds. Twenty are standard in biological sequences; synthetic peptides may additionally incorporate non-standard residues introduced to alter stability or resistance to enzymatic cleavage.
A modified version of a reference sequence.
A peptide carrying substitutions or additions relative to a reference sequence, commonly introduced to resist enzymatic cleavage or extend stability. The term describes a relationship to that reference rather than any property of the compound itself.
How a purity figure is calculated from a chromatogram.
Purity by HPLC is computed as the area of the target compound’s peak divided by the total area of all peaks detected, expressed as a percentage. It describes chromatographic composition rather than mass, and depends on where the integration boundaries were drawn — which is why the chromatogram is more informative than the figure alone.
Sterile water with a preservative that inhibits microbial growth.
Sterile water containing benzyl alcohol, which suppresses microbial growth in a solution accessed more than once. It addresses contamination only — oxidation, deamidation, and hydrolysis proceed at the same rate as in unpreserved water, so it does not extend chemical stability.
A quantity of material produced in one run under the same conditions.
Analytical results apply to the batch tested. Two batches of the same compound are separate material with separate certificates, which is why records here are organised by lot rather than by product.
The date to which the supplier expects the material to remain within specification.
Distinct from a pharmaceutical expiry date, which carries a regulatory meaning that does not apply to research material. A best-by date is a storage expectation, not a guarantee, and depends on the material having been stored as directed.
Preferential activation of one signalling pathway at a receptor.
A compound favouring one downstream pathway over another at the same receptor rather than activating all pathways proportionally. It is characterised experimentally through relative measures such as beta-arrestin recruitment against cAMP accumulation.
A mixture of two or more compounds, not a chemical substance.
A preparation containing multiple distinct compounds. Because it is a mixture rather than a single substance, it has no single CAS number, molecular formula, or molecular weight, and identity confirmation addresses each component separately.
An interim 503A designation for substances flagged as raising safety concerns.
One of three interim categories the FDA uses while evaluating nominated bulk drug substances. Category 2 identifies substances the agency has determined may present significant safety risks, which in practice prevents their use in compounding. Removal from Category 2 lifts that designation but does not authorise use — that requires separate placement on the affirmative list.
A laboratory document reporting what was measured on one specific batch.
A certificate of analysis reports the measurements a laboratory made on a named batch of material. It applies only to the batch tested — not to the product line, and not to a later batch of the same compound. A certificate without a lot number on it cannot be matched to anything you hold.
A statement that a batch meets a requirement, without the data.
A document stating that a batch meets a stated requirement, without necessarily disclosing the measurements supporting that statement. Distinct from a certificate of analysis, which reports the measured results themselves. The two are sometimes used interchangeably in supplier copy despite carrying different evidential weight.
The plot of detector response over time produced by a chromatographic run.
Peaks correspond to components leaving the column, with position indicating retention time and area indicating relative quantity. It is the underlying evidence a purity percentage is derived from.
Two or more compounds prepared together in one solution.
A preparation holding multiple compounds in a single solution, formulated under controlled conditions rather than combined after separate preparation. Requires that all components be soluble and stable under shared conditions, which cannot be assumed and is the reason co-formulation is a formulation decision rather than a mixing step.
The salt form a peptide is isolated in. Counts toward net weight.
Synthetic peptides are isolated as salts, most commonly acetate or trifluoroacetate. The counterion contributes to the net weight in the vial without being peptide, which is why peptide content is reported as a separate figure from purity and why net weight alone does not give peptide mass.
A synthesis byproduct missing an internal residue.
A chain assembled without one or more internal residues because a coupling step did not go to completion. Deletion and truncated sequences are the principal impurities in synthetic peptide material and the main contributors to a purity figure below 100 percent.
An enzyme that cleaves and inactivates many peptides.
Dipeptidyl peptidase-4, an enzyme that removes dipeptides from the N-terminus of susceptible sequences and rapidly inactivates several endogenous peptides. Resistance to DPP-4 cleavage is a common design objective in synthetic analogues and is achieved by substitution at the cleavage site.
A bacterial cell-wall component, quantified in endotoxin units per milligram.
Endotoxins are lipopolysaccharides shed by gram-negative bacteria. They are heat-stable and survive many processes that kill the organism itself, which is why endotoxin is measured separately rather than inferred from sterility.
A formulation component other than the active compound.
Any substance present in a preparation that is not the compound of interest — solvents, preservatives, buffering agents, and tonicity adjusters among them. Excipients contribute to the net contents of a container without being the material the certificate characterises.
A portion of a longer sequence, reproduced synthetically.
A synthetic peptide corresponding to part of a longer naturally occurring sequence. As with an analogue, the term describes a relationship to a reference and says nothing about purity, identity, or handling without the specific data.
A synthetic variant of growth hormone-releasing hormone.
A modified version of growth hormone-releasing hormone, or of its active 1-29 fragment, carrying substitutions that resist enzymatic cleavage and extend plasma stability relative to the native sequence. Research examines receptor binding and secretion in pituitary cell preparations.
High-performance liquid chromatography — separates a mixture into its components.
HPLC pushes a dissolved sample through a column that separates compounds by how strongly they interact with it. Each component emerges at a characteristic time, producing the peaks on a chromatogram. It is the basis of most purity determinations.
A gut hormone that stimulates insulin release after eating.
A hormone released from intestinal cells in response to nutrient intake that potentiates insulin secretion. The two principal incretins are glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide, and both are the basis of a synthetic agonist class.
The quantity stated on the label, as distinct from the quantity measured.
Label claim is what the product says it contains. Measured net weight is what the laboratory found. The two are reported side by side because the difference is the informative part; a certificate showing only one of them is difficult to interpret.
The test used to quantify endotoxin.
Limulus amebocyte lysate reacts with bacterial endotoxin, and the extent of that reaction is measured to give a value in EU/mg. It is a specific test that is performed or not performed on a given batch — where a certificate carries no LAL result, no endotoxin measurement was made on that batch.
The identifier tying a certificate to a specific batch of material.
A lot number appears on both the vial and the certificate, and is what makes an analytical record verifiable rather than decorative. If the lot on the label does not appear on the certificate you were given, the certificate describes different material.
Freeze-drying: removing water by sublimation under vacuum.
The sample is frozen and the pressure lowered so ice passes directly to vapour without melting. It produces a stable dry solid without the heat that would degrade a peptide, which is why most research peptides are supplied lyophilised.
The dry solid remaining after freeze-drying.
The porous solid left in the vial after freeze-drying. A compact cake at the base of the vial is the expected appearance; material dispersed around the walls, shrunken, or discoloured indicates a handling or seal problem. Normal appearance does not establish that material is chemically intact.
When the batch was produced.
Read alongside the best-by date and the date the certificate was issued. A large gap between manufacture and testing is worth noticing, since analytical results describe the material at the time it was tested.
Measures molecular mass to confirm identity.
Liquid chromatography separates the sample; the mass spectrometer then measures the mass-to-charge ratio of what emerges. Purity and identity answer different questions — HPLC tells you how much of the material is one thing, mass spectrometry tells you what that thing is. Neither substitutes for the other.
A receptor family responding to melanocortin peptides.
A family of five G protein-coupled receptors responding to melanocortin peptides, with distinct tissue distribution and downstream roles across the family. Synthetic agonists differ in their selectivity between receptor subtypes, which is the parameter of research interest.
A preparation supplying a fixed volume per actuation.
A container dispensing a set volume of a prepared solution each time it is actuated. Concentration is established by the formulation rather than at reconstitution, which fixes a parameter that a lyophilised vial leaves to the purchaser.
Concentration expressed in moles per litre.
Amount of substance per unit volume, calculated from peptide mass and molecular weight. Because label claim includes counterion and residual water, peptide content must be applied before the calculation — using label claim directly overstates molar concentration.
The mass of one molecule, in grams per mole.
A compound's molecular weight is fixed, so it is a useful check on a certificate: a reported mass that does not match the known value for the named compound means the sample, the label, or the certificate is wrong. Related forms differ — a copper-complexed peptide is heavier than the free peptide by the mass of the copper it carries.
The end of a peptide chain carrying a free amino group.
The end of the chain bearing an unbonded amino group, by convention written first when a sequence is stated. Modifications at this position, such as acetylation, are common in synthetic analogues and alter susceptibility to enzymatic cleavage.
A compound that raises cellular NAD+ through a salvage pathway.
A compound converted to nicotinamide adenine dinucleotide through one of several salvage routes. Precursors differ in which route they enter and in which tissues that route is active, so they are not interchangeable in experimental design.
The mass of material the laboratory actually recovered and weighed.
Reported in milligrams against the label claim. Small variances in either direction are normal in lyophilised material. Large ones are the reason to read the number rather than assume it.
A compound that inhibits nicotinamide N-methyltransferase.
A compound inhibiting the enzyme that methylates nicotinamide. Because that reaction consumes nicotinamide, inhibition is studied for its effect on nicotinamide availability and downstream metabolic pathways. Inhibitors in this class are small molecules rather than peptides and therefore carry no amino acid sequence.
The formal process required before a regulatory list changes.
The administrative procedure by which a federal agency issues a binding rule: a proposed rule is published, a public comment period runs, comments are reviewed, and a final rule is issued. Advisory committee recommendations do not bypass it, which is why a favourable committee vote does not itself change what is permitted.
The FDA advisory committee that reviews compounding substance nominations.
The Pharmacy Compounding Advisory Committee, which reviews bulk drug substances nominated for the Section 503A list and issues recommendations to the FDA. Its recommendations are advisory and non-binding; the agency must still proceed through notice-and-comment rulemaking before a substance is formally added.
A peptide of fifteen amino acid residues.
A peptide consisting of fifteen residues. The term is descriptive of length only and carries no implication about sequence, origin, or activity.
A chain of amino acids, shorter than a protein.
A chain of amino acids joined by peptide bonds, conventionally up to around fifty residues. The order of residues — the sequence — is the primary identifier, since chains containing the same residues in a different order are different compounds.
The proportion of a sample that is peptide rather than salts and water.
Distinct from purity. Purity describes the composition of the peptide fraction; peptide content describes how much of the total mass that fraction is. Lyophilised peptides commonly carry counter-ions and residual water, so material can be highly pure and still be a minority of the vial by mass. This matters for calculating molar concentration. The certificates published on this site report purity, identity and — where performed — endotoxin; they do not report peptide content.
An additive that suppresses microbial growth in solution.
A compound added to a solution accessed more than once to inhibit microbial growth, commonly benzyl alcohol. It addresses contamination only and has no effect on oxidation, deamidation, or hydrolysis, so it does not extend chemical stability.
A compound that binds a receptor and activates it.
A molecule that binds to a receptor and produces a signalling response, as distinct from an antagonist, which binds without activating and blocks the response. Agonist potency is measured per receptor, so a compound engaging several receptors has a separate potency at each.
Returning lyophilised material to solution.
Dry peptide is dissolved in an appropriate solvent before use. Which solvent is appropriate depends on the compound's solubility, listed per compound in the catalog.
A certificate from a different batch, offered as a stand-in.
A certificate reporting measurements from a different batch of the same compound rather than from the batch being supplied. It can be identified by a missing lot number, a lot number that does not appear on the vial, or the same document being supplied for material purchased at different times. Batch-to-batch variation is the thing a certificate exists to detect, which a representative certificate cannot do.
Material supplied for laboratory research and not for any human or animal use.
An RUO designation describes the terms on which material is supplied. It is not a drug approval, not a claim of safety, and not a statement that the compound is suitable for any diagnostic or clinical purpose. Everything on this site is supplied RUO.
A single amino acid within a chain.
An amino acid as it exists within a peptide chain, having lost the elements of water in forming the peptide bond. Chain length is expressed as a residue count, which is why a fifteen-residue peptide is described as a pentadecapeptide.
How cleanly adjacent chromatogram peaks separate.
The degree of separation between neighbouring peaks in a chromatogram. Poor resolution means impurity peaks overlap the main peak, and area assigned to the target may include material that is not the target. A purity figure derived from a poorly resolved chromatogram is less reliable than the same figure from a well-resolved one.
How long a component takes to travel through the column.
Characteristic of a compound under a given method, so a shifted retention time can indicate a different compound or a changed method. Comparable only between runs using the same conditions.
A compound that prompts release of a stored substance.
A substance that stimulates secretion of another substance. In peptide research the term usually refers to compounds prompting growth hormone release from the pituitary, acting either at the GHRH receptor or at the ghrelin receptor depending on the compound.
The statutory provision governing pharmacy compounding for individual patients.
A section of the Federal Food, Drug, and Cosmetic Act setting the conditions under which a state-licensed pharmacy may compound a preparation against an individual prescription. The associated bulk drug substances list determines which raw substances may be used. It governs pharmacy practice and has no application to the supply of material for laboratory research.
The order of residues in a peptide chain.
The order in which residues appear, conventionally written from N-terminus to C-terminus. Sequence determines solubility, degradation susceptibility, and storage requirements, and is the reason handling guidance cannot be generic across compounds.
A family of NAD+-dependent enzymes.
A family of enzymes that remove acetyl and related groups from protein substrates using NAD+ as a required cofactor. Because the reaction consumes NAD+, sirtuin activity is coupled to cellular NAD+ availability, which is why NAD+ precursors are studied in the same context.
A low molecular weight compound with no amino acid chain.
A compound of low molecular weight that is not a peptide and therefore has no sequence. Small molecules appear in adjacent research contexts as enzyme inhibitors, cofactors, and metabolic precursors, and differ from peptides in solubility behaviour and degradation pathways.
The standard method for producing synthetic peptides.
Assembly of a peptide chain one residue at a time on an insoluble resin support, followed by cleavage from the support. Incomplete coupling at any step produces truncated and deletion sequences, which are the principal impurities a chromatographic purity figure measures.
Absence of viable microorganisms. Not the same as purity.
Sterility describes whether viable microorganisms are present and is established by a dedicated test. It is unrelated to purity, which describes chemical composition, and unrelated to endotoxin, which measures bacterial cell-wall fragments that persist whether or not organisms are viable. A certificate reporting purity says nothing about sterility.
Reconstituted material held at a known concentration.
A solution of known concentration prepared from lyophilised material, from which working dilutions are made. Concentration is mass divided by volume; for molar concentration, peptide content must be applied to label claim before dividing by molecular weight.
A period spent outside the specified storage range.
An event in which material is held outside its stated storage conditions. The term describes the event rather than an outcome — whether degradation resulted depends on the temperature reached, the duration, and the sequence. Lyophilised material tolerates excursions that solution does not.
Analysis performed by a laboratory independent of the manufacturer.
Independence matters because the party with an interest in the result is not the party producing it. The analytical results published on this site were generated by AxisPharm, LLC, an independent laboratory; the manufacturer did not produce them.
A synthesis byproduct shorter than the target peptide.
A peptide chain that stopped short of the full target sequence because a coupling step did not go to completion during synthesis. Truncated and deletion sequences are the most common impurities in synthetic peptides and are what a purity figure is primarily measuring the absence of.
A substance that has not completed FDA approval for a human indication.
A compound that has not been through the approval process establishing safety and effectiveness for a specific indication in a specific population. The designation describes regulatory status rather than scientific merit or analytical quality, and applies regardless of how extensively a compound has been studied.
The proportion of UV-absorbing material attributable to the main peak.
A detector measures ultraviolet absorbance as compounds leave the HPLC column. Purity is the main peak's area as a percentage of total peak area. The important limitation: it only counts what absorbs at the chosen wavelength. Anything transparent at that wavelength — many salts, some solvents — is invisible to the measurement and does not reduce the figure.
These definitions describe what the terms mean, not what any particular batch contains. For the measured results on a specific lot, see the COA library. For longer treatments of how the methods work and where they fall short, see the analytical guides.
