Glossary

The ordered list of amino acid residues that defines a peptide's identity.

A peptide is identified by its sequence, written from the N-terminus to the C-terminus using one-letter or three-letter residue codes. Two preparations with the same sequence, the same modifications and the same stereochemistry are the same compound; a single substituted residue makes a different one.

Sequence is a claim about identity, and identity is established by measurement rather than by the label on a vial. Mass spectrometry confirms that the observed mass matches the mass the sequence predicts, which is strong evidence but not a full reading of the sequence. Sequencing methods, including tandem mass spectrometry and Edman degradation, read residue order directly and are used when identity is contested.

See alsoMolecular weightMass spectrometryLC-MS

A document reporting the measured results for one specific lot, against a stated specification, by stated methods.

A certificate is a record of measurement, not a statement of intent. It names the lot it describes, the laboratory that performed the work, the date of testing, the method used for each assay, the specification each result is judged against, and the measured result itself. Without the lot number it describes nothing in particular, and without the method the results are not comparable to anyone else's.

The distinction that matters most is between a specification and a measurement. A specification is a threshold we require before a lot is released. A measurement is what that lot actually returned. A document that prints a target where a result should be is a marketing sheet with a laboratory's letterhead on it.

See alsoLot / batchPeptide purityNet peptide content

The unbroken sequence of controlled-temperature storage and transport conditions a material passes through, together with the record proving it.

A cold chain is a documented condition rather than a description of equipment. It covers storage at the manufacturing site, transit to the distributor, storage at the distributor, transit to the customer, and the handover at each boundary. The chain is only as good as its weakest interval, and an undocumented interval is a broken one for practical purposes.

Lyophilized peptides are more forgiving than solutions, which is why they tolerate ambient shipping over a few days without measurable loss. That tolerance is not unlimited: repeated warming, condensation on a cold vial opened in humid air, and prolonged exposure to light all accumulate as degradation, and none of it is visible in the vial.

See alsoDegradationLyophilizationHalf-life

The ion paired with the charged groups on a peptide, carried over from purification.

Peptides purified by reversed-phase HPLC are eluted in a solvent containing an ion-pairing acid, most often trifluoroacetic acid, and the peptide is isolated as a salt of that acid. An ion-exchange step can convert it to another form, commonly acetate or hydrochloride. The sequence is unchanged; the mass of material in the vial is not.

Counterion mass is part of why gross vial mass overstates peptide mass. Together with residual water and residual solvent it accounts for the difference between a weighed 10 mg of powder and 10 mg of peptide, which is what net peptide content measures.

The salt form is worth publishing because it varies between suppliers of nominally identical material and it changes the molar quantity obtained from a given weight.

See alsoNet peptide contentHPLCMolecular weight

Chemical change in a peptide over time that reduces the proportion of intact target compound.

The dominant routes are hydrolysis of the peptide backbone, which is accelerated by water and by extremes of pH; oxidation of methionine, cysteine and tryptophan residues, driven by dissolved oxygen, light and trace metals; deamidation of asparagine and glutamine, which converts a residue to its acid form and shifts mass by one dalton; and aggregation, where molecules associate into species that may no longer dissolve.

Each route leaves a signature that analysis can read. Oxidation appears as a new peak at plus sixteen daltons per oxygen added. Deamidation appears as a plus-one mass shift and usually a small shift in retention time. Hydrolysis produces fragments at lower masses. A stability-indicating method is one whose separation resolves these products from the intact peak, so that purity falls when degradation occurs rather than being masked by co-elution.

Degradation is not visible. A vial that looks unchanged after a year at the wrong temperature can have lost several percent of intact compound, and the only way to know is to re-assay it.

See alsoCold chainHPLCHalf-life

Lipopolysaccharide from Gram-negative bacterial cell walls, quantified by the limulus amebocyte lysate assay and reported in endotoxin units per milligram.

Endotoxin is a heat-stable fragment of the outer membrane of Gram-negative bacteria. It survives autoclaving and sterile filtration removes it only partially, so a preparation can be sterile and still carry a significant endotoxin load. It is introduced by water, glassware and handling rather than by synthesis.

The LAL assay uses a clotting cascade from horseshoe crab amebocyte lysate that is triggered by lipopolysaccharide. Three formats are in use: gel-clot, which is a threshold test read as pass or fail against a defined sensitivity; turbidimetric, which tracks the onset of turbidity; and chromogenic, which releases a colored product and gives the most precise quantitation. Recombinant factor C assays use an engineered enzyme in place of the lysate and are increasingly accepted.

Endotoxin is invisible to HPLC and to mass spectrometry. It is only ever known to be low because it was measured, which is why the assay appears on expanded panels and not on the minimum two-assay certificate that is standard at the cheap end of this category.

A result is reported as endotoxin units per milligram of peptide, with the method and its sensitivity stated. A pass against an unstated limit is not a result.

See alsoCertificate of analysisPeptide purity

The time taken for the quantity of a compound in a defined system to fall to half its initial value.

The term carries a different meaning depending on the system being described, and the two are often conflated. A chemical or storage half-life describes how quickly a compound degrades under stated conditions of temperature, pH and light, and is a property of the material in storage. A biological half-life describes how quickly a compound is cleared from a defined compartment in an experimental model, and is a property of that model rather than of the material.

Neither figure transfers between systems. A half-life measured in buffer at 37 degrees Celsius does not predict behavior in serum, and a value determined in one animal model does not carry to another species. Published half-lives should always be read together with the system, the temperature and the analytical method used to obtain them.

See alsoDegradationCold chain

Trace elemental contaminants, quantified by inductively coupled plasma mass spectrometry and reported in parts per million or parts per billion.

Elemental impurities enter a preparation from reagents, from catalysts, from column hardware and from glassware rather than from the sequence itself. The elements usually specified are lead, arsenic, cadmium and mercury, with the scope set by the applicable elemental-impurity guidance rather than by preference.

ICP-MS atomizes the sample in an argon plasma and counts ions at each element's mass, which gives element-specific quantitation several orders of magnitude below what any chromatographic method would notice. Like endotoxin, elemental impurities are entirely invisible to HPLC and to peptide mass spectrometry, so a panel that omits the test carries no information about them either way.

See alsoEndotoxin / LAL assayCertificate of analysis

A separation technique that resolves a mixture into components by their affinity for a solid stationary phase, used as the primary purity assay for peptides.

In the reversed-phase mode used for peptides, the sample is carried by a water and acetonitrile mobile phase over a non-polar stationary phase, typically C18-bonded silica. More hydrophobic species are retained longer. A detector, usually a UV detector set to 214 nm where the peptide bond absorbs, records absorbance against time, producing peaks whose area is integrated.

HPLC establishes homogeneity: how much of the detected material is one species. It does not establish what that species is. A truncated sequence, a deletion analog or an unrelated compound of similar hydrophobicity can co-elute or simply form its own clean peak and be reported as pure. Identity requires an orthogonal method, which is why a credible panel pairs HPLC with mass spectrometry.

See alsoPeptide purityLC-MSMass spectrometry

Chromatographic separation coupled directly to a mass spectrometer, giving purity and identity from a single chromatographic run.

The column eluate passes straight into the ion source, so every chromatographic peak carries a mass spectrum. That coupling is what makes the technique more informative than either method alone: a peak can be quantified by its area and identified by its mass in the same run, and impurity peaks can be characterized rather than merely counted.

Because impurities are assigned masses, LC-MS distinguishes between failure modes that HPLC alone reports identically. A minor peak at the target mass minus eighteen daltons indicates dehydration; plus sixteen indicates oxidation; a mass matching a shortened sequence indicates a deletion from synthesis. A purity result accompanied by that detail is materially stronger than a bare percentage.

See alsoHPLCMass spectrometryPeptide purity

A quantity of material produced in one manufacturing run under uniform conditions, carrying a single identifier.

The lot is the unit that testing applies to. A result obtained on one lot describes that lot and no other, because a subsequent synthesis of the same sequence is a separate event with its own impurity profile, its own residual solvent and its own content figure. Treating a compound as the testable unit rather than the lot is the error that makes a shared PDF look like evidence.

For a certificate to mean anything, the identifier has to be traceable in both directions: printed on the vial the customer holds, and printed on the certificate the customer reads. Where either end is missing, the document cannot be checked against the material.

See alsoCertificate of analysisCold chain

Removal of water from a frozen solution by sublimation under vacuum, leaving a dry solid.

The solution is frozen, then held under vacuum while the ice sublimes directly to vapour without passing through a liquid phase. A secondary drying stage at slightly higher temperature removes residual bound water. Peptides are supplied this way because a dry solid is far more chemically stable than a solution: hydrolysis, deamidation and oxidation all proceed much faster in water.

The result is a porous cake or a thin film on the vial wall. Cake appearance carries limited information about quality. A collapsed or shrunken cake indicates that the formulation warmed above its collapse temperature during drying, which is a process observation rather than a purity result, and a peptide present at only a few milligrams may form a film that is barely visible against the glass. Neither observation substitutes for an assay.

See alsoDegradationCold chain

Measurement of the mass-to-charge ratio of ionized molecules, used to confirm that a peptide's observed mass matches its sequence.

The sample is ionized, most often by electrospray, and the resulting ions are separated by mass-to-charge ratio. Peptides typically appear in several charge states, and the deconvoluted spectrum gives a neutral mass that is compared with the mass calculated from the sequence. Agreement within the instrument's stated tolerance is the identity confirmation on most certificates.

Mass is necessary evidence of identity but not sufficient on its own. Isomers, several isobaric residue substitutions and some rearrangements leave the mass unchanged. A mass match therefore rules out a great deal and proves rather less than it appears to. Tandem mass spectrometry, which fragments the ion and reads the resulting ladder, is the step that addresses sequence directly.

See alsoLC-MSMolecular weightAmino acid sequence

The mass of one mole of the compound, in daltons or grams per mole, calculated from its molecular formula.

Two figures appear in peptide documentation and they are not interchangeable. The monoisotopic mass uses the lightest stable isotope of each element and is what a high-resolution mass spectrometer reports. The average mass uses natural isotopic abundance and is the figure normally printed on a specification sheet. For a peptide of a few thousand daltons the two differ by roughly one to two units, which is enough to look like a discrepancy if the two are compared directly.

Molecular weight also depends on salt form. A figure quoted for the free base or free acid does not match the figure for an acetate or trifluoroacetate salt of the same peptide, and a certificate should state which it refers to.

See alsoMass spectrometryCounterion / salt formNet peptide content

The fraction of the material in a vial that is peptide, as distinct from water, counterion and residual solvent.

Purity and content answer different questions. Purity asks what proportion of the detected peptide material is the target. Content asks what proportion of the weighed solid is peptide at all. A preparation can be 99% pure and 78% peptide by content at the same time, with the remainder being trifluoroacetate, adsorbed water and inorganic salt, and both figures would be honestly reported.

Content is determined by amino acid analysis, by quantitative nitrogen determination, or by quantitative HPLC against a reference standard of known concentration. Amino acid analysis is the reference method: the sample is hydrolyzed to free amino acids, which are derivatized and quantified, giving both content and a composition check against the expected sequence.

Content is the figure most often omitted from certificates in this category, and its absence is the reason two vials labeled identically can differ substantially in the amount of peptide they contain.

See alsoPeptide purityCounterion / salt formCertificate of analysis

The proportion of peptide-related material in a sample that is the target peptide, normally measured by HPLC peak area.

A purity of 99% by RP-HPLC at 214 nm means the target peak accounts for 99% of the total absorbance area detected in that run. It is a relative measurement within one chromatogram, so the denominator matters: purity is expressed against everything the method detected, not against everything in the vial.

The consequences are specific. Anything that does not absorb at the chosen wavelength, does not retain on the column or elutes with the solvent front is outside the measurement. Water, counterion, inorganic salts and endotoxin are all in that category. This is why a certificate showing 99% purity says nothing at all about endotoxin load or about how much peptide a vial contains.

Purity figures are only comparable when the method is. Wavelength, column chemistry, gradient and run length all move the number, which is why a defensible certificate prints the method alongside the result.

See alsoHPLCNet peptide contentCertificate of analysis