Peptide laboratory testing methods

No single test tells you everything about a peptide. Purity, identity, how much peptide is present, water content and microbial screening are separate questions, each answered by a different method — and each method has a clear limit. Being explicit about those limits is what separates a credible certificate from a reassuring one.

This page describes the common analytical methods used to characterise research peptides, what each one demonstrates, and — just as importantly — what each one cannot tell you.

Reverse-phase HPLC

Reverse-phase HPLCReverse-phase HPLCThe most common HPLC mode for peptides, separating molecules by how strongly they interact with a non-polar column.Full definition → separates the components of a dissolved sample by how strongly each interacts with a non-polar column, so they elute at different times and form a chromatogramChromatogramThe plot produced by a chromatography run, showing detector signal over time as separated components pass through.Full definition →. It is the standard method for peptide purity because it resolves closely related sequences and degradation products well. The purity figure is the main peak's area as a percentage of total peak area, following chromatographic principles in USP–NF <621> (USP ↗) and the validation principles of ICH Q2(R2) ↗.

What it cannot tell you: HPLC does not confirm the compound's identity on its own, and it does not report how much peptide is present by mass. It measures composition, not quantity.

Mass spectrometry (ESI-MS and MALDI-TOF)

Mass spectrometryMass spectrometryAn analytical technique that measures the mass-to-charge ratio of ionised molecules to confirm a compound's identity by its molecular weight.Full definition → ionises the sample and measures the mass-to-charge ratio of the resulting ions, yielding a molecular weightMolecular weightThe mass of one molecule of a compound, expressed in daltons (Da) — the value mass spectrometry checks against the expected figure.Full definition →. The common formats for peptides are electrospray ionisation (ESI-MS) and MALDI-TOFMALDI-TOFMatrix-assisted laser desorption/ionisation time-of-flight, a mass-spectrometry technique used to confirm a peptide's molecular weight.Full definition →. Identity is confirmed when the observed molecular weight matches the value calculated from the peptide's sequenceSequenceThe specific order of amino acids in a peptide, which defines its identity and its theoretical molecular weight.Full definition →.

What it cannot tell you: a mass match confirms identity but says nothing about purity or quantity. A correctly identified peptide can still contain impurities or less peptide than its label implies.

Amino acid analysis

Amino acid analysisAmino acid analysisA quantitative method that hydrolyses a peptide into its constituent amino acids to measure how much peptide is actually present.Full definition → hydrolyses the peptide into its constituent amino acids and quantifies them against reference standards (USP–NF <1052>). Because it measures the amino acids directly, it reports net peptide contentNet peptide contentThe actual mass of peptide in a sample, excluding salt, residual water and excipients — measured by amino acid analysis, not HPLC.Full definition → — the true mass of peptide in a sample.

What it cannot tell you: it quantifies peptide mass but does not, by itself, resolve which impurities are present or their individual sizes the way a chromatogram does.

Water content: Karl Fischer and loss on drying

Karl Fischer titrationKarl Fischer titrationA titration method that measures water content specifically, used to determine residual moisture in a lyophilised powder.Full definition → measures water specifically through a chemical reaction, while loss on dryingLoss on dryingA method that measures the total mass lost when a sample is dried, used as an estimate of residual moisture and volatiles.Full definition → measures total volatile mass lost on heating. Residual moisture matters because it adds to gross weight and drives degradation such as hydrolysis during storage; water determination is standardised in USP–NF <921>.

What it cannot tell you: loss on drying cannot distinguish water from other volatiles; neither method says anything about identity or purity.

Endotoxin testing (LAL)

The LAL testLAL testThe Limulus amebocyte lysate assay, a test that detects and quantifies bacterial endotoxin.Full definition → detects and quantifies bacterial endotoxinEndotoxinA heat-stable lipopolysaccharide from the outer membrane of Gram-negative bacteria, screened for because it is not removed by sterilisation.Full definition →, a heat-stable molecule that survives sterilisation. It is a quality parameter for laboratory materials and is standardised for bacterial endotoxins in USP–NF <85>.

What it cannot tell you: an endotoxin result is not a sterility result — a material can pass one and fail the other — and it says nothing about chemical purity or identity.

Sterility and bioburden

In a laboratory-materials context, sterility testing (USP–NF <71>) checks for viable microorganisms and bioburden quantifies the microbial load present. Sterile filtrationSterile filtrationPassing a solution through a 0.22-micron filter to remove bacteria, a sterilisation step that does not remove endotoxin.Full definition → through a 0.22-micron membrane is the usual sterilisation route for heat-sensitive peptides.

What it cannot tell you: sterility removes whole organisms but not the endotoxinEndotoxinA heat-stable lipopolysaccharide from the outer membrane of Gram-negative bacteria, screened for because it is not removed by sterilisation.Full definition → they may have already released, which is why the two are tested separately.

Independent versus in-house testing

Third-party testingThird-party testingAnalysis performed by an independent laboratory with no commercial stake in the result, a stronger evidence claim than in-house testing.Full definition → means the certificate is produced by an external laboratory — for example Janoshik Analytical ↗ — rather than by the supplier. Independence matters: a laboratory with no commercial stake in the result has no incentive to report a flattering number. An in-house figure is a weaker claim than one from a named, contactable independent lab that hosts its own record of the result.

Summary: test, question, limit

The final column is the one that builds trust. A supplier that is candid about the limits of its own testing is describing reality rather than selling it.