HPLC and Mass Spectrometry: How Peptide Purity Is Actually Measured

"99% pure" is a claim about one specific measurement made one specific way. Two suppliers can both truthfully claim 99% and be selling materials of noticeably different quality, because purity depends entirely on how you look.

RP-HPLC: the purity question

Reverse-phase high-performance liquid chromatography separates molecules by hydrophobicity.

The sample is injected onto a column packed with silica particles coated in hydrocarbon chains — C18 is the workhorse. A mobile phase, typically water and acetonitrile with 0.1% trifluoroacetic acid, flows through under pressure. The acetonitrile proportion is increased over the run: this is the gradient.

Hydrophilic components elute early. Hydrophobic ones stick to the stationary phase and only come off as the organic content rises. A UV detector at 214 nm — where the peptide bond absorbs — records what comes out and when.

Purity is the area of the main peak as a percentage of total peak area. That is it. Which immediately raises the question of what the method can see.

Why the method matters as much as the number

Gradient slope. A shallow gradient spreads the run out and resolves impurities that elute close to the main peak. A steep gradient compresses everything and can bury a 2% impurity inside the main peak entirely. A supplier reporting purity from a fast, steep method is not lying — but they are looking through a wider lens.

Detection wavelength. 214 nm detects the peptide bond and therefore sees essentially everything peptidic. 280 nm detects aromatic residues only and will miss impurities lacking tryptophan, tyrosine or phenylalanine. Reputable COAs report 214 nm.

Column chemistry and temperature. Different stationary phases resolve different impurity pairs. A single method can never guarantee it separated everything.

Injection load. Overload the column and peaks broaden and merge.

This is why a COA should state its method, not just its result. "99.2% by RP-HPLC" is a headline. "99.2% by RP-HPLC, C18 4.6×250 mm, 5–65% MeCN over 30 min, 0.1% TFA, 214 nm, 40 °C" is data.

Mass spectrometry: the identity question

HPLC cannot tell you what the main peak is. It could be 99% pure something-else. Mass spectrometry answers that.

Electrospray ionisation (ESI) sprays the sample from a charged capillary, producing multiply charged ions. A peptide of mass 3000 Da might appear at m/z 1001 (3+), 751 (4+) and 601 (5+). Software deconvolutes this charge envelope back to a single neutral mass. ESI is the standard for peptides because it is gentle and couples directly to HPLC.

MALDI-TOF embeds the sample in a matrix, hits it with a laser and measures flight time to a detector. It produces mainly singly charged ions, so the spectrum reads more directly, and it tolerates salts and impurities better than ESI. It is generally less accurate on mass.

Either way, the question is the same: does the observed mass match the theoretical mass calculated from the sequence? For a peptide of a few thousand daltons, agreement within one or two daltons is expected from a good instrument.

What the impurity masses tell you

The real value of MS is diagnostic. Common findings map to specific synthesis failures:

• -18 Da — loss of water; dehydration or aspartimide formation
• -17 Da — loss of ammonia; often from an N-terminal glutamine
• +16 Da — oxidation, usually at methionine
• Mass of one residue missing — a deletion sequence; a coupling step failed
• +42 Da — acetylation, sometimes from capping during synthesis
• Doubling of mass — dimerisation, frequently through a disulfide bridge

A supplier who publishes the mass spectrum lets you diagnose these yourself. A supplier who publishes only "MS: conforms" does not.

The techniques that fill the gaps

Karl Fischer titration measures water content. Essential for accurate concentrations, and routinely omitted.

Amino acid analysis hydrolyses the peptide and quantifies the constituent amino acids, giving both composition confirmation and net peptide content.

Elemental analysis confirms nitrogen content, which cross-checks peptide content independently.

Chiral analysis detects D-amino acid racemisation, which HPLC and MS both miss entirely — a racemised peptide has identical mass and often near-identical retention time, but different biology.

What to ask for

For most bench work, RP-HPLC at 214 nm plus ESI-MS with published traces is a reasonable standard. For quantitative work — anything where concentration precision drives your conclusions — add Karl Fischer and net peptide content. For structure-activity work, ask about chiral purity.

Every batch we sell publishes HPLC purity with the chromatogram, ESI-MS identity with the spectrum, and water content by Karl Fischer, tested by an independent laboratory.

For laboratory research use only.