HPLC and Mass Spectrometry: How Peptide Purity and Identity Are Verified

Two analytical questions sit behind every research peptide: is this the right molecule, and how much of the vial is that molecule rather than something else? Mass spectrometry answers the first. Reversed-phase HPLC answers the second. Neither answers both, which is why a credible analysis reports both.

Reversed-phase HPLC: the purity measurement

In RP-HPLC the sample is dissolved and pushed through a column packed with a hydrophobic stationary phase, typically C18-bonded silica. A gradient of increasing organic solvent — usually acetonitrile against water, both with a small amount of trifluoroacetic acid — progressively elutes compounds in order of hydrophobicity. A UV detector at the column outlet records absorbance against time, producing the chromatogram.

The target peptide appears as the main peak. Synthesis-related impurities appear as separate peaks: deletion sequences missing a residue, truncated chains, incompletely deprotected intermediates, and oxidation or deamidation products. Purity is reported as the main peak’s area divided by total peak area.

Details that change the number

• Detection wavelength. Peptide bonds absorb around 214 nm; aromatic side chains absorb around 280 nm. A purity figure at 214 nm and one at 280 nm are not comparable, because the second under-reports anything lacking aromatic residues. Analyses should state the wavelength.
• Gradient shallowness. A steep gradient compresses everything into a narrow window and can hide a close-eluting impurity inside the main peak. A shallow gradient separates them. Two labs can report meaningfully different purity for identical material purely from gradient choice.
• Peak shape. A symmetrical peak suggests one compound. Shouldering, tailing or a flattened apex often indicates co-elution — two species leaving the column at nearly the same time and being counted as one.

This is why the chromatogram itself matters more than the percentage. A trace lets you see whether the non-target 2% is one substantial impurity or a scatter of trace peaks, and whether the main peak is genuinely clean.

Mass spectrometry: the identity measurement

HPLC establishes that a sample is homogeneous. It does not establish what the homogeneous thing is. Mass spectrometry ionises the sample and measures mass-to-charge ratio, giving an observed molecular weight to compare against the theoretical weight calculated from the sequence.

Two ionisation methods dominate peptide work. ESI-MS (electrospray) produces multiply charged ions and pairs naturally with liquid chromatography, so LC-MS can separate and identify in one run. MALDI-TOF embeds the sample in a matrix, ionises it with a laser pulse, and measures flight time to a detector; it tends to produce singly charged ions and tolerates salts better.

An analysis should report theoretical and observed mass together. Agreement within the instrument’s accuracy confirms identity. A discrepancy of one residue’s mass indicates a deletion sequence; a discrepancy of +16 suggests oxidation; +1 can indicate deamidation.

Net peptide content: the figure most often missing

Neither technique tells you how much peptide is in the vial. Lyophilised peptides are supplied as salts — commonly trifluoroacetate — and are hygroscopic, so a vial labelled 10 mg contains peptide plus counter-ion plus residual water. HPLC purity is a ratio among peptide species and says nothing about that mass balance.

Net peptide content is determined by amino acid analysis or elemental nitrogen analysis, and typically falls between 70% and 90%. Water content is measured by Karl Fischer titration. Without these figures, any concentration calculated gravimetrically from vial weight carries an unquantified error — and comparisons between suppliers quoting only HPLC purity are not like-for-like.

Reading the two together

A complete picture needs three numbers: identity confirmed by mass spectrometry, purity by HPLC at a stated wavelength with the chromatogram shown, and net peptide content. High purity with unconfirmed identity means a pure unknown. Confirmed identity with unstated purity means the right molecule in unknown company. Both, without net content, means unknown concentration.

Related reading: how to read a peptide certificate of analysis and peptide storage and stability.

All products referenced are supplied for laboratory research use only and are not approved for human consumption, clinical, or veterinary use.