HPLC vs mass spectrometry: how peptide identity is tested

Research-use notice: this article is for legitimate laboratory and educational research only. It does not provide guidance for human use, administration, dosing or treatment.

High-performance liquid chromatography (HPLC) shows how a peptide sample separates and how its signal divides between peaks; mass spectrometry measures the mass of the molecules that are present. An HPLC purity figure is a peak-area percentage, not proof of identity, and a correct mass is evidence of identity, not a measure of purity. That is why European guidance for synthetic peptides recommends at least two orthogonal identity methods. Neither test says anything about sterility or suitability for use in people.

What does HPLC measure?

HPLC pumps a dissolved sample through a packed column. For peptides the usual mode is reversed-phase HPLC: peptides stick to a water-repelling (hydrophobic) stationary phase and are washed off as the proportion of organic solvent in the mobile phase rises. A methods chapter by Mant and colleagues calls reversed-phase HPLC the most widely used mode for peptide separations. It describes peptides eluting in order of increasing hydrophobicity, and explains why mobile phases of aqueous trifluoroacetic acid and acetonitrile are preferred: they are volatile, and trifluoroacetic acid’s ion-pairing properties help separate complex peptide mixtures.

The detector is usually ultraviolet (UV). As the same chapter notes, peptide bonds absorb strongly in the far UV, below 220 nm, so detection is generally set at 210–220 nm. The output is a chromatogram: signal plotted against time. Each component appears as a peak at a retention time, and the purity figure on a report is normally the main peak’s area divided by the total integrated area, multiplied by 100.

That number has clear limits:

• Co-elution. Two species that leave the column together appear as one peak.
• Detector response. Different molecules absorb UV differently, so peak areas are an approximation of relative amounts.
• What is not seen. Water, counterions and salts are not peptide peaks. An area percentage therefore says nothing about how much of the powder is peptide.
• No structure. A retention time identifies nothing unless it is matched to a reference standard run under the same conditions.

Our guide 99% purity: what does that number actually tell you? explains area percentage in more detail.

What does mass spectrometry measure?

Mass spectrometry (MS) turns molecules into ions and measures them. As Steen and Mann put it in their 2004 review of peptide sequencing in Nature Reviews Molecular Cell Biology, mass spectrometry does not measure the mass of molecules but their mass-to-charge ratio, written m/z. Electrospray ionisation, the usual source for peptides, produces ions carrying several charges, so one peptide can appear at several m/z values. The observed m/z is multiplied by the charge and corrected for the added protons to give the molecule’s mass in daltons (Da).

Two levels of MS evidence matter for identity:

• Intact mass. The observed mass is compared with the theoretical mass of the expected sequence. High-resolution instruments measure mass closely enough to narrow the possible elemental compositions; the 2025 peptide-pool study described below searched within 5 parts per million (ppm) of each expected mass.
• Tandem MS (MS/MS). The instrument isolates an ion and breaks it, mostly at peptide bonds, giving fragment series known as b and y ions. The mass gaps between fragments read out the sequence and can show where a modification sits.

The European Medicines Agency (EMA) guideline on synthetic peptides describes MS as a powerful tool for structure elucidation: variants of the technique can determine molecular mass and confirm amino acid sequence.

MS also has blind spots. Molecules with the same elemental composition have the same mass. Leucine and isoleucine are a classic pair. Aspartate and isoaspartate, formed when asparagine deamidates, are another, and a 2026 study in RSC Medicinal Chemistry noted that conventional chromatography and standard MS often fail to tell such near-identical peptides apart. Mirror-image (D- and L-) forms of an amino acid, which can arise through racemisation during synthesis, also share a mass. MS signal strength depends on how readily each species ionises, so a spectrum on its own is not normally used as a purity percentage.

What is LC-MS?

Liquid chromatography–mass spectrometry (LC-MS) connects the two. The column separates the sample, and the mass spectrometer measures what elutes at each moment. This lets an analyst attach a mass to each peak instead of only to the sample as a whole. Many LC-MS reports still take the purity figure from a UV trace recorded in the same run, and the identity evidence from the mass spectrum.

Ultra-high-performance LC with high-resolution MS (UHPLC-HRMS) is the more demanding form. In its 2021 guidance on generic versions of certain synthetic peptides, the US Food and Drug Administration (FDA) recommends sensitive, high-resolution procedures, for example UHPLC-HRMS, to detect and characterise peptide-related impurities, and asks applicants to identify each peptide-related impurity at 0.10% of the drug substance or above.

HPLC vs mass spectrometry: what each can and cannot show

Why one test is not enough: orthogonal methods

“Orthogonal” means using a second method that relies on a different measurement principle, so that the weaknesses of one are covered by the other. ICH Q2(R2), the International Council for Harmonisation (ICH) guideline on validating analytical procedures, says that where one procedure does not discriminate well enough, a combination of two or more is recommended. It also says an identification test should be applied to closely related materials to confirm that they do not give a positive result.

The EMA guideline on the development and manufacture of synthetic peptides, legally effective from 1 June 2026, applies that principle to peptides. It lists identification by mass, relative retention time, LC-MS, peptide mapping, bioactivity, amino acid analysis or nuclear magnetic resonance (NMR) as appropriate, and recommends using at least two orthogonal methods to test identity. It also cites European Pharmacopoeia thresholds for peptide-related impurities: reported above 0.1%, identified above 0.5% and qualified above 1.0%.

The older ICH Q6B guideline, written for proteins made in cell-culture systems, makes a similar point: identity tests should be highly specific and more than one may be needed. Q6B explicitly excludes synthetic peptides, which the dedicated EMA guideline now covers. All of these documents set standards for medicines. They do not certify research materials, but they show what level of evidence regulators regard as a complete identity answer.

What the research covers

The evidence here is analytical: laboratory measurements on defined materials, plus guidance documents. There is no clinical dimension. Recent studies show why the second method matters.

• Low resolution can mislead. A 2026 study in the Journal of Pharmaceutical and Biomedical Analysis traced apparent +56 Da impurities in a 36-residue model peptide to four different origins. One was a +57 Da impurity from a double glycine insertion, which overlapped with a +56 Da tert-butylation product at low resolution. High-resolution MS also showed that some of the remaining signal came from non-covalent adducts and solvent clusters formed during electrospray ionisation. In vitro analytical study.
• UV and MS together. Earlier work had shown that UHPLC-HRMS can confirm the identity and relative amount of all the peptides expected in a peptide pool. A 2025 study in Separations went back to the UV traces and mass spectra of a pool of 32 synthetic peptides to look for impurities. It found dimers, pyroglutamate and aspartimide formation, deamidation, methionine oxidation and, in the sample examined, only very small traces of deletion peptides. In vitro analytical study.
• Reviews. A 2014 review by D’Hondt and colleagues sorts the related impurities found in peptide medicines by origin. Those linked to synthesis include deletion and insertion of amino acids, racemisation, oxidation and unwanted trifluoroacetate counterions. Review.

These findings apply to the materials and methods studied. They do not show that any particular research peptide contains these impurities. Our guides to sequence impurities and deamidation and isoaspartate look at two of these problems in depth.

How to read HPLC and MS data on a batch report

A batch laboratory report or supplier-provided certificate of analysis (COA) should let you see which question each number answers. Check:

• The batch number and date match the vial and the report reference.
• The HPLC method is described: column, detection wavelength and, ideally, the chromatogram itself with every integrated peak visible.
• The purity figure is read as an area percentage under that method, not as peptide content.
• The MS result gives observed and theoretical masses, states whether they are monoisotopic or average, and shows the ions used.
• It is clear what the MS confirms: usually the main peak only, not every smaller peak.
• Peptide content or counterion form is reported separately if it was measured. Neither HPLC purity nor intact mass gives it. See our article on TFA salts and net peptide content.

Thistle shows a batch laboratory report on each product page, and you can look up a batch on our batch report lookup. For the full method, see How to read a peptide COA and What a COA cannot tell you.

HPLC vs mass spectrometry FAQs

Is HPLC or mass spectrometry better for peptide testing?

Neither replaces the other. HPLC is the usual way to estimate chromatographic purity, and MS is the usual way to confirm that the expected mass and sequence are present. A complete identity answer normally uses both.

Can HPLC confirm the identity of a peptide?

Only indirectly, by matching retention time to a reference standard run under the same conditions. On its own, a chromatogram shows separation, not structure.

What does LC-MS stand for?

Liquid chromatography–mass spectrometry. The liquid chromatograph separates the sample and the mass spectrometer measures the mass-to-charge ratio of what comes off the column.

Does a correct mass mean a peptide is pure?

No. A matching mass shows that the expected molecule is present. It does not show how much of the sample it makes up, or rule out impurities and isomers that share the same mass.

Why does a mass spectrum show several peaks for one peptide?

Electrospray ionisation gives a peptide several different charges, and each charge state appears at its own m/z value. Isotopes add further small peaks. Software combines them to calculate one neutral mass.

Sources

• European Medicines Agency. Guideline on the development and manufacture of synthetic peptides. EMA/CHMP/CVMP/QWP/367182/2025. Legally effective 1 June 2026. Medicinal-product guidance.
• International Council for Harmonisation. ICH Q2(R2) Validation of analytical procedures. EMA/CHMP/ICH/82072/2006. Effective 14 June 2024. Guideline.
• International Council for Harmonisation. ICH Q6B Specifications: test procedures and acceptance criteria for biotechnological/biological products. Step 4, 10 March 1999. Guideline; excludes synthetic peptides.
• US Food and Drug Administration. ANDAs for Certain Highly Purified Synthetic Peptide Drug Products That Refer to Listed Drugs of rDNA Origin. Guidance for industry. May 2021.
• Mant CT, Chen Y, Yan Z, et al. HPLC analysis and purification of peptides. Methods in Molecular Biology. 2007;386:3–55. Methods chapter.
• Steen H, Mann M. The abc’s (and xyz’s) of peptide sequencing. Nature Reviews Molecular Cell Biology. 2004;5(9):699–711. Review.
• D’Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis. 2014;101:2–30. Review.
• Liu W, Zhu H, Wang Y, et al. Possible origin and suppression of apparent +56 Da impurities during peptide synthesis. Journal of Pharmaceutical and Biomedical Analysis. 2027;282:117720. Available online 3 September 2026. Model peptide.
• Bosc-Bierne G, Weller MG. Investigation of impurities in peptide pools. Separations. 2025;12(2):36.
• Erckes V, Chamera Rendueles L, Misiek A, Steuer C. Revealing deamidation and isoaspartate formation during peptide analysis, purification and storage by tandem mass spectrometry. RSC Medicinal Chemistry. 2026;17(2):1144–1154. Model peptides.

Published by Thistle BioLabs, a UK supplier of research materials. This article summarises external research, not studies conducted by Thistle. Educational information only; no medical advice or recommendation for personal use.