HPLC vs. Mass Spectrometry: Two Different Questions on a Peptide COA

A certificate of analysis for a synthetic peptide usually reports two results that are easy to blur together: an HPLC purity percentage and a mass spectrometry result. They are not two measurements of the same thing. HPLC separates and measures what is in the sample, while mass spectrometry weighs molecules to show whether the main component is the intended one. This Vial Post explainer sets out what each technique can and cannot see, and why the impurities found in synthetic peptides make both necessary.

Start With the Impurities

It helps to know what a purity test is looking for. A 2014 review in the *Journal of Pharmaceutical and Biomedical Analysis* notes that most peptides today are made by solid-phase peptide synthesis (SPPS) and groups the related impurities found in peptide products (D'Hondt 2014):

• Synthesis-related: deletion and insertion of amino acids, linked to inefficient Fmoc deprotection and excess amino acid reagents; racemization that gives diastereomeric impurities; peptide-protection adducts from incomplete side-chain deprotection; oxidation; and dimers or oligomers.
• Counter-ions: unwanted counter-ions such as trifluoroacetate from the synthesis or purification.
• Contamination: other, unrelated peptides, which the authors link to a lack of appropriate GMP.
• Degradation-related: β-elimination, diketopiperazine, pyroglutamate and succinimide formation.

The review also warns that such impurities can greatly influence early functionality studies and possibly lead to erroneous conclusions (D'Hondt 2014). That is the practical reason researchers care about the purity line on a COA.

What HPLC Contributes

Reversed-phase HPLC separates the components of a sample as they pass through a column, so the target peptide and its impurities elute as separate peaks. Purity is usually reported as the target peak's share of the total peak area.

A detail that matters for peptides is the mobile phase. Trifluoroacetic acid (TFA) is widely used as an ion-pairing reagent in reversed-phase liquid chromatography for both purification and analytical characterization of peptides (Erckes 2025). It improves the chromatographic separation of peptides (Mao 2020). The same TFA can also stay with the peptide: Erckes and colleagues note that lyophilizing peptide solutions produces a peptide–TFA salt (Erckes 2025).

Two limits follow from how the method works:

• An HPLC purity percentage describes the peaks the method resolves. It does not, on its own, prove that the main peak is the intended sequence.
• A counter-ion such as trifluoroacetate is not a related impurity in the chromatographic sense, so it needs its own test. Erckes and colleagues validated FT-IR, 19F-NMR and HPLC with an evaporative light-scattering detector (HPLC-ELSD) to quantify residual TFA (Erckes 2025).

What Mass Spectrometry Contributes

Mass spectrometry measures mass-to-charge ratio, which gives the molecular mass of what elutes. For a peptide with a known sequence, that mass is predictable, so a match confirms identity and a mismatch flags a problem. Coupled to chromatography as LC-MS, it can also characterize individual impurity peaks.

A 2021 review by Lilly researchers in the *Journal of the American Society for Mass Spectrometry* describes LC-MS as increasingly used to characterize synthetic peptide impurities (Lian 2021). The review:

• notes that synthetic peptides are subject to many complex structural modifications from starting materials, manufacturing and storage;
• discusses thresholds for identifying and controlling impurities based on regulatory guidance;
• singles out structural isomers and stereoisomers (peptide epimers) as needing specific LC-MS techniques;
• critiques absolute and relative quantitation methods and potential pitfalls, and proposes a systematic LC-MS workflow (Lian 2021).

The emphasis on epimers is telling. A diastereomeric impurity from racemization (D'Hondt 2014) has the same atoms as the target peptide, so its mass alone does not set it apart; it has to be resolved chromatographically.

There is also a practical tension. TFA's ion-pairing benefit in separation comes with significant signal suppression in electrospray ionization mass spectrometry (Mao 2020). Mao and colleagues reported that adding a small amount of glycine to TFA mobile phases boosted peptide signal-to-noise by more than an order of magnitude on average without compromising separation (Mao 2020).

Reading the Two Results Together

• HPLC purity high, mass correct: the main component is the expected molecule, and the method resolved few other peaks.
• HPLC purity high, mass wrong: a clean sample of the wrong compound.
• Mass correct, HPLC purity low: the right molecule is present, alongside a meaningful share of other material.
• Neither tells you the counter-ion content: that needs a separate test such as ion chromatography (Mrozik 2012) or 19F-NMR and HPLC-ELSD (Erckes 2025).

Frequently Asked Questions

Does 99% HPLC purity mean 99% of the powder is peptide? No. HPLC purity describes chromatographic peaks. Counter-ions such as trifluoroacetate can still be present in the solid (D'Hondt 2014; Erckes 2025).

Can mass spec alone replace HPLC? Not for stereoisomers. An epimer has the same composition as the target, which is why Lian and colleagues highlight specific LC-MS techniques for isomers (Lian 2021).

Why does TFA come up in both tests? It is a common HPLC ion-pairing reagent, it suppresses electrospray MS signal, and it can stay in the final product as a counter-ion (Erckes 2025; Mao 2020).

Further Reading

For a line-by-line walk-through of a certificate, see the Dynamite Research Peptides Certificate of Analysis guide, and our own explainer on reading a Certificate of Analysis.

For research and educational use only. This article is not medical advice and does not describe dosing or human use.