Research Guides

Peptide Purity Testing Explained: What HPLC and Mass Spectrometry Measure

By the Nexaph team · Updated 6 November 2026 · 4 min read

“99% purity” appears on a lot of research peptide labels, but peptide purity testing involves more than one number. Two techniques do most of the work: high-performance liquid chromatography (HPLC), which separates the peptide from related impurities, and mass spectrometry (MS), which confirms the molecule’s identity. A third measurement, net peptide content, answers a different question altogether. Understanding what each one measures helps you read results critically and compare suppliers fairly.

What “purity” means for a synthetic peptide

Most research peptides are made by solid-phase peptide synthesis, building the chain one amino acid at a time. No synthesis step is perfectly efficient, so the crude product contains the target peptide plus closely related by-products. Purification, usually by preparative HPLC, removes most of them. Purity testing then reports how much of what remains is the target peptide.

Crucially, “purity” in this sense is usually chromatographic purity: the proportion of the target peptide among the peptide-related species that the method can detect.

How HPLC peptide purity is measured

Reversed-phase HPLC (RP-HPLC) passes the dissolved sample through a column packed with a hydrophobic material. Peptides with different sequences or modifications interact with the column differently and elute at different times. A UV detector, typically set around 214–220 nm where the peptide bond absorbs, records each component as a peak.

• The main peak should be the target peptide.
• Minor peaks represent related impurities.
• Purity is reported as the main peak’s area as a percentage of the total peak area.

Because the result depends on the column, gradient and wavelength, two labs can report slightly different purities for the same sample. A credible result shows the chromatogram and states the method.

What HPLC purity does not tell you

• Identity: HPLC shows a dominant peak, not what that peak is. A wrong peptide can still produce one clean peak.
• Water and salts: residual moisture and counter-ions such as trifluoroacetate (TFA) or acetate do not appear as UV peaks in the same way, so they are not reflected in the purity percentage.
• Co-eluting impurities: an impurity that elutes at the same time as the main peptide can hide inside the main peak.

This is why HPLC is almost always paired with mass spectrometry.

Mass spectrometry and peptide identity

Mass spectrometry measures the mass-to-charge ratio of ionised molecules. For peptides, electrospray ionisation (ESI) and MALDI are the common techniques. The observed mass is compared with the theoretical mass calculated from the sequence and any modifications.

A close match confirms that the expected molecule is present. Mass differences are often diagnostic: a loss of one residue’s mass suggests a deletion sequence, +16 Da suggests oxidation, and roughly −1 Da can distinguish a C-terminal amide from a free acid. Coupling the two techniques as LC-MS lets the lab check the mass of the main HPLC peak directly and identify the minor peaks around it.

Net peptide content is a separate measurement

A peptide can be 99% pure by HPLC and still be well under 100% peptide by weight. Lyophilized peptides carry counter-ions from purification and some bound water. Net peptide content, the percentage of the powder’s weight that is actually peptide, is typically measured by amino acid analysis or by elemental nitrogen analysis.

For quantitative work this distinction matters: if you weigh out 10mg of powder, the amount of peptide you have depends on net peptide content, not on HPLC purity. When it is not reported, ask the supplier whether a stated amount refers to gross weight or to net peptide.

A worked example

Suppose a certificate reports 98% HPLC purity and 80% net peptide content, and you weigh out 10.0mg of the lyophilized powder. The two figures combine like this:

• Peptide in the powder: 10.0mg × 80% net peptide content = 8.0mg of peptide material.
• Target peptide within that: 8.0mg × 98% HPLC purity ≈ 7.8mg of the intended sequence.
• The rest: about 2.0mg of counter-ions and water, plus about 0.2mg of related peptide impurities.

The numbers here are illustrative, not typical values for any product. The point is that HPLC purity and net peptide content answer different questions, and quantitative work needs both. If only one is available, ask the supplier for the other before you calculate concentrations.

Common peptide impurities

A review of impurities in peptide products groups them into synthesis-related impurities and degradation products (D’Hondt et al., 2014). Examples include:

• Deletion and truncated sequences: chains missing one or more residues.
• Incomplete deprotection: protecting groups left attached after synthesis.
• Diastereomers: residues that racemised during synthesis.
• Oxidation: particularly of methionine, cysteine or tryptophan.
• Deamidation, pyroglutamate and diketopiperazine formation: degradation pathways that can occur during storage.
• Dimers and aggregates.

Some of these, such as oxidation and deamidation, can increase after manufacture, which is one reason storage matters. See our guide on how to store lyophilized peptides.

Peptide purity testing: comparing suppliers

• Ask for batch-specific results, not a generic specification.
• Check that the HPLC chromatogram and the mass spectrum are both provided.
• Compare methods: purity figures from very different methods are not directly comparable.
• Ask whether net peptide content has been measured.
• Treat precise-sounding but unsupported numbers with caution.

Our guide to reading a peptide certificate of analysis turns these points into a section-by-section checklist, and the supplier checklist covers what else to look for.

Purity information for Nexaph kits

Nexaph vial labels state the product name, the amount per vial and a 99% purity figure. The SKU on each product page is the batch code for the current lot. Email support@nexaphpeps.com with that SKU to request the batch documentation for a specific kit, such as TB4 10mg or MOTS-c 10mg. This guide is for laboratory research purposes only and is not medical advice.