Research Peptide Purity & Testing: A Reference Guide

In the lab, a peptide is only as good as it is correctly identified and pure. Misidentified or contaminated material is one of the biggest drivers of irreproducible results. This reference walks through how a research peptide's purity and identity are verified, what a certificate of analysis (CoA) really tells you, and how lyophilised peptides should be stored. It's written for qualified researchers; everything here is supplied for in-vitro research use only.

Measuring purity: reversed-phase HPLC

Purity is most often measured by reversed-phase high-performance liquid chromatography (RP-HPLC). The sample runs through a hydrophobic (usually C18) column on a water/acetonitrile gradient with an acidic modifier, and is read by UV absorbance — typically at 214 nm, where the peptide bond absorbs strongly, and sometimes 280 nm for aromatic residues.

The result is the share of total peak area belonging to the main peptide peak. A spec of ≥99% by HPLC means that principal peak is at least 99% of the integrated UV signal, with related impurities making up the rest. Those impurities are usually deletion or truncated sequences (residues dropped during synthesis), incompletely deprotected species, and oxidation products.

Confirming identity: mass spectrometry

HPLC tells you how much of one species is present, not what it is. Identity comes from mass spectrometry — typically electrospray (ESI-MS) or MALDI-TOF — comparing the observed molecular mass against the theoretical mass from the peptide's sequence. A match inside the instrument's tolerance confirms the right molecule was made; a mismatch flags a sequence error or modification.

What a Certificate of Analysis reports

A useful CoA is batch-specific, and usually lists:

Appearance — e.g. a white lyophilised powder.
Identity — observed vs theoretical mass (from MS).
Purity — HPLC % area, with the chromatogram.
Net peptide content — the true peptide fraction of the powder. It matters because synthetic peptides are hygroscopic and carry bound water and counter-ions, so the actual peptide mass is usually less than the gross powder mass. Measured by amino-acid analysis (AAA) or nitrogen analysis.
Water content — commonly by Karl Fischer titration.
Counter-ion content — solid-phase peptides are usually isolated as acetate or trifluoroacetate (TFA) salts; the counter-ion is quantified by ion chromatography or HPLC.
Batch number and test date — for traceability.

Net peptide content is the figure researchers most often miss: two vials both labelled "5 mg" can hold different real amounts of peptide if their salt and water content differ.

Why peptides ship lyophilised

Most research peptides come as a lyophilised (freeze-dried) powder. Pulling the water off under vacuum sharply slows the hydrolytic and enzymatic breakdown that hits peptides in solution, leaving a stable, long-shelf-life solid that's also lighter and tougher to ship.

Storing lyophilised peptides

Kept properly, lyophilised peptides last a long time. The essentials:

Keep the sealed powder cold — typically −20°C, or −80°C for the long term.
Keep it dry (desiccated) and out of the light; let sealed vials warm to room temperature before opening to avoid condensation.
Keep freeze–thaw cycles to a minimum — they degrade peptides over time.

(This covers storage of the supplied material only. Vosberg does not provide preparation, handling-for-use or dosing guidance — our products are reference materials for in-vitro research, not for human or veterinary use.)

Requesting a certificate of analysis

Every batch we list gets a traceable batch number and an HPLC/MS certificate of analysis, available on request. See quality & testing for our process, browse the catalogue, or read the companion explainer on what HPLC purity testing means.