Research Peptide Purity & Testing: A Reference Guide

For laboratory research, a peptide is only as useful as it is identified and pure. Impurities and misidentified material are a leading cause of irreproducible results. This reference explains how research peptide purity and identity are verified, what a certificate of analysis (CoA) actually reports, and how lyophilised peptides should be stored. It is written for qualified researchers; all material is supplied for in-vitro research use only.

How purity is measured: reversed-phase HPLC

Peptide purity is most commonly assessed by reversed-phase high-performance liquid chromatography (RP-HPLC). The sample is separated on a hydrophobic (typically C18) column using a water/acetonitrile gradient with an acidic modifier, and detected by UV absorbance — usually at 214 nm, where the peptide bond absorbs strongly, and/or 280 nm for aromatic residues.

Purity is reported as the percentage of the total peak area attributable to the main peptide peak. A specification of ≥99% by HPLC means the principal peak accounts for at least 99% of the integrated UV signal, with related impurities making up the remainder. Those impurities are typically deletion or truncated sequences (residues missing during synthesis), incompletely deprotected species, and oxidation products.

How identity is confirmed: mass spectrometry

HPLC tells you how much of one species is present, not what it is. Identity is confirmed by mass spectrometry — usually electrospray ionisation (ESI-MS) or MALDI-TOF — by comparing the observed molecular mass to the theoretical mass calculated from the peptide's amino-acid sequence. A match within the instrument's tolerance confirms the correct molecule was synthesised; a mismatch indicates a sequence error or modification.

What a Certificate of Analysis (CoA) reports

A meaningful CoA is batch-specific and typically reports:

Appearance — e.g. white lyophilised powder.
Identity — observed vs theoretical mass (from MS).
Purity — HPLC % area, with the chromatogram.
Net peptide content — the actual peptide fraction of the powder. This matters because synthetic peptides are hygroscopic and carry bound water and counter-ions, so the true peptide mass is usually less than the gross powder mass. It is determined by amino-acid analysis (AAA) or nitrogen analysis.
Water content — commonly by Karl Fischer titration.
Counter-ion content — peptides from solid-phase synthesis 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 overlook: two vials labelled "5 mg" can contain different actual quantities of peptide if their salt and water content differ.

Why peptides are supplied lyophilised

Most research peptides are supplied as a lyophilised (freeze-dried) powder. Removing water under vacuum greatly slows the hydrolytic and enzymatic degradation pathways that affect peptides in solution, giving a stable, long-shelf-life solid that is also lighter and more robust to ship.

Storage and stability of lyophilised peptides

Stored correctly, lyophilised peptides are stable for extended periods. General principles:

Keep the sealed, lyophilised powder cold — typically −20°C, or −80°C for long-term storage.
Keep it dry (desiccated) and protected from light; allow sealed vials to reach room temperature before opening to avoid condensation.
Minimise freeze–thaw cycles, which degrade peptides over time.

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

Requesting a certificate of analysis

Every batch we list is assigned 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 related explainer on what HPLC purity testing means.