HPLC in Peptide Analysis

High-performance liquid chromatography (HPLC) is the standard technique for measuring the purity of synthetic peptides. Nearly every peptide certificate of analysis reports an HPLC purity value, including all of the COAs on our Certificates of Analysis page. This guide explains how the method works, what the purity number means and what HPLC cannot tell you on its own.

How reversed-phase HPLC separates peptides

Peptide purity is almost always measured by reversed-phase HPLC (RP-HPLC). The sample is dissolved and loaded onto a column packed with silica particles bonded to hydrophobic chains, most commonly C18 (octadecyl). A mobile phase flows through the column, starting mostly aqueous and becoming progressively richer in an organic solvent, usually acetonitrile, over a programmed gradient. An ion-pairing acid such as trifluoroacetic acid (TFA) or formic acid is added to both solvents to sharpen peaks and control the charge state of basic residues.

Each molecule partitions between the hydrophobic stationary phase and the mobile phase. More hydrophobic peptides bind more strongly and elute later, when the organic fraction is higher; more polar peptides elute earlier. Because synthesis impurities such as deletion sequences, truncated chains, incompletely deprotected species and oxidized forms usually differ slightly in hydrophobicity from the target, they separate into their own peaks.

Detection at 214 nm

As compounds leave the column they pass through a UV detector. Peptide bonds absorb strongly near 205–220 nm, so detection at 214 nm (or 220 nm) responds to essentially every peptide-related species regardless of side-chain composition. This is why most peptide COAs, including the ILS Laboratories certificates for our products, state a detection wavelength of 214 nm. Aromatic residues (tryptophan, tyrosine, phenylalanine) add absorbance near 280 nm, and some methods record both wavelengths, but 214 nm gives the most uniform response for purity calculation.

Reading a chromatogram

The chromatogram plots detector response (y-axis) against retention time in minutes (x-axis). A high-purity sample shows one dominant, symmetrical peak with a small number of minor peaks. Features near the very start of the run are often the sample-loading front or solvent effects rather than sample components. A baseline rise during the gradient is normal. Peaks that are broad, split or tailing can reflect column condition, overloading or, for some sequences, slow conformational exchange; proline-rich peptides such as Selank can show this because of cis/trans isomerism at proline bonds.

How purity is calculated

HPLC purity is usually reported by area normalization: the integrated area of the main peak divided by the total area of all integrated peptide-related peaks, expressed as a percentage. For example, the COA for our AOD-9604 10mg lot (see the AOD-9604 research compound) reports 97.46% by RP-HPLC area normalization at 214 nm, and the COA for TB-500 10mg reports 99.85% by the same method.

This calculation has important assumptions. It assumes every peptide species has a similar UV response per unit mass, which is approximately but not exactly true. It counts only what elutes and is detected: material that does not elute, co-elutes with the main peak or lacks UV absorbance is not represented. And it typically excludes non-peptide components such as counter-ions, residual solvents and water. HPLC purity is therefore a relative measure of peptide-related impurities, not an absolute assay of everything in the vial.

Purity versus net content

A separate measurement, net peptide content, answers a different question: how much peptide is present. It is typically determined by quantitation against a calibrated reference standard or by amino acid or elemental analysis. Our COAs report both values. The GHK-Cu 50mg lot (see the GHK-Cu research compound), for example, shows 98.96% HPLC purity and 52.48 mg net content.

Blends and multi-component samples

When two peptides are combined in one vial, a well-developed HPLC method resolves them as separate peaks. Our CJC-1295 + Ipamorelin Blend COA (see the CJC-1295 + Ipamorelin Blend research compound) shows two main peaks and reports a blend purity of 99.44% along with per-component content (CJC-1295 5.18 mg, ipamorelin 5.40 mg). Ipamorelin and CJC-1295 differ greatly in size and hydrophobicity, which makes the separation straightforward.

Identity by retention time

HPLC can also support identity by comparing the sample’s retention time with that of a reference standard run under identical conditions. ILS certificates list this as “Identity (HPLC-RTM)”. A retention-time match is consistent with the expected compound but does not measure its structure or mass, and different molecules can share a retention time. Confirming molecular identity directly requires mass spectrometry, which is covered in LC-MS vs HPLC: How Molecular Identity is Confirmed.

What HPLC cannot tell you

HPLC purity does not establish sequence, does not detect impurities that co-elute with the main peak, does not quantify water or counter-ions, and does not by itself confirm the identity of the main peak. It also reflects only the sample tested on the date tested. For these reasons, read an HPLC result together with the identity method, the net content and the lot number. Our guide How to Read and Verify a Peptide COA shows how to put those pieces together.

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