HPLC and Peptide Purity: How It Is Measured

HPLC peptide purity is the area of the target peptide's peak expressed as a percentage of the total integrated peak area in a reversed-phase HPLC chromatogram, recorded by UV absorbance at about 210–220 nm, where the peptide bond absorbs. It tells you how much of the UV-absorbing material that the method separates is the intended sequence. It does not measure water, counter-ions or salts, and on its own it does not prove identity.

HPLC stands for high-performance liquid chromatography. This guide covers how the reversed-phase method separates peptides, the column and gradient choices behind a result, why detection sits at 214–220 nm, how the area % figure is calculated, and the impurities it is designed to catch.

How reversed-phase HPLC separates peptides

In reversed-phase HPLC (RP-HPLC), the column is packed with small silica particles whose surface carries bonded hydrocarbon chains, usually octadecyl (C18) or octyl (C8). That surface is non-polar. The mobile phase pumped through it is polar: water mixed with acetonitrile, with a little acid added.

The sample is loaded at the head of the column in a mostly aqueous mobile phase, and the peptides adsorb onto the hydrophobic surface. The share of acetonitrile then rises steadily over the run. This is the gradient. Each peptide desorbs and moves down the column once the mobile phase is non-polar enough to carry it. Peptides therefore leave the column broadly in order of overall hydrophobicity: short, polar or highly charged sequences first, sequences rich in leucine, isoleucine, phenylalanine or tryptophan later, and lipidated peptides later still.

RP-HPLC is the most widely used HPLC mode for peptides. It is fast and efficient, and its volatile mobile phases suit both analysis and preparative purification. Ion-exchange, size-exclusion and hydrophilic-interaction chromatography separate by other properties, namely charge, size and polarity, and serve as orthogonal checks.

The role of the acid

Most peptide methods run at about pH 2, with trifluoroacetic acid (TFA) in both mobile phases. At that pH the carboxyl groups of aspartate, glutamate and the C-terminus are largely protonated and uncharged. The positively charged basic groups pair with trifluoroacetate anions, a process called ion pairing, which helps separate closely related peptides. The acidic pH also keeps residual silanol groups on the silica uncharged, which prevents unwanted ionic interactions with basic residues.

Methods coupled directly to a mass spectrometer often use formic acid instead, because TFA weakens the electrospray signal.

Columns and gradients

A purity figure belongs to the method that produced it. These are the settings a method specifies, with common choices for peptides.

A shallower gradient spreads peaks further apart and resolves more closely related impurities, at the cost of a longer run. Because retention depends on every one of these settings, a retention time or a purity figure means little without the method behind it.

Why detection sits at 214–220 nm

Peptide bonds absorb ultraviolet light strongly below about 220 nm, so detection is generally set between 210 and 220 nm. Every peptide has a backbone of peptide bonds, so every peptide absorbs there, whatever its sequence.

The signal per molecule is not identical, however. Kuipers and Gruppen measured the molar extinction coefficient of the peptide bond at 214 nm as 923 M⁻¹ cm⁻¹. Against that baseline:

• tryptophan absorbs about 30 times more strongly than a peptide bond
• phenylalanine, tyrosine and histidine absorb about six times more strongly
• proline within a chain absorbs about three times more strongly
• methionine absorbs about as strongly as a peptide bond, and the other amino acids much less

Two peptides present in equal amounts can therefore give different peak areas. An impurity that has lost a tryptophan responds less strongly than the target, and one that has gained an aromatic residue responds more strongly. Area % is therefore an approximation of composition, not an exact mass or mole fraction.

At 280 nm the picture changes. Only the aromatic side chains of tyrosine, tryptophan and phenylalanine absorb in the 250–290 nm range. A peptide without them, such as BPC-157, gives almost no signal at 280 nm, so a purity figure recorded there would say little about it.

How area % purity is calculated

The data system integrates each peak, measuring the area between the signal and the baseline. Purity is the main peak's area divided by the sum of all integrated peak areas, multiplied by 100.

The worked example below uses invented numbers for a crude synthetic peptide, before purification, to show the arithmetic.

Here the target accounts for 72.4% of the total area. Preparative purification leaves the minor peaks far smaller, but the calculation is the same.

The number also depends on choices that belong to the method:

• Integration. Where the baseline is drawn, and how overlapping peaks are split, moves area between peaks.
• Reporting threshold. Peaks below a set size are often not integrated, so they drop out of the total.
• Wavelength. A different wavelength changes the relative response of peptides with different compositions.
• Run length. Material that elutes after the gradient ends, or never leaves the column, is not counted.
• System peaks. A blank run without sample identifies peaks that come from the mobile phase or the instrument.

What HPLC purity measures, and what it does not

Typical impurities in a synthetic peptide

Most research peptides are made by solid-phase peptide synthesis, and their impurities follow from the chemistry of that process. A review by D'Hondt and colleagues groups them into synthesis-related and degradation-related types. The mass differences below are calculated from monoisotopic atomic masses.

Most of these differ from the target by one residue or one small group. Their hydrophobicity is similar, so they elute close to the main peak, which is why a shallow gradient matters. It is also why HPLC and mass spectrometry are used together: HPLC shows that a minor component exists and how large its peak is, and LC-MS gives its mass, which usually reveals what it is. Mass spectrometry and peptide identity covers that side.

HPLC purity on our product pages

For our research compounds, purity is stated as ≥99% (HPLC, supplier specification): our supplier's specification for the material, with HPLC as the stated method. Where a certificate of analysis is published in the COA Library, it reports the analysing laboratory's own result for the sample it tested.

How to read a peptide certificate of analysis shows where the HPLC line sits on a certificate. Peptide purity vs net peptide content explains why HPLC purity and the peptide content of a vial are different numbers. Retention time, peak area and chromatogram are defined in the glossary.