HPLC for Peptides: How Purity Is Measured

The purity figure on a peptide certificate of analysis almost always comes from one technique: reverse-phase high-performance liquid chromatography (RP-HPLC). Knowing how the measurement is made lets you judge whether a "99%" result is meaningful, and what it does not tell you.

What chromatography does

Chromatography separates the components of a mixture by passing them through a column packed with a stationary phase while a liquid mobile phase flows over it. Components that interact more strongly with the stationary phase move more slowly and leave the column later. A detector at the outlet records a signal over time. The resulting trace is the chromatogram, and each separated component appears as a peak.

"High-performance" refers to the use of very small packing particles (typically 1.7–5 µm) and high pressure, which together give sharp, well-resolved peaks.

Why reverse phase suits peptides

In reverse-phase chromatography the stationary phase is non-polar and the mobile phase is polar. For peptides the standard column is silica bonded with C18 (octadecyl) chains, sometimes C8 or C4 for larger or more hydrophobic molecules. Wide-pore silica (around 300 Å) is often used for longer peptides so the molecules can reach the bonded surface.

Peptides bind to the C18 surface mainly through their hydrophobic side chains. They are released as the proportion of organic solvent in the mobile phase increases. Two peptides that differ by a single hydrophobic residue, or by an oxidised methionine, will usually elute at noticeably different times. That sensitivity to small structural changes is exactly what makes RP-HPLC the reference method for peptide purity.

Mobile phases and gradients

A typical peptide method uses two solvents:

• Mobile phase A: water with 0.1% trifluoroacetic acid (TFA)
• Mobile phase B: acetonitrile with 0.1% TFA

TFA acts as an ion-pairing agent. It pairs with positively charged groups on the peptide (lysine, arginine, histidine and the N-terminus), suppresses unwanted interactions with the silica, and sharpens peaks. Formic acid is used instead when the eluent goes directly into a mass spectrometer, because TFA suppresses the MS signal.

Peptides are almost always separated with a gradient: the percentage of B increases over the run. A common screening method moves from 5% to 65% B over 30 minutes. Shallow gradients (for example 0.5% B per minute) improve resolution between closely related species; steep gradients shorten run time at the expense of separation.

Parameter	Typical value	Why it matters
Column	C18, 4.6 × 150 mm or 2.1 × 100 mm	Sets selectivity and efficiency
Particle / pore size	1.7–5 µm / 100–300 Å	Smaller particles sharpen peaks; wide pores suit longer peptides
Mobile phases	Water/acetonitrile, 0.1% TFA	Controls retention and peak shape
Gradient	e.g. 5–65% B over 30 min	Determines resolution of near-identical species
Column temperature	25–60 °C	Affects selectivity and peak width
Detection	UV, 214 or 220 nm	Measures the peptide backbone

Detection at 214–220 nm

The peptide bond absorbs ultraviolet light strongly between about 190 and 230 nm. Detecting at 214 or 220 nm therefore measures the backbone that every peptide has, regardless of its side chains. This is important for purity calculations: impurities such as deletion or truncated sequences share the same backbone chemistry and absorb in roughly proportion to their size, so they are not invisible.

Some methods also record 280 nm, where tryptophan and tyrosine absorb. That wavelength is useful for confirming aromatic residues but is unsuitable for purity, because an impurity lacking those residues would be missed. Photodiode-array detectors record the full spectrum across each peak, which helps flag co-eluting species.

Reading a chromatogram

A chromatogram plots detector response (absorbance, often in mAU) against retention time (minutes). When you look at one, work through these points:

1. Find the main peak. It should be by far the largest and reasonably symmetric. Tailing or a shoulder can mean a hidden impurity.
2. Look at the baseline. It should be flat apart from a gentle drift with the gradient. Large disturbances suggest a poor method or a dirty system.
3. Check the small peaks. Peaks just before or after the main one are often deletion sequences, diastereomers or oxidised forms. Their presence is normal; their absence on a crude scale can mean the trace is zoomed out or not integrated.
4. Ignore the solvent front. The disturbance in the first minute or two is unretained material, not peptide, and is excluded from integration.
5. Confirm integration. A results table should list each integrated peak with retention time, area and area percent.

Purity by area percent

Peptide purity is reported as the main peak area divided by the total area of all integrated peaks, multiplied by 100. If the main peak has an area of 4,950 mAU·s and the impurities total 50 mAU·s, purity is 4,950 ÷ 5,000 × 100 = 99.0%.

This calculation rests on assumptions you should keep in mind:

• Similar response factors. Area percent assumes impurities absorb about as strongly as the main peptide per unit mass. At 214 nm this is a reasonable approximation for related peptides, not an exact truth.
• Everything elutes and is detected. Material that sticks to the column, or that does not absorb at the chosen wavelength, does not appear in the total.
• Co-eluting species are counted as product. If an impurity elutes under the main peak, the purity figure includes it. Orthogonal methods (a different column, pH or gradient) and mass spectrometry reduce this risk.
• Integration settings are honest. Raising the threshold so small peaks are not integrated inflates purity.

What purity does not measure

HPLC purity says how much of the peptide material is the target sequence. It does not report water, residual TFA or acetate counter-ions, or inorganic salts, none of which absorb at 214 nm. Those are captured by net peptide content, a separate measurement. And HPLC alone does not prove identity: a wrong peptide can still produce a single clean peak. Identity comes from mass spectrometry.

What a good HPLC report includes

• The chromatogram itself, with axes labelled
• Column type and dimensions, mobile phases, gradient, flow rate, temperature and wavelength
• A peak table with retention times and area percents
• The batch number and analysis date
• The name of the laboratory and analyst

Method validation for purity assays is described in ICH Q2(R2) and covers specificity, linearity, precision and limits of quantitation. For the wider context of how purity sits alongside identity and content on a certificate, see how to read a certificate of analysis.

References

• Snyder LR, Kirkland JJ, Dolan JW. Introduction to Modern Liquid Chromatography. 3rd ed. Wiley; 2010.
• Mant CT, Hodges RS, eds. High-Performance Liquid Chromatography of Peptides and Proteins: Separation, Analysis, and Conformation. CRC Press; 1991.
• Aguilar MI, ed. HPLC of Peptides and Proteins: Methods and Protocols. Methods in Molecular Biology, vol. 251. Humana Press; 2004.
• United States Pharmacopeia. General Chapter ⟨621⟩ Chromatography. USP–NF.
• European Pharmacopoeia. 2.2.46 Chromatographic Separation Techniques.
• International Council for Harmonisation. ICH Q2(R2): Validation of Analytical Procedures. 2023.