What Is HPLC Analysis?

HPLC separates the components of a dissolved sample by pumping it at high pressure through a column packed with microscopic particles. For peptides the column is almost always reversed-phase: the particles are coated with hydrophobic carbon chains (C18 is most common), and the mobile phase is a mixture of water and an organic solvent such as acetonitrile, both containing a small amount of acid, typically 0.1% trifluoroacetic acid.

Each component interacts with the hydrophobic surface to a different degree. More hydrophobic molecules cling to the column longer; more polar ones are washed through sooner. By gradually increasing the proportion of organic solvent over the run, a 'gradient', components are released in order of increasing hydrophobicity and arrive at the detector one after another.

The time between injection and a component reaching the detector is its retention time. Under fixed conditions it is characteristic of a molecule, which is why a certificate often compares the sample's retention time with that of a reference standard. A match supports identity; a shift indicates a different compound or a changed method. Retention time alone is not definitive, since different molecules can co-elute, but it is a useful first check.

A UV detector set at 214 or 220 nm, where the peptide bond absorbs, records absorbance against time. Each component produces a peak, and the detector software integrates the area under every peak. Purity is the area of the main peak divided by the total area of all peaks. Peaks that elute just before or after the main peak are usually closely related impurities: deletion sequences, oxidised forms, or incompletely deprotected variants.

Integration settings matter. The threshold below which a bump is not counted as a peak, and where the baseline is drawn under a cluster of peaks, both change the result. A good laboratory reports its integration parameters or uses a standard protocol so that results are reproducible.

A clean chromatogram shows one dominant, symmetrical peak with a flat baseline and only small satellite peaks. Broad or tailing peaks suggest aggregation or poor column performance. A second substantial peak indicates a significant impurity, a second compound or a degradation product. Peaks at the very start (the solvent front) are usually salts and non-retained material and are often excluded from the purity calculation; whether they are excluded should be stated.

Because the appearance of a chromatogram depends entirely on the method, the method parameters should be printed alongside it. Column, gradient, flow rate, wavelength and injection volume are the minimum. Without them a chromatogram is a picture, not data.

HPLC quantifies how much of the UV-absorbing material is a single component. It does not identify that component, does not measure non-absorbing content such as water and counter-ions, and does not detect endotoxin or microbial contamination. Identity requires mass spectrometry; net peptide content requires amino-acid analysis or a nitrogen assay; sterility and endotoxin require their own tests. On our certificates HPLC purity and mass-spectrometry identity are reported together.