HPLC-UV

What does an HPLC system actually do with a peptide sample?

An HPLC system pushes a liquid (the mobile phase) at high pressure through a narrow steel column packed with very small particles (the stationary phase). The dissolved sample is introduced into that flowing liquid by an autosampler. Each molecule in the sample spends a different amount of time stuck to the packing, so the components leave the column one after another. A detector at the outlet records them as they pass.

For peptides the usual mode is reversed-phase HPLC, where the packing is water-repelling and the mobile phase starts mostly watery and becomes steadily richer in an organic solvent such as acetonitrile. A methods review by Mant and colleagues calls reversed-phase the most widely used mode of HPLC for peptide separations, and describes linear gradients from aqueous trifluoroacetic acid to trifluoroacetic acid in acetonitrile as the authors' best approach to most analytical separations (Mant and colleagues, Methods in Molecular Biology, 2007).

Why is UV light used to detect peptides?

The peptide bond absorbs far-ultraviolet light strongly, so the same review notes that peptide detection is generally carried out at 210 to 220 nm. The aromatic side chains of tyrosine, phenylalanine and tryptophan also absorb in the 250 to 290 nm range, which is why some laboratories record a second wavelength.

Nearly every peptide-related impurity also contains peptide bonds, so a low wavelength sees most of them. The weakness is that UV says nothing about mass or structure: a peak is only a time and an area.

How does one run give both a purity and an amount?

Purity is internal to the sample: the main peak's area divided by the total area of all integrated peaks, with no standard needed. The lesson on what HPLC measures covers how to read that percentage.

Amount (often called assay or content) needs an outside reference. The analyst runs a reference standard of the same peptide at known concentrations, builds a calibration line of peak area against concentration, and reads the sample's main peak against it to get a mass. In a multi-laboratory study run by the USP Biologics Department on oxytocin, an HPLC assay using the same peptide bulk material as the standard gave the lowest inter-laboratory variability of the three methods compared (HPLC, quantitative NMR and amino acid analysis), with the caveat that the variability figure left out the uncertainty in the standard's own assigned purity (Li and colleagues, Journal of Pharmaceutical and Biomedical Analysis, 2019).

That caveat matters, because the amount figure is only as good as the standard. The European Medicines Agency's draft guideline on synthetic peptides notes that peptides are often very hygroscopic, so reference standards need protecting from moisture during storage and analysis, and that the assay is typically expressed on a counter-ion free, anhydrous basis (EMA draft guideline on synthetic peptides, 2023). A standard that has absorbed water, or whose own content was assigned loosely, shifts every result read against it.

What settings change the result?

Two laboratories can run the same vial and report different purity figures without either being wrong, because the result belongs to the method as much as to the sample.

How do labs show the method is working on the day?

Before samples are reported, a validated method runs a system suitability test, which ICH Q2(R2) describes as an integral part of analytical procedures and a regular check of performance (ICH Q2(R2), 2023). Typical checks include agreement between repeat runs of the standard and resolution of a critical pair of peaks.

ICH Q2(R2) also says that for separation techniques, specificity can be shown by the resolution of the two components that elute closest to each other, and that where one procedure does not give enough discrimination, a combination of two or more procedures is recommended. That is the formal reason a purity figure should travel with an identity result from a different technique.

What can HPLC-UV miss?

The main blind spot is co-elution. If an impurity has almost the same retention as the target peptide, it sits inside the main peak and is counted as product. The EMA draft guideline names the risk of co-eluting impurities as one reason peptide impurity methods can be a challenge to develop, and says additional independent methods may be needed where one method cannot separate all peaks.

Weakly absorbing impurities are the second gap: material without peptide bonds or aromatic rings may barely register at the chosen wavelength. Water, counter-ions and salts are not counted as peptide peaks, so a high purity can sit alongside a lower peptide share of the powder (see net peptide content and counterions).

Is HPLC-UV part of a Tested.ph order?

Yes. Purity by peak area and amount against a reference standard are both in the standard package, alongside identity by LC-MS, analysed by the partner laboratory when testing opens. See pricing for package contents and the chromatogram tool to practise reading a trace.