Understanding HPLC Purity Testing

What happens during an HPLC run

High-performance liquid chromatography moves a liquid sample through a column containing a stationary phase. Components interact differently with the mobile and stationary phases, so they leave the column at different times. A detector records the response and software displays a chromatogram.

The horizontal axis is typically time. The vertical axis is detector response. A peak’s retention time describes when it was detected under those conditions; its integrated area describes the accumulated detector response assigned to that peak. Neither value is meaningful without the method and sample context.

Evidence: [1], [2]

What “99% purity” commonly means

For a relative-area calculation, the software integrates selected peaks and reports the main peak as a percentage of the total integrated area. A 99% result therefore means that the main peak accounted for about 99% of the included detector response under that run and integration method.

That wording matters. Compounds may respond differently at the selected wavelength, some components may not be detected, and excluded solvent fronts or system peaks may not enter the calculation. The result does not mean that 99% of everything inside a vial is peptide by mass. Salts, counterions, water, and substances without comparable detector response require other measurements.

Evidence: [1], [2]

Method details that change interpretation

Comparing purity numbers from two laboratories is unreliable when these conditions differ. A scientifically useful comparison starts with whether the methods were designed and validated for the same analytical purpose.

• Column chemistry and dimensions, which determine the separation environment.
• Mobile phases, additives, gradient program, flow rate, and column temperature.
• Detector type and wavelength; peptide-bond detection is often performed in the low-UV range, while aromatic residues can support other wavelengths.
• Sample concentration, diluent, injection volume, and reference preparation.
• System-suitability criteria such as resolution, repeatability, or signal-to-noise.
• Peak-integration rules, including which peaks are included, excluded, or manually adjusted.

Evidence: [1]

Retention time is evidence, not complete identity proof

A main peak appearing near a reference standard can support identification, but unrelated compounds can co-elute or share a similar retention time. FDA’s Q6A guidance specifically notes that identification solely by one chromatographic retention time is not regarded as specific. Coupling chromatography with mass spectrometry or using another orthogonal identity procedure adds discriminating evidence.

Likewise, one tall main peak can conceal co-eluting material. Resolution between critical peaks and demonstrated selectivity are more informative than visual neatness alone.

Evidence: [2]

How to review a chromatogram

• Match the sample name, lot, report number, and run identifier to the COA.
• Find the stated method, detector, wavelength, and test date.
• Check whether the main peak is identified and whether all relevant peaks are integrated.
• Look for a peak table with retention times and areas rather than relying on the plot alone.
• Confirm the reported calculation and acceptance criterion.
• Ask whether system suitability passed and whether the method can separate expected impurities or degradants.

Evidence: [1]

The correct conclusion

A defensible statement is narrow: “This sample produced the reported chromatographic purity under the identified HPLC method.” Broader conclusions about identity, content, sterility, stability, or experimental suitability need their own evidence. This disciplined wording is not a weakness; it is what makes the result useful.

Evidence: [1], [2], [3]