HPLC Analysis Explained

From a sample to a chromatogram

A liquid mobile phase carries the sample through a column containing a stationary phase. Components interact differently with those phases and can leave the column at different times. In reversed-phase HPLC, separation uses a relatively non-polar stationary phase and a more polar mobile phase. Changing the column or mobile phase can change the separation. [A5]

The detector records a response as material passes through. On a typical chromatogram, the horizontal axis shows time and the vertical axis shows detector response. A peak’s retention time tells you when it appeared in that run; integration estimates the area beneath it. Retention time depends on the method, so compare it only in that context. [A5]

How an area percentage is calculated

This deliberately simplified example contains three integrated peaks. The numbers are invented signal units, not a laboratory result, specification or measured impurity profile.

Main-peak area percentage = 900 ÷ (900 + 90 + 10) × 100 = 90%. The calculation itself does not identify B or C. Nor does it tell you how many milligrams are in the container. Uncorrected area normalisation uses signal proportions; an approximation to relative amounts requires appropriate detector-response assumptions. [A4]

Imagine that B and C were omitted from the integration. The same main-peak area would then produce 100%. This is an arithmetic illustration of why the denominator and integration rules matter, not an accusation about any laboratory. An interpretable report should make the included peaks and any exclusions clear.

Three limits behind a clean-looking trace

Co-elution: two components can emerge together and contribute to an unresolved peak. A single peak therefore does not establish a single chemical species. Suitability work should address the separations relevant to the method’s purpose; ICH Q2(R2) describes checking discrimination between closely eluting components. [A2]

Detector response: a UV detector depends on light absorption. Response varies with the compound and wavelength, and substances with inadequate response may be missed. Changing wavelength can change the observed profile. The stated detection wavelength is therefore part of the result, rather than a dispensable instrument detail. [A5]

Data processing: baseline placement, peak boundaries and integration settings determine the areas used in the calculation. Request the trace and integration table together. The chromatogram reading guide covers their interpretation. [A4]

An extra peak is evidence of an additional detected feature, not a structural identification. Common impurities introduces possible categories; assigning a particular impurity requires supporting analytical evidence.

HPLC can also measure content

HPLC is not restricted to area-purity reporting. A suitable quantitative method can compare sample response with calibration standards to measure content. The same broad technique may therefore support an assay or an impurity test, depending on its design and calculation. A report headed “HPLC” does not tell you which was performed. [A1]

Keep the distinction explicit when reviewing identity, purity and assay. An area percentage should not be converted into a content result unless the required quantitative basis is established.

What to request with the percentage

Use mass spectrometry to understand one possible identity method, including its own limits. A purity percentage alone does not prove identity.

For a Knockout Labs item, begin with the quality documentation hub and ask which records exist for the lot offered. The testing methods overview distinguishes the separate measurements. The 99%+ purity claim page explains why packaging wording needs lot-specific support.