HPLC Testing Explained

Introduction

Almost every peptide purity figure comes from HPLC. Understanding the technique makes it possible to evaluate a chromatogram rather than simply accept a percentage.

What Is HPLC?

HPLC pumps a liquid mobile phase at high pressure through a column packed with small particles (the stationary phase). Components in the sample interact differently with the stationary phase and therefore exit the column (elute) at different times, called retention times.

Scientific Principles: Reverse-Phase Separation

For peptides, reverse-phase HPLC (RP-HPLC) is standard. The stationary phase is hydrophobic (often C18-bonded silica); the mobile phase starts mostly aqueous and becomes increasingly organic (usually acetonitrile) over a gradient. More hydrophobic peptides are retained longer. An ion-pairing agent such as 0.1% TFA sharpens peptide peaks.

Detection

UV detection at 214–220 nm captures peptide-bond absorbance. Some methods add 280 nm for aromatic residues or couple the HPLC to a mass spectrometer.

Reading a Chromatogram

• X-axis: retention time (minutes).
• Y-axis: detector response (absorbance).
• Main peak: the target peptide.
• Smaller peaks: related impurities or artifacts.
• Purity = main-peak area ÷ total integrated area × 100.

Look for a stable baseline, a symmetrical main peak, and resolution between the main peak and nearby impurities. How this figure is interpreted is covered in What Does Peptide Purity Mean?.

Method Reliability

USP <621> describes system suitability parameters such as resolution, tailing factor and repeatability. ICH Q2(R2) describes validation characteristics — specificity, linearity, accuracy, precision and range — used to show a method is fit for purpose.

Limitations

HPLC cannot identify a peak on its own; retention time alone is not proof of identity. It also does not detect water or counter-ions, which is why purity differs from quantity.

Frequently Asked Questions

Scientific References

1. United States Pharmacopeia. General Chapter <621> Chromatography. [Compendial standard] View source (opens in new tab)
2. International Council for Harmonisation. ICH Q2(R2): Validation of Analytical Procedures. 2023. [Standards guidance] View source (opens in new tab)