HPLC

Overview

High-performance liquid chromatography (HPLC) is an analytical technique used to separate, identify, and quantify individual components in a mixture. In peptide science, it serves as the gold standard method for determining purity — the percentage of the desired peptide relative to all other compounds present in a sample.

When a peptide's purity is reported as "98% by HPLC," it means that 98% of the detectable material in the sample is the target peptide, with the remaining 2% consisting of synthesis byproducts, incomplete sequences, degradation products, or other impurities.

How HPLC Works

Basic Principles

HPLC separates compounds based on their differential interactions with two phases:

• Stationary phase — A column packed with solid particles, most commonly C18-bonded silica (octadecylsilane). This non-polar surface retains hydrophobic molecules more strongly than hydrophilic ones.
• Mobile phase — A liquid solvent system pumped through the column at high pressure (typically 50-400 bar). In peptide analysis, this is usually a gradient of water and acetonitrile with 0.1% trifluoroacetic acid (TFA) as an ion-pairing agent.

When a sample is injected into the mobile phase stream, different molecules interact with the stationary phase to varying degrees. More hydrophobic molecules are retained longer on the column, while more hydrophilic molecules pass through faster. This differential retention separates the mixture into its individual components.

Detection

As separated components elute from the column, they pass through a detector — most commonly an ultraviolet (UV) absorption detector set to 214 nm or 220 nm, wavelengths at which peptide bonds absorb strongly. The detector generates a signal proportional to the amount of each component, producing a chromatogram.

The Chromatogram

A chromatogram plots detector signal (y-axis) against time (x-axis). Each component in the sample appears as a peak at its characteristic retention time. The area under each peak is proportional to the quantity of that component.

Key features to examine:

• Main peak — Should be a single, well-defined, symmetrical peak representing the target peptide
• Peak area percentage — The ratio of the main peak area to the total area of all peaks gives the purity percentage
• Impurity peaks — Smaller peaks before or after the main peak represent contaminants
• Baseline — Should be stable and flat; a drifting baseline suggests instrument or method issues

Types of HPLC Used in Peptide Analysis

Reversed-Phase HPLC (RP-HPLC)

The most common mode for peptide purity analysis. Uses a non-polar stationary phase (C18 or C8) and a polar mobile phase. Peptides are separated primarily by hydrophobicity. Nearly all peptide Certificates of Analysis report RP-HPLC purity.

Size-Exclusion Chromatography (SEC)

Separates molecules by size. Useful for detecting aggregation — a concern for larger peptides and proteins where molecules can form dimers or higher-order aggregates.

Ion-Exchange Chromatography (IEX)

Separates molecules by charge. Useful for detecting deamidation products or other charge variants that may co-elute in RP-HPLC.

Ultra-High-Performance Liquid Chromatography (UHPLC)

A modern advancement using smaller particle sizes (sub-2 micron) and higher pressures, providing improved resolution, sensitivity, and speed compared to conventional HPLC.

Interpreting HPLC Data on a Certificate of Analysis

When reviewing a Certificate of Analysis, look for:

• Method description — Column type, mobile phase composition, gradient conditions, detection wavelength. Without this information, the purity value lacks context.
• Purity percentage — Typically calculated as the area percentage of the main peak. Values above 95% are standard for research-grade peptides; above 98% for high-purity applications.
• Chromatogram — The actual graphical output. A clean chromatogram with a dominant single peak and minimal impurity peaks is ideal. If only a number is provided without the chromatogram, independent verification is not possible.
• Retention time — Should be consistent with expected values for the peptide's hydrophobic character.

Common Impurities Detected by HPLC

Limitations

HPLC purity values, while essential, have limitations:

• They represent relative UV-absorbing purity, not absolute chemical purity
• Compounds that do not absorb UV light (some salts, solvents) are invisible to UV detection
• Co-eluting impurities (different molecules with identical retention times) will be counted as part of the main peak, artificially inflating purity
• Different HPLC conditions can yield different purity values for the same sample
• Purity at the time of testing may not reflect purity at the time of use if the peptide has degraded during storage

For comprehensive quality assessment, HPLC should be paired with mass spectrometry for identity confirmation and additional tests as described in Peptide Purity and Testing.