Principle and Operation of HPLC

High-Performance Liquid Chromatography (HPLC) is a chromatographic analysis technique, and its basic principles and operating steps are as follows:

Basic Principle of HPLC:

The principle of HPLC is that the sample solution passes through the HPLC column in a process of liquid phase flow, interacts with the surface of the column packing, and achieves separation and detection. The packing material in the liquid chromatography column is usually solid particles, which have certain surface properties and can interact with sample molecules through adsorption, chelation, ion exchange, and other interactions. In the liquid chromatography column, the mobile phase is continuously injected into the column, and when the sample solution passes through the column, it interacts with the packing material surface. Different substances can be separated based on the different types of interactions with the packing material.

  • Partition or Adsorption:
    • Components in the sample are separated in the chromatographic column based on their interactions with the stationary phase and mobile phase (such as adsorption, distribution, etc.). The stationary phase is typically porous particle material in the column, while the mobile phase is the solvent or solvent mixture passing through the column under high pressure. Separation is based on partitioning (distribution of analytes between the mobile and stationary phases) or adsorption (attachment of analytes to the stationary phase surface).
    • Example:
  • Selective Retention:
    • The functional groups on the stationary phase dictate its selectivity for specific analytes.
    • Example: A C18 column (hydrophobic) retains non-polar compounds longer, while polar compounds elute faster.
  • Efficiency and Resolution:
    • The column’s performance is determined by:
      • Particle Size: Smaller particles increase the surface area and improve separation efficiency but create higher backpressure.
      • Pore Size: Determines which analytes can access the stationary phase. Large pores are required for macromolecules like proteins.
  • Elution Order:
    • The retention time of analytes depends on:
      • Polarity: Non-polar analytes have longer retention in reverse-phase columns.
      • Molecular Size: Larger molecules may be excluded in small-pore columns.
      • Hydrophobicity: More hydrophobic compounds have longer retention in reverse-phase HPLC.
  • Detection: After separation, the components are detected by a detector, and their concentrations are converted into electrical signals, which are then transmitted to a recorder or computer software to generate a chromatogram.

Operating Steps:

  • Sample Preparation: Inject the sample into the instrument using a syringe and mix it with the mobile phase through valves connecting the sample loop.
  • Column Installation: Install the chromatographic column on the liquid chromatography instrument, paying attention to the flow direction and sealing.
  • Mobile Phase Preparation: Select an appropriate mobile phase and perform filtration and degassing to ensure analysis accuracy and reproducibility.
  • Analysis Run: After establishing a suitable analysis method (including selecting the right column, mobile phase, elution mode, and detector), start the analysis. During the analysis, adjust the composition, proportion, and pH of the mobile phase to control retention time and selectivity for different samples.
  • Data Collection: Collect the chromatogram obtained after analysis in HPLC software. Perform qualitative analysis based on peak retention times and sequence, and quantitative analysis based on peak heights and areas.
  • Result Interpretation: Based on peak retention times and concentrations in the chromatogram, perform qualitative and quantitative analysis of compounds.

HPLC features high pressure, high speed, high efficiency, and high sensitivity. Compared to classical liquid chromatography, HPLC offers faster analysis speed, higher resolution, and sensitivity.