Principles, Types, Components, and Applications of HPLC

Introduction

High-Performance Liquid Chromatography (HPLC) is a widely used and highly effective chromatographic technique with many applications in drug production and research. It separates or identifies different components in a mixture based on their molecular structure and composition.

HPLC is one of the most widely applied separation techniques, known for its sensitivity, specificity, and precision.

  • It is a special form of column chromatography used in biochemistry and analysis to separate, identify, and quantify active compounds in mixtures.
  • In HPLC, a column is packed with a stationary phase, a pump drives the mobile phase through the column, and a detector displays the retention time of the analytes.
  • The retention time is variable and primarily depends on the interactions between the stationary phase, the analytes, and the solvent used.
  • A small amount of the sample to be analyzed is introduced into the mobile phase stream and is retained through specific chemical or physical interactions with the stationary phase.
  • Retention depends on the nature of the analytes and the composition of the stationary and mobile phases.
  • The most commonly used solvents in HPLC are methanol and acetonitrile.

Principle

HPLC involves injecting a small volume of liquid sample into a tube packed with small particles (3 to 5 μm in diameter, called the stationary phase). The individual components in the sample are carried through the column by a high-pressure liquid (mobile phase) pumped through the system.

Column packing materials are used to separate the components. This involves various chemical and physical interactions between the molecules and the packing particles.

A detector at the column exit detects the separated components, which measures the quantity of these components. The output from the detector is called a “liquid chromatogram.”

Principles of HPLC

Advantages over Low-Pressure Column Liquid Chromatography

HPLC offers several advantages over traditional low-pressure column liquid chromatography:

  • Higher sensitivity (with various detectors available)
  • Higher resolution
  • Faster speed
  • Easier sample recovery (less mobile phase removed)
  • Applicability to a wide range of stationary phases

Branches

Chromatography is divided into gas, liquid, and supercritical fluid techniques. Gas chromatography is further subdivided into gas-liquid and gas-solid techniques.

Liquid chromatography includes various techniques, such as thin-layer chromatography. Pressurized liquid chromatography is further divided into ion-exchange chromatography, size-exclusion chromatography, partition chromatography, and liquid-solid chromatography.

Types

HPLC has several variants depending on the phase systems (stationary and mobile phases) used in the process.

  • Normal-Phase High-Performance Liquid Chromatography

    Also known as normal-phase chromatography or adsorption chromatography. This method separates analytes based on polarity. It has a polar stationary phase and a non-polar mobile phase. Typical stationary phases include silica, and typical mobile phases include hexane, dichloromethane, chloroform, diethyl ether, and mixtures thereof. This technique is suitable for compounds sensitive to water, geometric isomers, cis-trans isomers, category separation, and chiral compounds.

  • Reverse-Phase High-Performance Liquid Chromatography

    The stationary phase is non-polar (hydrophobic), and the mobile phase is aqueous or moderately polar. It operates on hydrophobic interactions, so the more nonpolar the material, the longer the retention time. This technique is suitable for non-polar, polar, ionizable, and ionic molecules.

  • Size-Exclusion High-Performance Liquid Chromatography

    Also known as gel-permeation chromatography or gel-filtration chromatography. The column is packed with material of precisely controlled pore size, and particles are separated based on their molecular size. Larger molecules pass through the column quickly, while smaller molecules pass through the porous packing material and are eluted later. Size-exclusion chromatography also helps determine the tertiary and quaternary structures of proteins and amino acids and is used to determine the molecular weight of polysaccharides.

  • Ion-Exchange High-Performance Liquid Chromatography

    In this chromatography, retention is based on the attraction between solute ions and charged sites on the stationary phase. Ions with the same charge are excluded. This technique is used for purifying water, protein-ligand and ion-exchange chromatography, high pH anion-exchange chromatography of carbohydrates and oligosaccharides, etc.

  • Bioaffinity High-Performance Liquid Chromatography

    In this type of chromatography, separation is based on reversible interactions between proteins and ligands.

Components of a Chromatographic System

The core of the system is the column where separation occurs.

  • Solvent Reservoir: Contains the mobile phase contents in glass containers. The mobile phase or solvent in HPLC is typically a mixture of polar and nonpolar liquid components, with concentrations varying according to the sample composition.
  • Pump: Draws the mobile phase from the solvent reservoir and forces it through the column and detector. The working pressure can reach up to 42,000 kPa (about 6000 psi) depending on factors like column size, particle size of the stationary phase, flow rate, and mobile phase composition.
  • Sample Injector: Can be a single injection or an automated system. HPLC sample injectors should be able to inject liquid samples ranging from 0.1-100 mL at high pressure (up to 4000 psi) with high reproducibility.
  • Column: Typically made of polished stainless steel, ranging from 50 to 300 mm in length and 2 to 5 mm in internal diameter. Columns are usually packed with stationary phase particles of 3-10 μm in diameter. Columns with an internal diameter of less than 2 mm are known as microbore columns. Ideally, the temperature of the mobile phase and the column should remain constant during analysis.
  • Detector: Located at the end of the column, it detects analytes as they elute from the column. Common detectors include UV-visible spectroscopic detectors, fluorescence detectors, mass spectrometry detectors, and electrochemical detectors.
  • Data Collection Device: The signal from the detector can be collected by a chart recorder or an electronic integrator, varying in complexity and ability to process, store, and reprocess chromatographic data. The computer integrates the detector responses for each component and places them into the chromatograph for easy reading and interpretation.

Applications

HPLC can provide information on compound resolution, identification, and quantification. It also aids in chemical separation and purification. Other applications of HPLC include:

  • Pharmaceutical Applications: Controlling drug stability, tablet dissolution studies, and drug quality control.
  • Environmental Applications: Detecting phenolic compounds in drinking water and biomonitoring pollutants.
  • Forensic Applications: Quantifying drugs in biological samples, identifying steroids in blood and urine, forensic analysis of textile dyes, and determining cocaine and other drugs of abuse in blood and urine.
  • Food and Flavor: Quality control of soft drinks and water, sugar analysis in fruit juices, analysis of polycyclic compounds in vegetables, and preservative analysis.
  • Clinical Testing: Analyzing urine, antibiotics in blood, bilirubin, and biliverdin in liver diseases, and detecting endogenous neuropeptides in extracellular brain fluid.
  • Chemical Research: Reaction monitoring, Compound identification, and purification.

Advantages of HPLC

  • High resolution and accuracy
  • Suitable for complex mixtures
  • Quantitative and qualitative analysis
  • Wide applicability across industries

Limitations

  • High equipment cost
  • Requires skilled operation
  • Solvent consumption and waste generation

Summary

HPLC works by separating compounds based on their interactions between a mobile liquid phase and a stationary solid phase. With various types and configurations, it has become an essential analytical tool across pharmaceutical, environmental, food, and biological fields.