Summary
Highlights
Chromatography is a technique for separation, purification, and testing of compounds. The term is derived from Greek words 'chroma' (color) and 'graphien' (to write), signifying its use in separating chemical substances into individual components for analysis. Various types exist, including absorption, thin-layer, column, and partition chromatography. HPLC finds applications in many industries, such as pharmaceuticals, food, medical devices, clinical and preclinical research, bioanalytics, textiles, leather, metals, and forensics.
HPLC separates compounds based on their interaction (affinity) with a stationary phase and a mobile phase. The mobile phase, a liquid or gas mixture containing the substances to be separated, percolates through the stationary phase, a porous solid matrix in a separation column. The mobile phase is forced at high pressure through this column.
An HPLC system comprises a solvent delivery pump, a degassing unit, a sample injector, a column oven, a detector, and a data processor. The pump delivers the mobile phase at a controlled flow rate, and the degassing unit removes air bubbles. The column, placed in a column oven, separates compounds, which are then detected and analyzed.
The mobile phase, a solvent or mixture of solvents, carries the sample through the column. It's often a mix of polar and non-polar liquid components. HPLC solvents must be pure, free of dissolved gases and particulates, and selected carefully for optimal elution strength to achieve desired separation. Elution can be isocratic (constant mobile phase composition) or gradient (varied composition during the run). Key selection criteria include solvent purity (HPLC or MS grade), optimal viscosity, appropriate refractive index, low boiling point for easy removal, non-corrosiveness to system components, non-toxicity, miscibility with the sample, transparency at detection wavelengths, and optimal elution strength.
The column is crucial for separation. Types include normal phase (polar stationary, non-polar mobile), reversed phase (hydrophobic stationary), ion exchange (separates anions and cations), and size exclusion (separates based on molecular size). Column base materials, such as silica gel or polymer, influence pH stability. The detector identifies and quantifies separated compounds. Common detectors are UV-Vis (measures light absorption), fluorescence (for compounds that fluoresce), and refractive index (detects differences in RI between eluent and reference stream, acting as a near-universal detector).
Separation utilizes differences in compound characteristics: polarity, electrical charge, and molecular size. The chromatogram, an electronic or hard copy output, displays the separation. A good peak shape is indicated by a tailing factor near 1.0 (USP requires no greater than 2.0; EP and BP specify 0.8-1.5 for symmetry factor), high efficiency, and narrow peak width. Factors affecting peak shape include column packing, mobile phase properties, and sample factors. Issues like flat-top peaks (detector overload), peak fronting (channeling, column overload, or incompatibility), shoulder peaks, and split peaks (unresolved compounds, frit blockage) require specific troubleshooting.
HPLC offers high sensitivity (identifying trace amounts), wide applicability, accurate quantification (high precision and detection of impurities), and minimal sample preparation. Advanced HPLC techniques often couple HPLC with other instruments like Mass Spectrometry (HPLC-MS, LC-MS/MS), Inductively Coupled Plasma Mass Spectrometry (HPLC-ICPMS) for enhanced analysis and drug metabolite profiling.
Various software programs control HPLC systems, acquire and analyze data. Examples include Waters Empower, ThermoFisher Scientific Chromeleon, Agilent OpenLab CDS, Shimadzu LabSolutions, Data Apex Clarity, Agilent ChemStation, Scientific Software Incorporated EasyChrome Elite, and Mettler Toledo LabX. These software packages offer features for instrument control, data acquisition, method development, peak integration, calibration, and reporting, aiding in optimizing HPLC methods and complying with regulatory standards.
HPLC has extensive applications: in the pharmaceutical industry for drug purity, stability studies, dissolution profiling, and chiral separation; in the food industry for analyzing additives, mycotoxins, pesticides, allergens, amino acids, and vitamins; in medical devices for biocompatibility testing, material characterization, and cleaning validation; in forensic laboratories for analyzing seized drugs, toxins, biological samples, explosives, and trace evidence; in the textile industry for fabric dye analysis, chemical residue testing, and fiber composition; in the leather industry for tanning agent analysis, chemical residue testing, and degradation studies; in material science for surface coating analysis and corrosion studies; and in bioanalytical laboratories for pharmacokinetic studies, metabolite profiling, and biomarker detection.
To ensure quality results, use high-purity reagents and solvents, filter solvents (0.45 µm) and store them properly, and prepare fresh buffers at correct pH. Samples should be filtered (0.2 µm or 0.45 µm). The HPLC system should be fully degassed. All glassware must be clean and in good condition. Prevent column vibrations and use end caps to prevent drying. Utilize guard columns and store columns in appropriate organic solvents. Wash columns before and after each use. Lastly, ensure staff are trained and follow standard operating procedures.