Principle of Chromatography
Chromatography is a powerful analytical technique used to separate components of complex mixtures. It is widely applied in chemistry, biology, and pharmaceutical analysis.
Liquid chromatography has significantly expanded analytical capabilities, enabling the separation of compounds that were previously difficult to analyze using thin-layer chromatography (TLC) or gas chromatography (GC).
Initially, liquid chromatography was performed using glass columns, where the mobile phase flowed through the stationary phase by gravity or low pressure. To improve flow rate and efficiency, the system evolved to operate under high pressure, leading to the development of High-Performance Liquid Chromatography (HPLC). Over time, “high pressure” became synonymous with high performance, thanks to improvements such as smaller particle sizes and more uniform stationary phases.
In HPLC, the sample (solute) is dissolved in a solvent and introduced into a liquid mobile phase (eluent). As the mixture passes through the chromatographic column, compounds interact differently with the stationary phase, resulting in separation.
A high-pressure pump drives the mobile phase through the system. As compounds elute from the column, they are detected and represented as peaks on a chromatogram, where each peak corresponds to a specific component.
HPLC System Components
1. Solvent Reservoir
The solvent reservoir contains the mobile phase. Multiple solvents with different polarities can be used to create gradient elution, allowing better separation of compounds.
2. Pump
The pump delivers the mobile phase at a controlled flow rate (from µL/min to mL/min) and supports:
- Isocratic mode: constant solvent composition
- Gradient mode: changing solvent composition during analysis
3. Injection System
Samples are introduced using an injection valve with sample loops ( 20 µL). This ensures a precise and reproducible injection volume, essential for quantitative analysis.
4. Column
The column is typically made of stainless steel or glass, with a constant internal diameter (4–20 mm) and length (15–30 cm). It contains the stationary phase and is the core of the separation process.
5. Stationary Phase
Normal Phase
- Polar stationary phase (silica gel)
- Non-polar mobile phase
- Polar compounds are retained longer
Limitation: Reduced stability and reproducibility over time.
Reverse Phase
- Non-polar stationary phase (e.g., C8, C18 bonded silica)
- Polar mobile phase (water, methanol, acetonitrile)
- Polar compounds elute first
Advantage: High stability and reproducibility (most commonly used in HPLC).
6. Mobile Phase
The mobile phase transports analytes through the column. Its polarity directly affects retention time:
- Polar stationary phase → non-polar mobile phase (normal phase)
- Non-polar stationary phase → polar mobile phase (reverse phase)
Adjusting solvent composition allows optimization of separation. Gradient elution is often used to improve resolution.
7. Detectors
UV-Visible Detector
- Measures absorbance of compounds at specific wavelengths
- Uses deuterium lamps (190–350 nm) or mercury lamps (254 nm)
- Requires analytes to absorb UV light
Refractive Index Detector
- Measures changes in refractive index
- Highly sensitive but temperature-dependent
- Only compatible with isocratic mode
Data is recorded using an integrator or data acquisition system.
Applications of HPLC in Analysis
1. Chromatogram Interpretation
A successful separation produces well-resolved peaks. Key parameters to report include:
- Column type and dimensions
- Mobile phase composition and flow rate
- Detection wavelength
- Injection volume
2. Qualitative Analysis
Each compound is identified by its retention time (tR) under specific conditions.
Other important parameters:
- Selectivity factor (α): distinguishes compounds
- Column efficiency: measured as theoretical plates
3. Quantitative Analysis
Quantification is based on the principle that peak area is proportional to analyte concentration.
External Calibration Method
- Calibration curve: Area vs concentration
- Unknown concentration determined from the curve
- Highly accurate and widely used
Standard Addition Method
- Known quantities of analyte are added to the sample
- Change in peak area is used to calculate concentration
- Useful for complex matrices
Internal Standard Method
- A known compound is added to all samples
- Results are expressed as ratios of analyte to standard
- Improves accuracy and reproducibility
Conclusion
HPLC is a highly efficient and versatile analytical technique used for both qualitative and quantitative analysis of complex mixtures. Its precision, reproducibility, and adaptability make it a cornerstone in modern laboratories, particularly in pharmaceutical, biochemical, and environmental applications.