What is High-Performance Liquid Chromatography (HPLC)?
High‑Performance Liquid Chromatography (HPLC) is an analytical chemistry technique used to separate, identify, and quantify components in a liquid mixture. A pump drives the mobile phase (solvent) at high pressure through a column packed with a stationary phase; compounds interact differently with the column and elute at different times. Detectors (e.g., UV‑Vis, refractive index, fluorescence) measure each compound as a peak on a chromatogram. Common types include reversed‑phase, normal‑phase, ion‑exchange, and size‑exclusion. (Fig. 1).
Applications include:
- pharmaceutical quality control
- food safety testing
- environmental monitoring
- forensic analysis.
Key system components:
- mobile phase
- pump, injector
- column, detector
- data system
The Beginning of Chromatography
Methods for separating components of a mixture include filtration, distillation, and extraction. Chromatography was invented by the Russian botanist Mikhail Semenovich Tswett. In the early 1900s, Tswett packed calcium carbonate in a standing tube, placed pigments extracted from plants on top, and then flushed the tube with petroleum ether as a solvent (Fig. 2).
HPLC was originally known as high‑pressure liquid chromatography. By applying high pressure to push solvents through tightly packed columns, HPLC achieves faster, more precise separations than traditional low‑pressure column chromatography.
Separation Mechanism in Chromatography
A mixture is placed in a stream of liquid (petroleum ether in Fig. 3) called the mobile phase and moved through a solid medium (calcium carbonate powder in Fig. 3) called the stationary phase. The components in the mixture move with the flow of the mobile phase and interact with the stationary phase. The speed of movement depends on the strength of the interaction between each component and the stationary phase. That is, components that interact strongly with the stationary phase move slowly, whereas components that interact weakly move quickly, so allowing the components to be separated.
The separated components can be analyzed using different types of detectors. A UV detector, for example, can detect components based on UV absorption. The chromatogram is obtained by measuring the elution time on the X axis and the intensity of the UV signal on the Y-axis. If the measurement conditions are the same, the elution time (peak position) for the standard sample whose components are known and that for the unknown sample can be compared to identify the components for qualitative analysis. In addition, since the absorption intensity is proportional to the concentration, a calibration curve can be prepared using a standard sample, and the component concentration can be determined by measuring the peak area or height for quantitative analysis.
How to Read an HPLC Chromatogram
Understanding how to interpret a chromatogram is an essential skill for anyone new to HPLC. A chromatogram is the visual output of an HPLC analysis, showing how compounds in a sample separate over time as they pass through the detector.
Below is a simplified example of a typical HPLC chromatogram with key features labeled.
Key Components of a Chromatogram
Baseline
The baseline is the flat signal observed when no analyte is being detected. It represents the detector response to the mobile phase alone. A stable, smooth baseline indicates good system performance.
Retention Time (tR)
Retention time is the time it takes for a compound to travel from injection to detection. It is measured from the moment of sample injection to the top (apex) of a peak.
- Each compound has a characteristic tR under fixed conditions.
- Retention time helps identify compounds when compared to standards.
Dead Time (t0)
Dead time, also called void time, is the time required for an unretained compound to pass through the system.
- It represents the fastest possible elution time.
- Compounds that do not interact with the stationary phase will elute at approximately t0.
- t0 is useful for understanding column performance and calculating retention factors.
Peak Height
Peak height is the vertical distance from the baseline to the top of the peak.
- It reflects detector response intensity at maximum concentration.
- Peak height can be used for quantification in some methods, but it is more sensitive to peak shape changes.
Peak Area
Peak area is the total area under the peak.
- It represents the total amount of analyte reaching the detector.
- Peak area is most commonly used for quantitative analysis because it is less affected by peak broadening than peak height.
Quick Comparison: Peak Height vs. Peak Area
- Peak height measures how tall the signal is at its maximum.
- Peak area measures the total signal across the entire peak.
- Peak area is generally more accurate for determining concentration.
- Peak height may be useful when peaks are very sharp and well-resolved.