How is HPLC different from GC column?

HPLC (High Performance Liquid Chromatography) and GC (Gas Chromatography) are both powerful analytical techniques used to separate, identify, and quantify compounds in a wide variety of samples. However, they differ greatly in terms of operation, equipment, and applications. This article outlines the key differences between HPLC and GC columns, focusing on their design, functionality, and suitability for different types of analysis.

Column Design

HPLC Columns

HPLC columns are typically shorter and wider than GC columns. They are typically up to 30 cm in length and have an internal diameter ranging from 2.1 mm to 8 mm. The packing within HPLC columns consists of small particles (typically less than 5 microns in diameter) that provide a large surface area to interact with sample components. The packing properties of these columns allow them to efficiently separate compounds based on their chemical properties.

Main features:

Length: up to 30 cm

Diameter: typically between 2.1 mm and 8 mm

Packaging materials: small particles (e.g. silica) with various surface modifications suitable for different separation mechanisms (e.g. reversed phase, normal phase).

GC columns

GC columns, by contrast, are longer and narrower, typically up to 100 m in length and have an internal diameter ranging from 0.1 mm to 1 mm. They can be divided into two main types: packed columns and capillary columns. Packed columns contain a solid stationary phase or a liquid coated on a solid support, whereas capillary columns have a thin film of the stationary phase coated on the inner wall.

Main features:

Length: up to 100 m

Diameter: typically between 0.1 mm and 1 mm

Types: packed columns (solid or liquid stationary phase) and capillary columns (open tubular structure).

Mobile phase

High performance liquid chromatography

In HPLC, the mobile phase is typically a liquid solvent or a mixture of polar or non-polar solvents. Common solvents include water, methanol, acetonitrile, and various buffers. The choice of mobile phase is critical because it affects the interaction between the analyte and the stationary phase within the column.

Gas Chromatography

GC uses a gaseous mobile phase, most commonly an inert gas such as helium or nitrogen. The sample must be volatile enough to evaporate when introduced into the column. This requirement means that GC is primarily suitable for analyzing volatile compounds, while HPLC can handle a wider range of substances, including nonvolatile compounds.

Separation Mechanism

High Performance Liquid Chromatography

HPLC separates compounds based on their affinity for the stationary phase relative to the mobile phase. Various modes of chromatography can be employed:

Reversed Phase Chromatography: Nonpolar stationary phase with polar mobile phase.

Normal Phase Chromatography: Polar stationary phase with nonpolar mobile phase.

Ion Exchange Chromatography: Separates charged species based on their interaction with the charged stationary phase.

Size Exclusion Chromatography: Separates molecules based on size.

Gas Chromatography

In gas chromatography, separation is primarily achieved by differences in the volatility and boiling points of the analytes. Compounds that evaporate easily will elute from the column first, while less volatile compounds will take longer to pass through. Interactions between the analyte and the stationary phase can also affect retention time.

Sensitivity and Resolution

HPLC Sensitivity

HPLC generally has higher sensitivity for non-volatile compounds because it is able to analyze lower concentrations of samples without evaporation. Using smaller particle sizes in HPLC columns provides a larger surface area for interaction, which improves resolution.

GC Sensitivity

Since gas chromatography is able to concentrate analytes through evaporation, it is able to achieve high sensitivity for volatile compounds. Capillary columns generally have better resolution than packed columns due to their longer length and smaller diameter.