Gas Chromatography vs Liquid Chromatography: Principles & Components

Posted on Jan 21, 2025 in Chemistry

Gas Chromatography (GC) vs. Liquid Chromatography (LC)

The fundamental difference between GC and LC lies in the state of aggregation of the mobile phase. In LC, the mobile phase is a liquid, while in GC, it’s a gas. They also differ in viscosity and their ability to penetrate.

GC Components:

  • Gas supply system (carrier gas)
  • Nano-reducer
  • System cell regulation and flow measurement
  • Sample introduction system
  • Thermostat system (oven)
  • Column
  • Detection system
  • Microprocessor temperature programming

LC Components:

  • Solvent tank
  • High-pressure pump
  • Injection system
  • Detector
  • Continuous microprocessor

Improvements include pulse suppression and temperature control of the pump, ensuring free flow to the injector. Systems also include solvent reservoirs and electronic gradient programming with high or low-pressure pumps, and fraction collectors.

Stationary Phase

The stationary phase is a solid (adsorption) or liquid (partition) retained in a column through which the mobile phase passes.

Carrier Gas in GC

The carrier gas is the mobile phase in GC. Its role is to transport the solute mixture from the injection point through the column to the detector. It must be chemically inert and not interact with the column or the solutes. Common carrier gases include N2, He, H2, and Ar. The choice depends on the sample, stationary phase, detector, cost, purity, and safety.

  • N2: Cheap, safe, and easy to purify, but has low thermal conductivity.
  • H2: High thermal conductivity, low viscosity, and inexpensive, but may reduce or alter solutes.
  • He: Combines advantages of N2 and H2, but is more expensive. Less dense than H2, allowing for greater flow.
  • Ar: Relatively inexpensive and easy to purify.

Mobile Phase Effects in GC

The mobile phase in GC affects the duration and resolution of the chromatographic process. Lower solute diffusivity requires lower gas speeds, increasing analysis time. Chromatographic resolution depends on the carrier gas, as it affects column efficiency (AETP). High solute diffusivity in the carrier gas is desirable for shorter retention times.

Sample Introduction

Liquid Samples:

More commonly used in GC. Introduction methods include rotating cells and injection needles.

Gas Samples:

While special needles exist, gas sampling cells are preferred.

Solid Samples:

Usually dissolved in a suitable solvent and introduced like liquid samples. Encapsulation in a glass capillary, broken in a heated injection block, is also used.

Headspace Analysis

Headspace analysis is used for samples that cannot be injected directly or contain non-volatile compounds. It analyzes the vapor phase in equilibrium with the sample in a closed container. This is useful for determining trace amounts of volatile substances in diluted samples.