The main difference between gas and liquid chromatography is that the mobile phase of gas chromatography is a gas, which is most often helium, whereas the mobile phase of liquid chromatography is a liquid, which can be either polar or non-polar. Furthermore, the stationary phase of gas chromatography is often a liquid silicone-based material while the stationary phase of liquid chromatography is mainly silica. Moreover, gas chromatography is carried out in a column while liquid chromatography is either carried out in a column or a plane.
Gas and liquid chromatography are the two types of chromatography techniques classified based on the physical state of the mobile phase. Generally, the mobile phase is the phase that flows through the stationary phase.
Key Areas Covered
1. What is Gas Chromatography
– Definition, Principle, Importance
2. What is Liquid Chromatography
– Definition, Principle, Importance
3. What are the Similarities Between Gas and Liquid Chromatography
– Outline of Common Features
4. What is the Difference Between Gas and Liquid Chromatography
– Comparison of Key Differences
Key Terms
Column Chromatography, Gas Chromatography, Liquid Chromatography, Mobile Phase, Stationary Phase
What is Gas Chromatography
Gas chromatography is the type of analytical chromatography whose mobile phase is a gas. Generally, this carrier gas is either an inert gas such as helium or a non-reactive gas such as nitrogen. However, hydrogen is preferred over helium for better separation, although helium is the common carrier gas in 90% of the instruments. Moreover, the stationary phase of gas chromatography is a liquid. Therefore, the full name for gas chromatography is gas-liquid chromatography. Here, a microscopic layer of liquid stationary phase occurs on inert solid support inside a tiny glass tube. Thus, gas chromatography operates as a column chromatography technique.
Figure 1: Gas Chromatography
Furthermore, gas chromatography is responsible for analyzing compounds in the form of vapor. Also, its separation of compounds depends on the partition equilibrium of components between the mobile and the stationary phase. However, the usage of high temperature in gas chromatography makes it unsuitable for separating polymers of high molecular weights. Basically, this is due to the inability of these polymers to become a vapor. In preparative chromatography, gas chromatography is an important tool to prepare pure components from a mixture.
What is Liquid Chromatography
Liquid chromatography is the other type of chromatography classified based on the physical state of the mobile phase. Significantly, its mobile phase is a liquid. For instance, the stationary phase of the liquid chromatography is solid. Therefore, the basic chromatography structure can be either a column or plane chromatography. Generally, in column chromatography, the stationary bed occurs within a tube. In contrast, in planar chromatography, the stationary phase occurs on a plane.
Figure 2: Liquid Chromatography
Moreover, present-day liquid chromatography is mainly the high-performance liquid chromatography (HPLC), which uses a very small packing of articles. Also, HLC operates under high pressure. Therefore, the stationary phase is mainly a porous membrane or a porous monolithic layer, composed of spherical or irregular shaped particles. Meanwhile, the liquid mobile phase flows on the stationary phase under high pressure. However, there are two types of HPLC techniques according to the polarity of the mobile and stationary phases. They are the normal phase and reverse-phase liquid chromatography. Typically, in the normal phase liquid chromatography, the mobile phase is non-polar (e.g. toluene) while the stationary phase is polar (e.g. silica). On the other hand, in reverse-phase liquid chromatography, the mobile phase is polar (e.g. water-methanol mixture) while the stationary phase is non-polar (e.g. C18). However, both types of HPLC operate under room temperature.
Similarities Between Gas and Liquid Chromatography
- Gas and liquid chromatography are the two types of chromatography techniques classified according to the type of mobile phase.
- Both are laboratory techniques for the separation of a mixture. Also, both are analytical separation methods.
- Generally, the mixture to be separated is dissolved in the mobile phase, which carries it through the stationary phase.
- However, the separation occurs depending on the properties of the components of the mixture, determining the variable interactions towards the mobile or stationary phase.
- Both can be column chromatography.
- Mass spectrometry (MS) is the most powerful detection method for both types of chromatography.
Difference Between Gas and Liquid Chromatography
Definition
Gas chromatography refers to the chromatography technique which separates and analyzes volatile compounds in the gas phase while liquid chromatography refers to the chromatography technique useful for separating ions or molecules dissolved in a solvent.
Also Known as
Another name for gas chromatography is gas-liquid chromatography while another name for liquid chromatography is liquid-solid chromatography.
Type of Mobile Phase
The mobile phase of gas chromatography is a gas while the mobile phase of liquid chromatography is a liquid.
Examples
The mobile phase of gas chromatography is most often helium, while the mobile phase of liquid chromatography can be either polar or non-polar.
Mobile Phase Gradient
While the mobile phase has no gradient in gas chromatography, the mobile phase has a gradient in liquid chromatography.
Stationary Phase
Moreover, the stationary phase of gas chromatography is often a liquid silicone-based material while the stationary phase of liquid chromatography is mainly silica.
Chromatographic Bed Shape
Gas chromatography is carried out in a column while liquid chromatography is either carried out in a column or a plane.
Columns
Long and narrow packed or capillary columns are used in gas chromatography while short and wide packed columns are used in liquid chromatography.
Sample
Components of the sample are volatile in gas chromatography, while the components of the sample are less volatile.
Chromatographic Conditions
Gas chromatography operates under high temperatures while liquid chromatography operates under high pressure.
Resolution
The resolution of gas chromatography depends on the volatility of the components of the mixture while the resolution of liquid chromatography depends on the polarity of molecules and the composition of the mobile phase.
Detectors
The two main types of detectors used in gas chromatography are flame ionization detector (FID) and thermal conductivity detector (TCD) while the two main types of detectors used in liquid chromatography are ultraviolet-visible (UV/Vis) spectroscopic detector and refractive index detector (RID).
Importance
Gas chromatography is mainly used in analytical chemistry while high-performance liquid chromatography is the mainly used form of liquid chromatography.
Relative Cost
Also, gas chromatography is a low-cost technique, while liquid chromatography is a high-cost technique.
Applications
Gas chromatography is used for the separation of oils, plant pigments, pesticides, fatty acids, toxins, air samples, drug abuse testing, etc. while liquid chromatography is used for inorganic ions, polymers, sugars, nucleotides, vitamins, peptides, proteins, lipids, tetracyclines, etc.
Conclusion
Gas chromatography is the type of chromatography, using a gas mobile phase. Generally, the mobile phase is helium. Also, the stationary phase of gas chromatography is a liquid with a silicone base. Therefore, it is a type of column chromatography. Liquid chromatography is another type of chromatography, using a liquid mobile phase, which is mainly silica. Moreover, liquid chromatography can be either column or plane chromatography. Hence, the main difference between gas and liquid chromatography is the physical state of the mobile phase.
References:
1. Clark, Jim. Gas-Liquid Chromatography. Available Here.
2. “Liquid Chromatography.” ELGA LabWater, Available Here.
Image Courtesy:
1. “Gas chromatograph-vector” By Offnfopt – Own work based on: Gas chromatograph.png (Public Domain) via Commons Wikimedia
2. “Preparative HPLC” By GYassineMrabet. This W3C-unspecified vector image was created with Inkscape. – Own work (CC BY 3.0) via Commons Wikimedia
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