Difference Between Gas and Liquid Chromatography (with Comparison Chart)

Both gas and liquid chromatography serve the same purpose, i.e., separating the components of any mixture. But they vary greatly if we talk about their working mechanisms, components used and the required conditions.

Gas chromatography uses gas as its mobile phase for moving the particles of the sample inside the column. In contrast, in liquid chromatography, the mobile phase used to move sample particles is liquid in nature.

The principle behind gas chromatography is that it uses, separates and analyses the compounds in their volatile or gaseous state. In contrast, the liquid chromatography technique separates the dissolved ions, particles or molecules in a liquid.

The gas chromatography works under a very high temperature to keep the sample in its volatile state. In comparison, liquid chromatography works under high-pressure conditions that can push and separate the constituent particles of a sample.

In analytical chemistry, people often get confused between these two important chromatographic techniques. Thus, here we aim to provide you with all the major differences between gas and liquid chromatography.

Comparison Chart

Basis of Characteristics	Gas Chromatography	Liquid Chromatography
Meaning	It is the chromatographic technique that aids the separation and analysis of volatile compounds in a gaseous phase.	It is the chromatographic technique which separates ions or molecules present in the dissolved state in a solvent.
Also Known as	Gas-liquid chromatography or vapour-based chromatography	Liquid-solid chromatography
Mobile phase	Gas Example: Helium	Liquid Example: Silica
Stationary phase	Liquid molecules present in the solid support	Solid adsorbent
Sample	Volatile	Comparatively less volatile
Chromatographic Bed Shape	Column	Column or plane
Detectors	The quality of resolutions depends on the volatility of the components present in the mixture	Here the detectors are: • Ultraviolet-visible (UV-Vis) spectroscopic detector • Refractive Index detector (RID) • LC/MS
Column	Thin, long and narrow capillaries or packed columns	Short and wide-sized packed columns
Dependency of Resolution	The quality of resolutions depends on the volatility of the components present in the mixture	Here, the resolution relies on the composition of the mixture and the polarity of the molecules present.
Operative Conditions	Takes place in high temperatures	Works under high pressure
Temperature Control	Required	Not-required
Use	In the separation of the volatile compounds	In the separation of the soluble constituent molecules
Expense	Low-cost	High-cost
Time consumption	Takes comparatively less time	Very time -a consuming process
Application	• Separation of oils and fatty acids • Separating the plant’s pigment • Drug abuse testing • Isolating the volatile pesticides from the sample • Testing the toxins and other harmful chemicals in the air	• Separation of inorganic ions from the sample • Separation of sugars, polymers, nucleotides, vitamins, peptides, proteins, lipid molecules etc.

What is Gas Chromatography?

Gas chromatography is a very renowned technique in analytical chemistry used to separate and analyse the constituents of the compounds in their volatile state.

Gas chromatography uses gas as a mobile phase to carry the sample molecules over the stationary phase. This gaseous phase carries the sample molecules on the stationary phase inside the column.

Separation of Components

The separation of the components in gas chromatography relies on the partition equilibrium of the components between the mobile and stationary phase.

The level of separation depends upon the boiling point of the components. The higher the boiling point, the more slowly the component moves through the stationary phase.

The molecules with a low boiling point interact less with the stationary phase and more with the mobile phase, moving faster through the column.
Whereas those with higher boiling points interact less with the mobile phase and more with the stationary phase, moving slowly within the column.

Components of Gas Chromatography

Column

It is a long-tube-like coil that remains compactly arranged in a chamber. The column contains an adsorbent material that acts as the stationary phase for the chromatography.

There are two types of columns:

Packed column
Capillary column

Sample

The sample should be volatile if you want to analyse it with gas chromatography. It can be either in gaseous or liquid form.
If it is in the liquid state, the sample needs to get converted into its vaporised form.

The sample used should be able to tolerate higher temperatures. Also, we should keep in mind the original nature and activity of the sample do not change during volatilisation.

Stationary Phase

The stationary phase provides the platform on which the molecules run. The inner walls of the column have the lining of adsorbent material that acts as a stationary phase.

Both liquids, as well as solid materials, can serve the role of the stationary phase. Moreover, the material used must be able to bear very high temperatures in the chamber or oven.

If the stationary phase is solid, we refer to this technique as gas-solid chromatography. Whereas, when the stationary phase is liquid, we refer to this technique as gas-liquid chromatography.

Mobile Phase

The gas used for the mobile phase is generally inert or unreactive by nature. This avoids any chemical activity between sample and gas or gas and stationary phase.

Mostly the analytical laboratories prefer helium or nitrogen gas for this purpose since the former is inert while the latter is unreactive.

Molecular sieve

This remain attached between the gas cylinder and the chamber. It filters out all the unwanted contaminations like hydrocarbons, oxygen or water content that could hamper the process.

Detector

A detector detects the elute molecules coming out of the column at the end of the column.

There are several types of detectors present in gas chromatography like:

Flame ionisation detector (FID)
Thermal conductivity detector (TCD)
GS/MS

Working on Gas Chromatography

The sample to be separated is prepared by mixing with an appropriate volatile solvent such as heptane, acetone or methanol. The sample is injected into the injecting port.

The injection port temperature remains 20-50 degrees higher than the column. This allows the rapid volatilisation of the sample.

The mobile gas is then released into the column, and the sample starts.

The constituents separate according to their respective boiling points.

Detection of results in Gas chromatography

As soon as the samples run via column, the detector detects the elute. After it sends the signals to the computer to generate a peak.

They give a peak concerning the retention time of the sample.

The area under the peak gives information about the concentration of the sample.

Understanding Gas Chromatography with example

Suppose if we run three samples,

Containing methanol: Sample 1
Containing acetone: Sample 2
Unknown: Sample 3

The peak in methanol came at 5 min while that of acetone came at 3 min.

So, when the unknown sample shows the peak at 5 min and 8 min, then it makes it confirms that the one obtained at 5 min is methanol. And, since the graph indicates no peak at 3 min, this indicates the absence of acetone.

What is Liquid Chromatography?

Liquid chromatography is the most advanced type of chromatography which uses liquid as its mobile phase. Currently, liquid chromatography refers to high-performance liquid chromatography (HPLC).

As the name suggests, high performance means high resolution, while liquid chromatography indicates the liquid mobile phase. Moreover, HPLC is referred to as high-pressure liquid chromatography in some places since it works under very high pressure.

Principle Behind Liquid Chromatography

The working principle of Liquid chromatography or HPLC is that of others. The principle starts with equilibration, where the column is equilibrated with buffer followed by sample binding, wash, and ultimately elution.

Types of HPLC

There are two primary types of HPLC based on the polarity of stationary and mobile phases:

Normal phase: The mobile phase is non-polar while stationary is polar.
Reverse phase: The mobile phase is polar, and the stationary phase is non-polar.

Components of Liquid Chromatography