Gas chromatography (GC) is a commonly used instrumental method in different fields for analytical purposes. GC is an analytical separation and purification technique used to analyze a volatile compound or a compound in the gaseous phase. This technique is based on the principles of column chromatography where gas is used as the mobile phase.
The stationary phase in GC can be liquid or solid and based on this the GC can be categorized as Gas-Liquid Chromatography(GLC) or Gas-Solid Chromatography(GSC). The separation in GLC is based on the principle of partition of the analyte into liquid and gas whereas in GSC the separation is based on the adsorption of the analyte on the solid stationary phase.
GAS Chromatography was first developed by APJ Martin and AT James in 1951 and it was gas-liquid chromatography. The first gas-solid chromatography was developed by Erika Cremer(German Physical chemist) and Fritz Prior(an Austrian graduate student) in 1947.
Principle of Gas Chromatography
The basic principle of GC is the separation of the analyte based on its affinity with the stationary and mobile phases.
In GSC the separation is based on the laws of Freundlich and Langmuir which are represented by:
x/m = Kc¹⁄n
x/m = K₁c + K₂c , respectively
x = mass of the gas
m = mass of the sorbent
K, K₁, K₂ = constant
c = vapor concentration in the gas
Freundlich’s law describes the adsorption isotherm which is a curve that expresses the variation in the amount of gas adsorbed by the adsorbent with the temperature at constant pressure.
Langmuir law explains the adsorption isotherm which describes the equilibrium between adsorbate and adsorbent systems, where the adsorbate adsorption is limited to one molecular layer at or before a relative pressure of unity is reached.
The principle of GLC is based on Henry’s law of partition which states that at a constant temperature, the amount of a given gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. It is represented by the equation:
x/m = Kc
Instrumentation and Working of Gas Chromatography
The instrumentation of GC is composed of the following components:
- Carrier gas chamber
- Sample injector
- Separation column
- Thermostat chamber
I. Carrier Gas Chamber
Carrier gas is the soul of this instrument. It acts as the mobile phase which carries the vapourised analyte from the separating chamber to the detector.
The carrier gas should be inert, pure, not cause any hazardous explosion, easily available and cost-effective, detectable to the detector, and have consistent column speed. The most frequently used carrier gases are- hydrogen, nitrogen, and helium. These gases can be used as single carrier gases or in combinations.
II. Sample Injector
The sample injector is connected to the head of the separating column used to inject a small amount of sample in the column. The sample is injected by a hypodermic syringe through a self-sealing septum into an inlet vaporizing chamber which is heated to vaporize the sample immediately.
The amount of sample injected in the column should be small but must be reproducible so that it can be detected by the detector. The vaporization chamber is typically heated 50 °C above the lowest boiling point of the sample and subsequently mixed with the carrier gas to transport the sample into the column.
Liquid samples are directly injected into the chamber whereas the solid samples are first dissolved or mixed in the volatile liquids and then injected in the separating column. A special gas stream is required for the injection of gaseous samples.
III. Separating Column
The separating column is the place where the separation of analytes is carried out. It is a long tube made up of glass or metal or teflon which is wound in a spirally coiled fashion placed in the thermostat chamber. The separating columns used in GC are of two types namely- open tubular column and packed column.
Open-tubular columns, also known as capillary columns, are thin, fused silica glass tubes, lined with a liquid phase or adsorbent material or having a chemical bonding layer. They are further classified as WCOT and SCOT.
WCOT, i.e. Wall Coated Open Tubular columns are the capillary tubes coated with a thin layer of stationary phase along the walls. The sample holding capacity of this column is low but still, it has better efficiency. FSWC(Fused Silica Wall Coated) is a special WCOT column in which the walls are prepared from fused silica containing minimal amounts of metal oxides. These columns are highly inert, require small amounts of sample, and have higher column efficiency.
SCOT or Support Coated Open Tubular columns are capillary tubes coated with a thin layer of adsorbent solid such as diatomaceous earth. The sample holding capacity of SCOT is higher but the efficiency is lower than WCOT.
Packed columns are metal or glass tubings that are densely packed with solid material like diatomaceous earth. The diameter of these columns is usually larger than the open tubular column because they need to be densely packed. This results in their low efficiency in comparison to open tubular columns. There is a semi-permanent deposition of impurities in the adsorbent causing the deactivation of the column
IV. Thermostat Chamber
The thermostat chamber is responsible for the continuous heating of the columns which is necessary for the maintenance of the gaseous phase of the carrier gas and sample throughout the column. The chamber operated either in isothermal programming or temperature-based programming.
Isothermal programming maintains a constant temperature throughout the separation process and can be useful for samples with narrow boiling point differences whereas temperature-based programming requires a regular increase of temperature to separate the samples having a broad range of boiling points.
Detectors are located at the end of the column that gives quantitative measurements of the analyte in the mixture. It is composed of a sensor (which is placed near the column that detects the signal) and electronic equipment (which digitizes the received analog signal).
There are a wide variety of detectors used in Gas Chromatography. Some of them are :
- Flame Ionisation Detectors(FID)
- Thermal Conductivity Detector(TCD)
- Electron-Capture Detector(ECD)
- Atomic Emission Detectors (AED)
- Mass Spectrometry(MS) Detector
- Chemiluminescence (CS) Detector
- Photoionization Detector (PID)
The last components of a GC instrument are amplifiers and recording systems. The signals detected by the detectors are amplified multiple times by an amplifier and recorded by the computer systems in the form of graphical representations.
Advantages of Gas Chromatography
GC is a highly sensitive technique with high resolving power. The instrument can detect the components of a mixture easily with a small amount of sample. It provides high accuracy and precision. The instrument is cost-effective and time-effective.
Limitations of Gas Chromatography
Though GC is a frequently used analytical method, it has certain drawbacks that limit its use. Gas Chromatography is performed only for volatile compounds even semi-volatile compounds cannot be detected by it. It requires strong electric fields for rapid temperature variation which may affect the signal detection. Also, frequent calibration of the instrument is needed regularly.
Uses of Gas Chromatography
Gas Chromatography is an analytical technique with high applications in multiple disciplines. It is frequently used in the pharmaceutical and cosmetics industry, petrochemical industry, environmental monitoring, food industry, etc.
From the perspective of forensic science, Gas Chromatography is a frequently used instrumental method. It has a wide application in forensic toxicology for the analysis of poisons, pesticides, poisonous gases, etc. It is often used for the drug profiling of the seized drugs. It is also helpful in the analysis of fibers, paints, explosives, inks, toners, dyes, etc.
GC is a destructive analytical technique that has a wide application in multiple industries. It is a separation, purification, and identification technique, also known as hyphenated analytical method. It is coupled with mass spectroscopy to get enhanced quantifying results.
Though it is an advanced technique it still has a few limitations that need to be worked upon so that it can give more effective, reliable, and robust results.