Gas chromatography is an analytical technique that is used for separating and analyzing thermally stable compounds. It is to be noted that only those compounds which can easily vaporize without decomposition and do not undergo any change at high temperatures can be analyzed.
In this technique, the sample is vaporized and injected into a column, then eluted with a gas. Note that, the mobile phase is a gas rather than a liquid. However, if we compare this technique with others, it has a more complicated and expensive apparatus. Yet the advantages are more than these limitations.
The key characteristics of a sample to be tested in gas chromatography are as follows:
- Thermally stable
Gas Chromatography works on the principle of the relative affinity of sample analyte with the stationary and gas mobile phase. In general, it is used solely as well as coupled with mass spectrometry (GC-MS) for best results. This sophisticated technique finds hundreds of applications in analytical and research labs for the analysis of materials.
Principles of Gas Chromatographic Techniques
In gas-liquid chromatography (GLC), the principle is similar to liquid partition chromatography (LPC) except that the mobile phase is a gas. Whereas, in gas-solid Chromatography (GSC) the analyte is adsorbed directly on the solid surface of the stationary phase. The sample components distributed themselves between the stationary and mobile phases. The separation occurs inside the column based on the retention time.
The sample is converted into vapors and injected through a silicon rubber septum with a syringe needle and enters a carrier gas septum. These are forced through the separation column by the carrier gas. Such as Ar, He, and N2. The injection port is maintained at a higher temperature than the boiling point of the least volatile component in the sample mixture.
In addition to it, It involves separating components with a wide range of boiling points is accomplished by starting at a low oven temperature. After that, the temperature is increased over time to elute the high boiling point components.
Types of Gas Chromatography
Gas chromatography has two types based on the types of the stationary phase.
Gas-solid chromatography (GSC)
In this type, the stationary phase is a solid that acts as an adsorbent. The components of the sample distribute themselves between the mobile phase and the solid stationary phase. As GSC can be used at high temperatures, therefore, it is more frequently used for the analysis of gases having no active functional groups which interact with the adsorbent surface.
Gas-liquid chromatography (GLC)
In this type, the stationary phase is the liquid. The separation takes place by the partition of the analyte between the gas phase and liquid phase. A large range of liquid coatings provides a large range of separation and good resolution of peaks.
Instrumentation of Gas chromatography
Gas chromatography consists of the following components which are detailed below:
Carrier gas (mobile phase)
The mobile phase is known as the carrier gas. This is because its purpose is to transport solute through the column. The mobile phase should have the following properties:
- It should be inert.
- It should be non-flammable.
- It is preferably cheap.
- The carrier gas must be purified.
Helium and nitrogen are mostly used. However, helium is used in the capillary column whereas nitrogen is used in the packet column. Note that, helium gives separation efficiency due to faster mass transfer. Thus, high-pressure carrier gas tanks are attached to the pressure regulator and flow meter.
Source of carrier gas
The carrier gas is supplied from a high-pressure cylinder, with a purity of 99.999% via a pressure-reducing valve at 10-45psi, which provides flow rates between 1 and 50 cm3/min.
The mass-flow controller ensures constant flow rates regardless of back pressure and temperature. So, a bubble flow meter or rotameter is used to check the flow rate. The most basic configuration of a bubble flow meter is calibrated tube, a stopwatch, and a good eye.
Common contaminants of the carrier gas
The most common contaminants of the carrier gas are the following:
- Air or oxygen: At a level above about 10 ppm can oxidize sample components. However, these can be removed by using a cartridge containing a molecular sieve.
- Hydrocarbons: Their presence affects the detector’s performance by producing a large background signal. This can be removed by using a cartridge containing activated carbon.
- Water vapors: Water vapors can affect some solid and bonded liquid stationary phases. They can be removed by using a molecular sieve.
Sample injection system
Liquid samples are introduced into the flowing mobile phase using a micro-syringe with a hypodermic needle. This needle is inserted through a self-sealing silicon rubber septum and the sample is injected smoothly into a heated metal block at the head of the column.
The temperature of the sample port should be appropriate such that the liquid is rapidly vaporized but without decomposing the sample. hence, the rule of thumb is to set the sample port temperature approximately to the boiling point of the least volatile component.
Columns in Gas Chromatography
A column is a place where the separation process occurs. There are two main classes of columns in gas chromatography which are discussed below:
Capillary (open tubular) columns
These are the most widely used column. They are made of stainless steel tube, glass, copper, or fused silica material which is usually between 5-500 meters in length and an internal diameter of a few tenths of a millimeter. In capillary columns, the inner wall of the tube is coated with the liquid or solid stationary phase. However, the most widely used stationary phase are polyethylene glycols, polysiloxane, alumina, etc.
Open tubular columns have the following properties:
- Provide better resolution.
- a large number of theoretical plates.
- has greater sensitivity.
- smaller sample capacity.
These are stainless steel or glass tubes filled with the stationary phase. It contains a finely divided inert, high surface area solid granular support material coated with the liquid stationary phase. Mostly, the packed column is 1.5-10 m long and has an internal diameter of 3 to 8 mm.
The main advantages of using packed columns in Gas chromatography are enlisted below:
- The major advantage is that packed columns have a uniform particle size, which decreases the multiple path term in the Van Deemter equation.
- The small particle size also reduces the time required for solute equilibrium, thus improving column efficiency.
- Packed columns have a high sample capacity and are durable.
- They are best suited to the separation of a mixture of up to ten or twenty components.
- They are much cheaper than the capillary columns. But, their overall efficiencies and resolving power are limited.
Stationary phases in Gas chromatography
GLC stationary phases
These stationary phases are the coatings of very high boiling liquids, oils, waxes, or some with polymeric structures such as polysiloxane and polyethylene glycol.
GSC stationary phases
GSC stationary phases are solid adsorbents and polymers. Since, they can be classified according to their polarity, varying from nonpolar hydrocarbons to polar polyesters.
There the following detectors are used in gas chromatography:
Advantages and limitations of gas chromatography
Gas chromatography has several advantages, for example, it required very small samples with little preparation time. Additionally, It is good at separating complex mixtures into components. Further, the results are rapidly obtained with high precision.
However, there are a few limitations of gas chromatography, such as only gaseous substances can be analyzed. In addition, it is a fairly complicated and expensive apparatus.
Applications of Gas chromatography
There are the following applications of gas chromatography:
- Separation of hydrocarbons and refinery gases in petroleum industries
- To analyze environmental hazardous substances like automobile exhaust gases, blood, saliva, urine, air, and drinking and wastewater.
- To separate the aromas of flowers
- In pharmaceuticals industries to check the intermediates, purity of drugs samples
- Forensic and clinical analysis, toxicological cases, fatty acids, steroids, and body secretions.
- To study the reaction mechanism
- Separation of radioactive products.
How does gas chromatography work?
Gas chromatography involves these four steps:
What is gas chromatography used for?
Gas chromatography is an analytical technique that is used for separating the mixture into its components.
What are the gases used in gas chromatography?
Chemically inert gases are used in gas chromatography such as helium.
What is the first peak in gas chromatography?
The first peak in the GC is the peak of the solvent.
Why do the results of gas chromatography not equal 100%?
No technique is 100% accurate. This is because a large number of errors can add up. Such as human error, temperature fluctuation, measurement errors, etc.
How are compounds with different boiling points separated in Gas Chromatography?
The GC involves the separation of the compounds based on the difference between boiling points. This is because this technique involves the vaporization of the sample.