Table of Contents
Gas chromatography Definition-
Performance of gas chromatography characteristics has made process gas chromatography the workhorse of the online chemical analysis industry. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase which is adsorbed onto the surface of an inert solid. Gas chromatography is also sometimes called as vapor-phase chromatography or gas–liquid partition chromatography. in short, Gas chromatography is also referred as GC. A process GC analyzes a vapor or volatile liquid sample and then separates the various chemical components in the sample for individual identification and measurement. The discrete separation and positive identification of components and the measurement process of composition enable the gas chromatograph to be one of the few analyzer types available that minimizes the potential for cross interference during measurement. This allows the analyst to measure many chemical compounds during each analysis to levels that reach parts-per-million and even parts-per-billion levels.
Gas chromatography (GC) principle –
Gas chromatography is based on the principle that components with higher affinity for the stationary phase have longer retention times because they take longer to come out of the column. However, components with higher affinity for the stationary phase have shorter retention times as they move along the mobile phase. The mobile phase is a gas, mostly helium, hydrogen that is sampled through the column. The sample is passed through a detector to determine the retention time once it has converted to the vapor state. The components are collected separately as they come out of stationary phase at different times. The various components are separated inside the column. The detector measures the amount of components exiting the column. To measure a sample with an unknown concentration, a standard sample of known concentration is injected into the instrument. The standard sample peak retention time (appearance time) and area are compared with the test sample to calculate the concentration.
Gas chromatography diagram-
Basic diagram for GC is shown in the figure.
The carrier gas must be chemically inert. Commonly used gases include nitrogen, hydrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependent upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities. To provide carrier gas to chromatographic column. Impure and moisture can harm the column , worse performance of detectors, adversely affect quantification of trace analysis. In flow regulator set requirement carrier pressure .There are two general types of column, packed and capillary (also known as open tubular). Packed columns contain a finely divided, inert, solid support material (commonly based on diatomaceous earth) coated with liquid stationary phase. Most packed columns are 1.5 – 10m in length and have an internal diameter of 2 – 4mm.Capillary columns have an internal diameter of a few tenths of a millimeter. They can be one of two types; wall-coated open tubular (WCOT) or support-coated open tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated with liquid stationary phase. In support-coated columns, the inner wall of the capillary is lined with a thin layer of support material such as diatomaceous earth, onto which the stationary phase has been adsorbed. In this part, it has two oven which is column oven and isothermal oven . Temperature controller keeps constant temperature by measuring of thermostats or other temperature sensor . Column oven and isothermal oven is different temperature and maintained constant temperature of both column and isothermal oven. GC Detector mostly used thermal conductivity detector for communicate display and remote system.
Gas chromatography detectors-
There are many detectors which can be used in GC. Different detectors will give different types of selectivity.
Thermal Conductivity Detector (TCD)-
Thermal Conductivity Detectors (TCD) are the most widely used in gas chromatography. These detectors use heated metal filaments (or thermistors made from certain semiconductors of fused metal oxides) to sense small changes in the thermal conductivity of the carrier gas. The thermal conductivity of the carrier gas only gives an essentially constant signal. The presence of vapors of the various components of the mixture in the carrier gas causes a change in thermal conductivity in proportion to their volume in the stream. This brings about a change in the resistance of the filament which is measured. The recorder that records these changes is equipped with an automatic device that tracks the magnitude of these changes on a graph sheet along with the retention time.The thermal conductivity detector utilizes the difference in the thermal conductivity between the measured gas and the carrier gas and detects the unbalanced voltage produced in a bridge circuit as a measure of concentration. The fundamental principle of the thermal conductivity detector (TCD) As shown in figure, there are two streams, each having two filaments. One stream passes through carrier gas only and the other connected to the column outlet allows sample with carrier the measured gas to pass during analysis. Thermal conductivity detector is frequently used to measure the component concentration of the measured gas (liquid).
Flame Ionization Detector (FID):-
FID uses the phenomenon that the carbon molecules in the measured component (hydrocarbons) are ionized in a hot hydrogen flame. i.e it detects the ionization current that flows between the electrodes to which a high voltage is applied. The ionization current is proportional to the concentration of the measuring component. FID is used to measure the component concentrations of gases with low concentrations of hydrocarbons.
As the sample gas containing the sulfur component is taken to an additional hydrogen flame, the component containing the sulfur atoms is excited. The FPD detects the luminous intensity of the emitted light when this excited component returns to its original position using a multiplier phototube and converts it into a voltage. This voltage proportional to the concentration of the sulfur component in the measured gas. FPD can measure on the sulfur component with a high sensitivity of 1 ppm and only works for sulfur.
- H2S, COS, CS2
An electron capture detector (ECD) is a device used in a gas chromatography to detect trace amounts of chemical compounds in a sample. Typically, it consists of a 10 milli-Curie nickel (Ni-63) metal foil.Gas chromatography is used in analytical chemistry to separate and analyze compounds that can be vaporized without decomposition. The ECD used in gas chromatography detects electron-absorbing samples such as halogen compounds. Nickel (Ni-63), a radioactive beta emitter, is a component of an ECD. Beta particles ionize a gas causing a background current to flow in the ECD circuitry. If a sample absorbs some negative electron ions (a process called ionization electron capture), the current decreases. Hence current fluctuations suggest the presence of sample components. The largest application of an ECD is in the analysis of halogenated compounds, making it possible to detect compounds such as pesticides and chlorofluorocarbons at levels of only one part per trillion.
Example of gas chromatography-
Some examples of GC are given which is used for monitoring and quality control in industries and also monitor in medical field.
- Petrochemical :- Ethylene, polypropylene , polythylene, butadine , alcohol, vinyl chloride
- Petroleum refining:- distillation point analysis ,sulfur recovery, FCC.
- Identification of performance-inducing drug in athlete’s urine.
- Separation and quantification of a solid drug in soil and water samples
Gas chromatography applications-
Followings are given some important gas chromatography applications.
- This technique is used to calculate the concentrations of different chemicals in different samples.
- It is used in the analysis of air pollutants, oil spills and other samples.
- Gas chromatography can also be used in forensic science to identify and quantify various biological samples found in a crime scene.
Advantage of Gas chromatography-
The major advantage of gas chromatography is its high sensitivity, resolution and separation capability, which allows it to separate a wide range of volatile compounds.
- It can be used to determine the mass-to-charge ratio of ions by a upgraded mass spectrometer (MS).
- It comes with a variety of detectors and injectors that can be used for various pharmaceuticals as well as other applications.
- Gas chromatography can analyze a sample of the process much faster than other chromatography techniques.
- It is a robust method of separation that gives a better signal-to-noise ratio.
- It only takes a very small amount of sample to be injected, and its detectors are extremely sensitive, allowing it to detect extremely low concentrations (ng-pg).
- According to the requirement of the molecule, different types of GC columns are available in many diameters and lengths.
- Gas chromatography is easy, automated, and involves quick analysis of data which gives comparatively high precision, accuracy, and reproducible results.
Disadvantage of Gas chromatography-
- The major disadvantage of gas chromatography is that only volatile and thermally stable compounds can be separated using gas chromatography.
- The detectors used in GC are destructive except in MS( mass spectrometer).
- The selectivity is also better in HPLC or TLC as the mobile phase can be changed easily. In GC, you can only modify the temperature of the column and the oven, but you can not change the mobile phase because it has a constant flow of carrier gas (helium, nitrogen).
- Since hydrogen gas, which is used for flames, is highly flammable, caution should be exercised when using it.
- It is impossible to recover the individual sample components.
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