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GAS LIQUID CHROMATOGRAPHY
A gas chromatograph is a chemical analysis instrument for separating chemicals in a complex sample. It is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition.
A gas chromatograph uses a sample flow-through narrow tube known as the column, through which different chemical constituents of a sample pass in a gas stream (carrier gas, mobile phase) at different rates depending on their various chemical and physical properties and their interaction with a specific column filling, called the stationary phase.
Gas chromatography is typically used in testing the purity of a particular substance, or in separating the different components of a mixture. It can also be used to prepare pure compounds from a mixture.
Gas chromatography, utilizes the principle of partitioning between molecules in the mobile phase or “moving phase” and the stationary phase. The mobile phase, is usually an inert carrier gas, such as helium or unreactive gas e.g. nitrogen.The stationary phase is a microscopic layer of liquid on an coated or suspended on inert solid support placed inside a long piece of glassor metal tubing called a column
The gaseous compounds being analyzed interact with the walls of the column, which is coated with a stationary phase. This causes each compound to elute at a different time, known as the retention time of the compound. The comparison of retention times is what gives GC its analytical usefulness.
Components of Gas chromatograph
Gas chromatograph consist of a carrier gas supply, an injection port, a column, an oven and a detector. The detector output will be received by an integrator which is an instrument that will integrate the output signal. and the integral signal is then recorded from a recorder as shown in the diagram below
Components of Gas chromatograph
Care and maintenance
Preventive maintenance of gas liquid chromatograph includes replacing consumable items and performing maintenance procedures on a regular basis. Specific precautions for working with gas chromatographs include:
- Perform periodic visual inspections and pressure leak tests of the sampling system plumbing, fittings and valves.
- Follow the manufacturer’s instructions when installing columns. Glass or fused capillary columns are fragile: handle them with care and wear safety glasses to protect eyes from flying particles while handling, cutting or installing capillary columns.
- Turn off and allow heated areas such as the oven, inlet and detector, as well as connected hardware, to cool down before touching them.
- To avoid electrical shock, turn off the instrument and disconnect the power cord at its receptacle whenever the access panel is removed.
- Turn off the hydrogen gas supply at its source when changing columns or servicing the instrument.
- When using hydrogen as fuel (flame ionization FID and nitrogen-phosphorus detectors NPD), ensure that a column or cap is connected to the inlet fitting whenever hydrogen is supplied to the instrument to avoid buildup of explosive hydrogen gas in the oven.
- Measure hydrogen gas and air separately when determining gas flow rates.
- Perform a radioactive leak test (wipe test) on electron capture detectors (ECDs) at least every 6 months for sources of 50MBq (1.35 mCi) or greater.
- Ensure that the exhaust from (ECDs) is vented to the outside.
- When performing split sampling, connect the split vent to an exhaust ventilation system or appropriate chemical trap if toxic materials are analyzed or hydrogen is used as the carrier gas.