Use of gas chromatograph

Gas chromatographs are widely used in fire investigation, petroleum, chemical, biochemical, medical and health, food industry, and environmental protection. In addition to its quantitative and qualitative analysis, it can also determine the physicochemical constants such as partition coefficient, activity coefficient, molecular weight and ruthenium specific surface area of ​​the sample on the stationary phase. An instrument for analyzing and detecting each component in a mixed gas. Let us follow the work of the gas chromatograph with Wuhan Titworth.

1. Requirements for the chromatograph analysis room

(1) There must be no strong magnetic field, flammable or strong corrosive gas around the analysis room.

(2) The indoor ambient temperature should be in the range of 5 to 35 degrees, the humidity should be less than or equal to 85% (relative humidity), and air circulation should be maintained indoors. Conditional factory zui is good to install air conditioning.

(3) Prepare a working platform that can withstand the entire instrument, moderate width and easy operation. The general factory is better with cement platform (0.6~0.8 m high), the platform can't be close to the wall, and should be 0.5~1.0 m away from the wall, which is convenient for wiring and maintenance.

(4) The power line capacity for the instrument should be around 10KVA, and the power supply of the instrument should not share a line with the high-power power consumption equipment or the power equipment that often changes greatly. The power supply must be well grounded. Generally, the iron rod (wire) with a length of about 0.5~1.0 m is nailed on the wet ground (or the salt solution is poured), and then the power grounding point is connected with it. In general, the grounding resistance is less than 1 ohm.

2. Gas source preparation and purification

(1) Gas source preparation

Pre-prepared high-pressure cylinders (usually available in large and medium-sized cities) that require gas. A cylinder containing a certain gas can only contain such a gas. The color of each cylinder represents a gas and cannot be exchanged. Generally, three gases of nitrogen, hydrogen and air are used, and each cylinder is ready for two cylinders for use. Some plants use hydrogen generators and air compressors, but air compressors must be oil-free. When the cylinder pressure drops to 1~2Mpa, the cylinder should be replaced. Generally, the manufacturer can use 99.99% of the above gas, and the electron capture detector must use a high purity gas source of 99.999% or more.

(2) Gas source purification

In order to remove the moisture, ash and organic gas components that may be contained in various gases, the gas should be subjected to strict purification before entering the instrument. If all the cylinder gas is used, some chromatographs are equipped with a purifier, and the inside is filled with 5A molecular sieve, activated carbon, silica gel, which can basically meet the requirements. If a general hydrogen generator is used, the water purification treatment must be strengthened, so the drying pipe area should be increased (the volume is more than 450 cubic centimeters, the filler is preferably 5A molecular sieve), and the volume after the generator is larger. The reservoir barrel reduces or overcomes the effects on the instrument baseline when the source pressure fluctuates. If an air compressor is used as the air source, the air inlet of the air compressor should be reinforced with air filtration, and the volume of the purification pipe should be increased. Half of the 5A molecular sieve and half of the activated carbon should be filled in the drying pipe. Generally, domestic oil-free gas compressors can meet the needs.

3. Chromatograph set inspection and placement

After the instrument is unpacked, check the list of accessories in the kit, and check the spare parts of the consumable parts. Then, according to the requirements of the instrument's instruction manual, place it on the working platform, connect the parts of the instrument to the wiring diagram and each plug, and connect the recorder to the data processor. Be careful not to connect the connectors.

4. Connection of the external gas path

(1) Installation of pressure reducing valve

Some instruments come with a pressure reducing valve, if not, you have to buy it. Two oxygen and one hydrogen pressure reducing valve were used. Install 2 oxygen pressure reducing valves and 1 hydrogen pressure reducing valve on the nitrogen, air and hydrogen cylinders (note that the hydrogen pressure reducing valve threads are reversed and the attached O-shaped plastic gasket is added at the interface. In order to seal), after tightening the nut, close the pressure reducing valve adjustment handle (ie, loosen) and open the cylinder high pressure valve. At this time, the pressure gauge high pressure gauge should have an indication. After the high pressure valve is closed, the indicated pressure should not drop. Otherwise, there is leakage, it should be eliminated in time (with gasket or raw material tape seal), sometimes the high pressure valve will leak, pay attention. Then turn the adjustment handle to drain the remaining air.

(2) External airway connection method

The gas in the cylinder is introduced into the chromatograph, and some are made of stainless steel tubes (φ2×0.5 mm), and some are made of pressure-resistant plastic tubes (φ3×0.5 mm). Plastic tubes are easy to handle, so plastic tubes are generally used. If a plastic tube is used, a stainless steel liner (φ2×20 mm) and some plastics for sealing should be used at the joint. The length of the plastic tube from the cylinder to the instrument is determined as needed. It should not be too long, and then the gas source and the instrument (gas inlet) are connected by a plastic tube.

(3) Leak detection of external gas path

Upload the gas on the main airway panel, shut off the valve knob of hydrogen and air, then open the high pressure valve of each cylinder and adjust the output pressure of the low pressure gauge on the pressure reducing valve to make the carrier gas and air pressure 0.35~0.6Mpa (about 3.5~). 6.0kg/cm3), hydrogen pressure is 0.2~0.35 Mpa. Then close the high pressure valve. At this time, the indication value of the low pressure gauge on the pressure reducing valve should not be lowered. If it is lowered, it indicates that there is a leak in the connecting gas path and should be excluded.

5. Chromatograph gas path air tightness check

Air tightness inspection is a very important task. If the airway leaks, it will not only directly lead to unstable or weak sensitivity of the instrument, but also the risk of explosion. Therefore, this work must be carried out before operation. The inspection is generally to check the carrier gas flow path. If the hydrogen and air flow paths have not been removed, they may not be checked. The method is to open the column cover, remove the column from the detector, block the column, and then open the carrier flow path, adjust the low-pressure output pressure to 0.35~0.6Mpa, and open the carrier gas knob on the host panel. The pressure gauge should have instructions. After the zui, the gas carrier knob is turned off, and the pressure indication value before the column should not decrease within half an hour. If there is a decrease, there is a leak, which should be excluded. If there is a leak in the air circuit inside the main unit, remove the side plate of the main unit and use soapy water (zui is sodium dodecyl sulfonate solution) to check for leaks one by one (hydrogen, air can also be leak tested), after Zui will Dry the soapy water.