This experiment is from the official website of College of Chemistry, Qingdao University of Science and Technology.
Operation method
Connection of air circuit system, leakage detection, measurement and calibration experiment of carrier gas flow rate
Principle
1、Gas circuit system Gas circuit system is an extremely important part of gas chromatograph. The gas system mainly refers to the continuous operation of the carrier gas closed pipeline, including the connection pipeline, regulating the measurement of gas flow of the various components, as well as the vaporization chamber, column, detector and so on. When using a hydrogen flame detector, auxiliary gases, such as hydrogen, air, etc., also need to be introduced. The piping through which they flow also belongs to the gas circuit system. The carrier gas supplied from the high-pressure cylinder is first reduced to an appropriate value by a pressure reducing gauge, and then enters the chromatograph through a purging tube. Pressure stabilizing valve, pressure gauge, regulating valve, flow meter and other components on the chromatograph are used to regulate, control and measure the pressure and flow rate of the carrier gas. The hydrogen and air gas line systems are also equipped with corresponding regulation, control and measurement components. The gas circuit system must be kept clean and airtight, and the performance of each regulating and controlling component must be normal and reliable.2. Carrier gas flow rateThe carrier gas flow rate is one of the important operations affecting the chromatographic separation, and must be measured frequently. Chromatograph on the rotor flow meter to measure the gas volume flow rate, but the height of the rotor and the flow rate is not a simple linear relationship, and with the media. Therefore, the need for soap film flow meter to be corrected. (1) visual volume flow rate (F'CO) with a soap film flowmeter in the column directly after the measured volume called visual volume flow rate. It includes not only the carrier gas flow rate, and includes the saturated steam flow rate under the conditions. (2) actual volume flow rate (FCO) PO: atmospheric pressure, mmHg; PW: saturated water vapor pressure at room temperature, mmHg (3) corrected volume flow rate (FC) due to the volume of gas with the change in temperature, and the column temperature is different from the room temperature, it is necessary to make temperature correction. (4) Average volumetric flow rate (Fc) Gas volume is pressure dependent. However, the pressure inside the column is not uniform, there is a pressure gradient, and pressure correction is required. pi: carrier gas pressure at the inlet of the column; Po: carrier gas pressure at the outlet of the column, the units of Pi and Po should be the same in the calculations. Move Gas chromatography often uses high-pressure cylinder gas as the gas source, the use of cylinders must be installed with a decompression gauge. Caveat 1、Hydrogen decompression gauge is only allowed to be installed on cylinders of self-combustible gases. 2、Oxygen decompression gauge can be installed on non-spontaneous combustible gas cylinder. 3、When installing the pressure-reducing gauge, all tools and connectors should be oil-free. 4、When opening the cylinder, the mouth of the cylinder is not allowed to face to the person and the instrument. 5、Purge pipe should be installed vertically, with the upper port inlet and lower port outlet. 6、Any place where soap has been applied should be wiped dry with filter paper. For more product details, please visit Aladdin Scientific website.
1. Select the decompression gauge correctly.
The interface nut of the pressure-reducing gauge and the thread of the cylinder nozzle must be matched. There are two spring pressure gauges on the decompression gauge, the larger value indicates the gas pressure inside the cylinder, and the smaller one indicates the output pressure. When opening the cylinder, the pressure gauge indicates the pressure inside the cylinder, and use soapy water to check whether there is any gas leakage at the interface.
2. Prepare the purging tube
(1) Clean the purging tube: soak it in 10% NaOH solution for half an hour, and then rinse and dry it with water.
(2) activation cleaning agent: silica gel - 120 ℃ baked to blue; activated carbon - 300 ℃ baked 2 hours; molecular sieve - 550 ℃ 3 hours. Shall not exceed 600 ℃
(3) fill the purification tube: three equal amounts of purification agent in order to fill the purification tube, separated by glass wool. Mark the gas inlet and outlet, and plug a glass wool at the outlet. Silicone is installed at the outlet.
3. Connection of pipes: Use a piece of pipe to connect the purification pipe to the outlet of the decompression table, and connect the other end of the purification pipe to a pipe that can reach the chromatograph. Turn on the gas supply, flush with gas, then turn off the gas supply and connect the pipe to the mouth of the instrument.
4. Leak detection: It is very important to ensure the tightness of the whole gas circuit system, and it should be checked carefully, and it is easy to leak for the joints and interfaces.
Leak detection method
(1) open the gas source, import the carrier gas, adjust the decompression standard for 2.5 kg/cm2, first close the instrument on the inlet pressure regulator valve. Use a small brush dipped in soapy water to check all the connections from the gas source to the interface.
(2) Connect the column to the thermal conductivity detector, open the inlet pressure reducing valve, and adjust the pressure gauge on the instrument: 2 kg/cm. adjust the rotameter flow rate to the maximum, block the exhaust port on the outside of the host, such as the rotameter float can fall to the bottom, there is no leakage of gas; otherwise, you need to check the internal interfaces of the instrument with soapy water.
(3) Hydrogen, air check as before.
(4) the elimination of air leakage phenomenon: tighten the screw interface, such as ineffective, unload the screw, check whether the pad is flat, can not be used when the need to replace.
5, the carrier gas flow rate measurement and correction
(1) will be connected to the column outlet and thermal conductivity detector, in the soap film flowmeter loaded with an appropriate amount of soap liquid. So that the liquid level is exactly in the center line of the mouth of the branch, with a hose connected to the carrier gas.
(2) Turn on the carrier gas, adjust the carrier gas pressure to the required value, adjust the rotor height. A minute later, gently pinch the rubber tip, so that the soap rises to seal the branch tube that will produce a soap film.
(3) Use a stopwatch to write down the time required for the soap film to pass through a certain volume, which is converted to the carrier gas flow rate in mL/min.
(4) Using the above method, sequentially measure the volume flow rate when the height of the rotameter is 0, 5, 10, 15, 20, 25, 30 cells, and then measure the flow rate of the other gas path.
(5) Measure the flow rate of the gas path with hydrogen as the carrier gas.
(6) Take the height of the rotor on the rotor flowmeter as the horizontal coordinate and the apparent volume flow rate as the vertical coordinate to draw the calibration curve of the rotor flowmeter, and at the same time, record the parameters such as the type of carrier gas, column temperature, room temperature and air pressure.
(7) According to the apparent volumetric flow rate, the actual volumetric flow rate can be calculated according to the following formula 
(8) According to the following formula to find out the corrected volumetric flow rate of the gas in the column under the condition of column temperature 