How to install and calibrate gas chromatography

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:) I. Installation of the chromatograph 1. Requirements for the chromatograph analysis room (1) There should be no strong magnetic fields, flammable, or highly corrosive gases in the vicinity of the analysis room. (2) The indoor environmental temperature should be within the range of 5 to 35 degrees, with humidity at or below 85% (relative humidity), and air circulation should be maintained indoors. Factories with the means should preferably install air conditioning. (3) Prepare a work platform that can support the entire set of instruments, with appropriate width and height to facilitate operation. For ordinary factories, a cement platform is preferred (with a height of 0.6~0.8 meters). The platform should not be placed right against the wall; it should be kept at a distance of 0.5~1.0 meter from the wall to facilitate wiring and maintenance work. (4) The power supply capacity for the instruments should be around 10 KVA, and the power supply used by the instruments should, as much as possible, not share the same circuit as high-power-consuming devices or equipment with significantly variable power consumption. The power supply must be properly grounded; generally, an iron rod (wire) about 0.5 to 1.0 meter long is driven into damp ground (or soaked in a salt solution), and then the grounding point of the power supply is connected to it. In short, the grounding resistance should be less than 1 ohm. (Note: It is recommended to ground both the power supply and the enclosure for better results). 2. Gas source preparation and purification (1) Gas source preparation: Prepare in advance the high-pressure gas cylinders required (which can generally be purchased in large and medium-sized cities). A cylinder designed for a specific type of gas can only hold that type of gas; each cylinder has a color that indicates its contents, and they cannot be used interchangeably. Generally, nitrogen, hydrogen, and air are used. It’s advisable to have two cylinders for each of these gases as a backup. Some factories can use hydrogen generators and air compressors as well, but the air compressor must be oil-free. The gas cylinder should be replaced when its pressure drops to 1–2 Mpa. Generally, manufacturers can use gases with a purity of 99.99% for their purposes, but electron capture detectors require gas sources with a purity of over 99.999%. (2) Gas source purification: In order to remove moisture, ash, and organic gas components that may be present in various gases, they must undergo strict purification before entering the instrument. If only cylinder gas is used, some chromatographs are equipped with purifiers that already contain 5A molecular sieve, activated carbon, and silica gel, which is generally sufficient to meet the requirements. When using a conventional hydrogen generator, it is necessary to enhance the purification of moisture. Therefore, the surface area of the drying tube should be increased (a volume of at least 450 cubic centimeters is recommended; 5A molecular sieve is preferred as the filler). Additionally, a storage tank with a relatively large volume should be connected after the generator, in order to minimize or eliminate the impact of fluctuations in the gas source pressure on the instrument’s baseline. If an air compressor is used as the source of air, its intake should be equipped with enhanced air filtration; the volume of the purification pipe should be increased. Inside the drying pipe, half of it should be filled with 5A molecular sieve and half with activated carbon. Generally, domestic oil-free gas compressors (produced in Tianjin) can meet the requirements. 3. Inspection of the chromatograph set and its installation: After unpacking the instrument, check each item according to the list provided in the documentation packet, and store spare parts for vulnerable components properly. Then, as specified in the instrument’s user manual, place it on the work platform; using the wiring diagram and various plugs and sockets, connect all the parts of the instrument together, and finally connect the recorder and the data processor. Be careful not to connect the connectors incorrectly. 4. Connection of the external air circuit (1) Installation of the pressure reducing valve: Some instruments come equipped with a pressure reducing valve; if not, it needs to be purchased. Used were 2 oxygen and 1 hydrogen pressure reducers. Install 2 oxygen pressure regulators and 1 hydrogen pressure regulator on the nitrogen, air, and hydrogen cylinders respectively (note that the threads of the hydrogen pressure regulator are reversed, and an attached plastic O-ring should be used at the connection point to ensure sealing). After tightening the nuts, turn the adjustment knob of the pressure regulator clockwise (i.e., loosen it). Open the high-pressure valve on the cylinder; at this point, the gauge on the pressure regulator should show a reading. After closing the high-pressure valve, the indicated pressure should not drop – if it does, there is a leak that needs to be addressed promptly (seal it using a gasket or tape). Sometimes the high-pressure valve itself can leak, so this aspect should also be taken into consideration. Then turn the adjustment knob to release any remaining air. (2) External gas line connection method: The gas from the cylinder is introduced into the chromatograph; some systems use stainless steel tubes (φ2×0.5mm), while others use pressure-resistant plastic tubes (φ3×0.5mm). Plastic pipes are easy to use, so they are generally employed. If plastic pipes are used, stainless steel lining pipes (φ2×20mm) and some plastics for sealing, etc., are required at the joints. The length of the plastic tube from the gas cylinder to the instrument should be determined as needed; it should not be too long. Then, use the plastic tube to connect the gas source to the instrument (the gas inlet). (3) Leak detection of the external gas circuits: Turn off the valve knobs for the carrier gas, hydrogen, and air on the gas circuit panel of the main unit. Then open the high-pressure valves of each gas cylinder, and adjust the pressure output indicated by the low-pressure gauge on the pressure regulator so that the pressure of the carrier gas and air is 0.35~0.6 Mpa (approximately 3.5~6.0 kg/cm3), and the pressure of hydrogen is 0.2~0.35 Mpa. Then close the high-pressure valve; at this point, the reading on the low-pressure gauge attached to the pressure reducing valve should not drop. If it does drop, it indicates a leak in the gas connection, which must be fixed. 5. Inspection of the gas path airtightness in the chromatograph. Checking the airtightness is a very important task; if there are leaks in the gas path, it not only leads to unstable operation or reduced sensitivity of the instrument, but also poses a risk of explosion. Therefore, this check must be carried out before using the instrument (the airtightness check generally applies to the carrier gas path, while the hydrogen and air paths do not need to be checked if they have not been altered). The method is to open the cover of the chromatography column box, remove the column from the detector, seal the column inlet, then turn on the carrier gas flow path and set the low-pressure output pressure to 0.35–0.6 Mpa. Open the carrier gas knob on the main unit’s panel; at this point, the pressure gauge should show a reading. Finally, turn off the carrier gas knob; the pressure reading in front of the column should not decrease within half an hour. If it does decrease, there is a leak that must be addressed. If there is a leak in the gas circuit inside the host, remove the relevant side panels of the host and use soapy water (preferably a sodium dodecyl sulfate solution) to check for leaks at each connection (this method can also be used to detect leaks in hydrogen and air), and finally wipe away the soapy water. II. Instrument commissioning: After connecting and positioning the gas lines, instruments, etc., as described above, the following inspection and commissioning procedures can be carried out. 1. Inspection of various components in the chromatograph circuit: Before starting the instrument, the carrier gas flow path should be connected first. Adjust the carrier gas knob on the main unit’s panel (i.e., the carrier gas flow control valve) to set the carrier gas flow rate at 20–30 ml/min. (1) Start the host: Turn on the main power switch of the host; the motor inside the column chamber begins to operate. Check for any abnormal noises, and if any are present, cut off the power immediately and conduct further checks to resolve the issue. Some chromatographs perform self-diagnosis upon startup, indicating whether the instrument is operating normally or not; if it is not operating properly, it shows which part has a problem, such as wiring errors, etc. (2) Temperature control checks for various components: In accordance with the instructions, check the temperature of the column (including programmed temperature changes), the temperature of the injector, and the temperature of the detector one by one to ensure that they remain constant at high, medium, and low temperatures. In particular, the accuracy of column temperature control is required to be 0.01 degrees.