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How to Properly Install and Use a Gas Chromatograph

Leak testing is an extremely important task. If there is a leak in the gas line, it will not only directly cause the instrument to operate unstably or reduce sensitivity but also pose a risk of explosion. Therefore, this task must be performed before operation (leak testing generally checks the carrier gas flow path; if the hydrogen and air flow paths have not been disassembled, they may not need to be checked).

1. Installation of the Chromatograph

1. Requirements for the Chromatograph Analysis Room

(1) There should be no strong magnetic fields, flammable gases, or highly corrosive gases around the analysis room.

(2) The indoor ambient temperature should be between 5–35°C, with humidity ≤85% (relative humidity), and the room should be well-ventilated. Factories with conditions should preferably install air conditioning.

(3) Prepare a work platform that can support the entire instrument, with appropriate width and height for easy operation. Generally, a cement platform is best (0.6–0.8 meters high). The platform should not be placed flush against the wall; it should be 0.5–1.0 meters away from the wall to facilitate wiring and maintenance.

(4) The power line capacity for the instrument should be around 10 KVA. The power source for the instrument should not be shared with high-power consumption equipment or devices that frequently cause significant voltage fluctuations. The power supply must be well-grounded. Generally, a iron rod (or wire) about 0.5–1.0 meters long can be driven into damp ground (or a salt solution灌注), and the power supply ground point can be 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 casing for better效果).

2. Gas Source Preparation and Purification

(1) Gas Source Preparation: Prepare high-pressure cylinders of the required gases in advance (generally available in large and medium-sized cities). A cylinder for one type of gas can only contain that gas; each cylinder's color represents a specific gas and cannot be interchanged. Generally, nitrogen, hydrogen, and air are used. It is best to prepare two cylinders of each gas for backup. Some factories use hydrogen generators and air compressors, but the air compressor must be oil-free. When the cylinder pressure drops to 1–2 MPa, the cylinder should be replaced. Generally, 99.99% purity gas is sufficient for most manufacturers, but an electron capture detector requires high-purity gas sources of 99.999% or higher.

(2) Gas Source Purification: To remove possible moisture, ash, and organic gas components from various gases, the gases should undergo strict purification treatment before entering the instrument. If all gases are from cylinders, some chromatographs come with purifiers filled with 5A molecular sieves, activated carbon, and silica gel, which基本 meet the requirements. If using a general hydrogen generator, the purification of moisture must be strengthened. Therefore, increase the drying tube area (preferably with a volume of 450 cubic centimeters or more, filled with 5A molecular sieve), and connect a larger storage tank after the generator to reduce or overcome the impact of gas source pressure fluctuations on the instrument baseline. If using an air compressor as the air source, strengthen air filtration at the compressor intake, increase the purifier tube volume, and fill half of the drying tube with 5A molecular sieve and half with activated carbon. Generally, domestic oil-free air compressors (e.g., those produced in Tianjin) can meet the needs.

3. Chromatograph Completeness Check and Placement

After unpacking the instrument, check each item against the附件清单 in the资料袋 and store spare parts for vulnerable components properly. Then, according to the instrument's user manual, place it on the work platform. Connect all parts of the instrument according to the wiring diagram and各插头,插座, and finally connect the recorder and data processor. Be careful not to misconnect any joints.

4. External Gas Line Connection

(1) Installation of Pressure Regulators

Some instruments come with pressure regulators; if not, they must be purchased. Typically, two oxygen regulators and one hydrogen regulator are used. Install the two oxygen regulators and one hydrogen regulator onto the nitrogen, air, and hydrogen cylinders respectively (note: the hydrogen regulator has reverse threads, and an attached O-ring plastic gasket should be added at the interface for sealing). After tightening the nuts, close the regulator adjustment handle (i.e., loosen it), open the cylinder high-pressure valve. At this point, the high-pressure gauge on the regulator should show a reading. After closing the high-pressure valve, the indicated pressure should not drop; otherwise, there is a leak that should be promptly addressed (sealed with a gasket or生料带). Sometimes the high-pressure valve may also leak, so be cautious. Then, rotate the adjustment handle to release the residual gas.

(2) External Gas Line Connection Method: To introduce gas from the cylinder into the chromatograph, either stainless steel tubes (φ2×0.5mm) or pressure-resistant plastic tubes (φ3×0.5mm) can be used. Plastic tubes are easier to handle, so they are generally preferred. If using plastic tubes, stainless steel liners (φ2×20mm) and some sealing plastic materials are needed at the joints. The length of the plastic tube from the cylinder to the instrument should be as needed but not too long. Then, connect the gas source to the instrument (gas inlet) with the plastic tube.

(3) External Gas Line Leak Test: Close the carrier gas, hydrogen, and air valve knobs on the host gas line panel. Then, open the high-pressure valves of each cylinder and adjust the output pressure on the regulator's low-pressure gauge so that the carrier gas and air pressure are 0.35–0.6 MPa (approximately 3.5–6.0 kg/cm³), and the hydrogen pressure is 0.2–0.35 MPa. Then, close the high-pressure valves. At this point, the indicated value on the regulator's low-pressure gauge should not drop. If it drops, it indicates a leak in the connected gas line, which should be排除.

5. Chromatograph Gas Line Leak Test

Leak testing is an extremely important task. If there is a leak in the gas line, it will not only directly cause the instrument to operate unstably or reduce sensitivity but also pose a risk of explosion. Therefore, this task must be performed before operation (leak testing generally checks the carrier gas flow path; if the hydrogen and air flow paths have not been disassembled, they may not need to be checked).

The method is: Open the chromatographic column oven cover, disconnect the column from the detector, and block the column outlet. Then, turn on the carrier gas flow path, adjust the low-pressure output pressure to 0.35–0.6 MPa, and open the carrier gas knob on the host panel. At this point, the pressure gauge should show a reading. Finally, close the carrier gas knob. The indicated pressure at the column front should not drop within half an hour. If it drops, there is a leak that should be排除. If there is a leak in the host's internal gas line, remove the relevant side panels of the host and use soapy water (preferably a sodium lauryl sulfate solution) to check each joint for leaks (hydrogen and air can also be checked this way). Finally, wipe dry the soapy water.

II. Instrument Debugging After connecting the gas lines and instruments as described above and setting them up, proceed with the following checks and debugging.

1. Check of Various Circuit Components of the Chromatograph Before starting the instrument, first connect the carrier gas flow path and adjust the carrier gas knob on the host panel (i.e., the carrier gas flow stabilizer valve) to set the carrier gas flow rate to 20–30 ml/min.

(1) Start the Host: Turn on the main power switch of the host. The motor inside the chromatographic column oven should start working. Check for any unusual noises; if any, immediately cut off the power and further inspect and resolve the issue. Some chromatographs perform self-diagnostics upon startup, displaying the instrument's operation status: normal or abnormal. Abnormal displays indicate which part has a problem, wiring errors, etc.

(2) Temperature Control Checks: According to the manual, check the constant temperature control for the column temperature (including programmed temperature), injector temperature, and detector temperature one by one. Ensure they can remain stable at high, medium, and low temperatures, especially requiring the column temperature control accuracy to reach 0.01°C.

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