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With the development of science and technology, gas chromatograph has become the mainstream detection equipment. This paper mainly introduces the related principle, operation flow and matters needing attention of gas chromatograph. This paper can provide relevant theoretical guidance and practical experience for gas chromatograph users. (only pay attention to the content covered by black fonts. )

Gas-liquid chromatography was first established in 1952, and has been widely used in petroleum smelting industry, chemical industry, biopharmaceutical industry, environmental monitoring and other fields. The gas chromatograph based on this method has become the main tool for gas chromatography analysis. However, there are certain rules in its operation and use, and the operator must have good operating skills in order to give full play to the function of gas chromatograph in practice.

A, gas chromatograph using method, program

Gas chromatograph is mainly composed of stationary phase and mobile phase: stationary phase and mobile phase have different adsorption and distribution functions; Through the relative motion of the two phases, the detected substance moves with the mobile phase, so that the substance is repeatedly distributed between the two phases, thus separating different tissues. The use and operation flow of gas chromatograph generally includes the following procedures.

1. heating

The gas chromatograph produced by different manufacturers gives the temperature in different ways. General temperature setting methods can be divided into: microcomputer setting method, knob positioning method and so on. If the temperature is given by microcomputer setting method, the set temperature can be directly specified. If the knob positioning method is adopted, its use is skillful. Using overtemperature positioning method, adjust the temperature control knob to about 30℃ below the working temperature, and heat the gas chromatograph.

When the temperature exceeds the working temperature, gradually adjust the temperature control knob to the appropriate position by observing the temperature indicator and the heating indicator. If the step-by-step positioning method is adopted, first rotate the temperature control knob by an angle in the direction of temperature rise, then the instrument starts to heat up and the indicator light is on; When the temperature reaches a certain temperature and is basically stable, turn the knob in the same direction to keep the temperature of the instrument rising; According to this method, the temperature is adjusted step by step until the required working temperature is reached.

2. Adjust the pool balance

Tank balance is also called thermal bridge balance, and its purpose is to make the output of the instrument more suitable. For instruments with cell balance, zero adjustment and recording function, we need to pay attention to certain adjustment skills.

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For hydrogen flame gas chromatograph, ignition is needed when starting. Or for some reason, the fire needs to be rekindled after it is extinguished. However, we often encounter ignition failures, so ignition also requires some skills. The general ignition method is to increase the flow of hydrogen first, then ignite, and then slowly adjust the instrument back to the working state.

4. Adjustment of gas proportion

According to relevant data, it is suggested that the flow ratio of three gases in hydrogen flame gas chromatograph is nitrogen: hydrogen: air = 1: 1: 10. However, due to the fact that in practical instruments, the rotameter can not measure accurately, so this standard gas ratio is difficult to achieve in practical operation. In the actual operation process, we can focus on the sensitivity and separation effect of the detector, and adjust the ratio according to the actual situation.

5. Sample injection

In gas chromatographic analysis, the common injection method is to inject samples with a syringe or a six-way valve. The main factors affecting the sample volume are gasification temperature, column capacity and linear response range of the instrument. The efficiency of chromatographic column is mainly affected by the injection time. If the injection time is too long, the chromatographic region will be widened, thus reducing the column efficiency. Therefore, for washing chromatography, the injection time should be as short as possible, generally not exceeding1s.

Second, the steps of using gas chromatograph:

1, turn on the regulated power supply.

2, open the nitrogen valve, open the carrier gas switch valve on the purifier, and then check whether there is air leakage, to ensure good air tightness.

3. Adjust the total flow to an appropriate value (measured by a calibrated flowmeter).

4. Adjust the diverter valve so that the diverter flow is the flow required by the experiment (actually measured on the gas path system panel with soap film flowmeter), and the chromatographic column flow is the total flow minus the diverter flow.

5, open the air and hydrogen switch valve, adjust the air and hydrogen flow to the right value.

6. Set the column temperature, inlet temperature and FID detector temperature according to the experimental requirements.

7. Turn on the computer and workstation.

8. When the temperature of FID detector reaches above 150oC, press the FIRE key to ignite the flame of FID detector.

9. Set FID detector sensitivity and output signal attenuation.

10. When the set parameters reach the set values, samples can be injected for analysis.

1 1. After the experiment, turn off the hydrogen and air, blow the chromatographic column clean with nitrogen, and then turn it off.

Three. Matters needing attention in the use of gas chromatograph

The following points should be paid attention to in the use of gas chromatograph.

1. Requirements for analysis room of gas chromatograph

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

(2) The indoor ambient temperature should be within the range of 5 ~ 35 degrees, the humidity should be less than or equal to 85% (relative humidity), and indoor air circulation should be maintained. Conditional factories should install air conditioners.

(3) Basically, there is a good working platform of 3000x800x600 (length× width× height) (mm), which can bear the whole set of instruments and is easy to operate. The platform should be 0.5 ~ 1.0m away from the wall to facilitate wiring and maintenance.

(4) In order to prevent interference caused by voltage fluctuation, a power line power supply with a separate capacity of about 10KVA shall be provided.

2. Preparation and purification of auxiliary gas source

When the instrument works, the gas is generally supplied by the gas supply cylinder, and the cylinder pressure reducing valve should be frequently leak-checked, which requires high gas purity, and the purity should be greater than 99.99%. For air and hydrogen generators, it is necessary to drain water and replace desiccant regularly.

(1) gas source preparation: prepare high-pressure gas cylinders that need gas in advance (generally available in large and medium-sized cities). Cylinders containing a certain gas can only contain this gas, and the color of each cylinder represents a gas and cannot be interchanged. Generally, high-purity nitrogen, high-purity hydrogen and oil-free air are used, and two steel cylinders should be prepared for each gas for replacement. Some factories can use nitrogen generator, hydrogen generator and air compressor, but the air compressor must be oil-free. When the air pressure of the cylinder drops to 1 ~ 2 MPa, the cylinder should be replaced. General manufacturers only need to use 99.99% of the above gases. For example, if the gas chromatograph is equipped with an electron capture detector, it is necessary to use a high-purity gas source of steel cylinders, and the purity is above 99.999%.

(2) Gas source purification In order to remove the moisture, ash and organic gas components that may be contained in various gases, the gases must undergo strict purification treatment before entering the instrument. At present, the development technology of domestic gas generators is relatively high, generally equipped with 5A molecular sieve or activated carbon filtration and purification device, which can basically meet the requirements of gas chromatograph. If the high-purity gas in the steel cylinder is used, it needs to be purified and filtered before use. Some high-end chromatographs are equipped with purifiers and have been filled with 5A molecular sieve, activated carbon and silica gel, which can basically meet the requirements.

3. Complete inspection and installation of gas chromatograph

After unpacking the instrument, it shall be counted item by item according to the list of accessories in the data package, and spare parts of wearing parts shall be properly kept. Then, according to the requirements of the instruction manual of the instrument, put it on the working platform, connect all parts of the instrument with the wiring diagram and plug socket, and connect the chromatographic workstation.

4. Gas path connection and gas path air tightness inspection of gas chromatograph

The gas path connection of gas chromatograph is generally carried out by the engineers and technicians of the chromatograph manufacturer. If it can't meet the requirements, it should be installed in strict accordance with the requirements of the manual under the guidance of professionals.

Air tightness inspection is a very important work. If the gas path leaks, it will not only directly lead to instrument instability or decreased sensitivity, but also the danger of leakage, especially hydrogen. Generally, the airtight inspection of gas chromatograph is to check the flow path of carrier gas, focusing on the joint of pipeline, and the flow path of hydrogen and air should also be checked accordingly.

5. Injection port

According to practical experience, the septa of most instruments will be damaged after 50 ~ 100 injections, and the peak retention time will sometimes change, and even ghost peaks will appear. At this point, it is necessary to update the spacer. Check the integrity of the gasket regularly. If cracks or more gasket fragments are found, please replace them. At the same time, the glass liner of the injection port should also be cleaned regularly.

6. Chromatographic column

When installing the capillary tube, make sure that the notches at both ends of the chromatographic column are flat. When the capillary column is not used for a long time, both ends of the capillary column should be cut off by about 2cm before connecting it to the sample port and detector.

7. detector

Detectors that are not in working state should not be turned on, but should be kept off. For ECD detectors, a duct should be provided when exhausting air, so as to exhaust air outside. When using ECD detector at ordinary times, you should also be careful not to bring in air.

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As a modern mainstream detection equipment, the working principle of gas chromatograph is not difficult to understand, but its use has strict operating procedures. From gas source, equipment to external environmental conditions, each use link has certain requirements. Only by understanding these requirements can we use the gas chromatograph efficiently and quickly.