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With the development of science and technology, gas chromatography has become the mainstream testing equipment, The article mainly introduces the principle of gas chromatography, operation and use of the process and its precautions. The article provides theoretical guidance and practical experience for users of gas chromatographs. (emphasis on the content of the black font)
Gas-liquid chromatography (GLC) was first established in 1952, and has been widely used in petroleum refining 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, its operation, the use of certain procedures, the operator must have good operating skills in order to better play the function of the gas chromatograph in practice.
A gas chromatograph method, procedure
Gas chromatograph is mainly composed of a fixed phase and mobile phase: fixed phase and mobile phase have different adsorption and distribution; through the relative motion of the two phases, the detected material with the mobile phase with the movement of the substance so that the material in the two phases of the distribution of repeated, so that the different tissues separated. Separation of different tissues. The use of gas chromatograph and operation process generally includes the following procedures.
1. Heating
The way to set the temperature of the gas chromatograph is different for different manufacturers. Temperature setting method can be generally divided into: microcomputer setting method, knob positioning method. If the microcomputer setting method is used to set the temperature, the temperature can be directly specified and set. If the knob positioning method is used, there are techniques for its use. With the over-temperature positioning method, the temperature control knob is adjusted to about 30℃ below the operating temperature to warm up the gas chromatograph.
When the over-temperature is about the operating temperature, gradually adjust the temperature control knob to the appropriate position by observing the temperature indicator and heating indicator. If the step-by-step progressive positioning method, the first temperature control knob towards the direction of temperature rise by turning an angle, the instrument began to warm up, the indicator light; when the temperature reaches a certain temperature is basically stable, and then turn the knob in the same direction, so that the instrument temperature continues to rise; repeatedly according to the method of progressive temperature adjustment, until the desired operating temperature.
2. Adjust the pool balance
The adjustment of the pool balance is also known as the adjustment of the thermal conductive bridge balance, the purpose of which is to make the output of the instrument is more appropriate. For the pool balance, zero function and recording function of the instrument, need to pay attention to certain adjustment skills.
3. Ignition
For the hydrogen flame gas chromatograph, ignition is required when the instrument is turned on. Or for some reason, the fire goes out after the need to re-ignite. However, we often encounter the situation of ignition failure, so ignition also needs some skills. The common ignition method is to increase the flow of hydrogen gas first, then ignite, and then slowly adjust the instrument back to the working state.
4. Adjustment of gas ratio
According to the relevant information, the flow rate of the three gases of the hydrogen flame gas chromatograph is recommended to be nitrogen:hydrogen:air=1:1:10, but because in the actual instrument, the rotor flow meter generally can not be done to measure very accurately, so it is very difficult to achieve the standard gas ratio in the actual operation. In the actual operation process, you can focus on the detector sensitivity and separation effect, according to the actual situation to adjust the ratio.
5. Sampling
In gas chromatography analysis, the common method of injection is to use a syringe or six-way valve injection. Factors affecting the injection volume are mainly the gasification temperature, column capacity and the linear response range of the instrument. Column efficiency is mainly affected by the length of the injection time. If the injection time is too long, it will cause the chromatographic region to become wider, thus reducing the column efficiency. Therefore, for the flushing method of chromatography, the injection time should be reduced as much as possible, and the general requirement is no more than 1s.
Second, the use of gas chromatography steps:
1, turn on the regulated power supply.
2, open the nitrogen valve, open the carrier gas switch valve on the purifier, and then check for air leakage to ensure good gas tightness.
3, adjust the total flow rate to the appropriate value (measured according to the graduated flow meter).
4, adjust the diversion valve so that the diversion flow rate for the flow rate required for the experiment (with a soap film flowmeter in the air system panel to actually measure), the column flow rate that is the total flow rate minus the diversion flow rate.
5, open the air, hydrogen switch valve, adjust the air, hydrogen flow for the appropriate value.
6. Set the column temperature, inlet temperature and FID detector temperature according to the experimental needs.
7. Turn on the computer and workstation.
8, FID detector temperature reaches 150oC or more, press the FIRE key to ignite the FID detector flame.
9, set the FID detector sensitivity and output signal attenuation.
10, to be set when the parameters set to achieve the settings, you can enter the sample analysis.
11, after the experiment is finished, turn off the hydrogen and air first, and use nitrogen to blow the column clean and then turn off the machine.
Three, the use of gas chromatograph precautions
Gas chromatograph in the use of the process should generally focus on the following places.
1. Requirements for the analysis room of the gas chromatograph
(1) The analysis room shall not be surrounded by strong magnetic field, flammable and strong corrosive gases.
(2) The indoor ambient temperature should be within the range of 5 to 35 degrees, humidity less than or equal to 85% (relative humidity), and the room should maintain air circulation. Conditional plant should be installed air conditioning.
(3) basic with a good 3000x800x600 (LXWXH) (mm) can withstand a full set of instruments, easy to operate the work platform. The platform can not be close to the wall, should be 0.5 ~ 1.0 meters from the wall, to facilitate wiring and maintenance.
(4) in order to prevent voltage fluctuations caused by the detection of interference should be equipped with a separate capacity of about 10KVA power line power supply.
2. Supporting the preparation and purification of gas source
Instrument work gas supply cylinder supply, cylinder pressure reducing valve should be frequently leak detection, the gas purity requirements are high, the purity should be greater than 99.99%. For air and hydrogen generator, it is necessary to drain and replace desiccant regularly.
(1) gas source preparation, prepare in advance the need for gas high-pressure cylinders (generally large and medium-sized cities can be purchased), the cylinder containing a certain type of gas can only be loaded with such gases, the color of each cylinder represents a gas, can not be interchanged. General use of high-purity nitrogen, high-purity hydrogen, oil-free air, these three gases, each gas should be prepared for two cylinders, in order to swap spare. Some factories use nitrogen generator, hydrogen generator and air compressor can also be, but the air compressor must be oil-free. Where the cylinder gas pressure drops to 1 ~ 2Mpa, the cylinder should be replaced. General manufacturers use the use of the above gas 99.99% can be, such as gas chromatograph equipped with electron capture detector should be used in cylinders of high-purity gas source, the purity of 99.999% or more.
(2) gas source purification in order to get out of the various gases may contain moisture, ash and organic gas components, the gas should be strictly purified before entering the instrument. Now the domestic gas generator has higher development technology, generally configured with 5A molecular sieve or activated carbon filtering and purification devices, can basically meet the requirements of gas chromatograph. If you use high-purity gas in cylinders, you need to purify and filter the gas after use. For some high-end chromatograph with purifier, and has been filled with 5A molecular sieve, activated carbon, silica gel, can basically meet the requirements.
3. Gas chromatograph complete set of inspection and placement
After the instrument is opened, according to the list of accessories in the kit, make an itemized inventory, and the spare parts of the perishable parts to be properly preserved. Then, according to the instruction manual of the instrument, place it on the working platform, and connect all parts of the instrument to the wiring diagram and each plug and socket, and connect the chromatographic workstation.
4. Gas chromatograph gas line connection and gas line air tightness check
For gas chromatograph gas line link is generally carried out by the chromatograph manufacturer's engineers and technicians, in the case of failure to meet the requirements should be strictly in accordance with the instruction manual under the guidance of the professionals to install the connection.
Gas tightness check is a very important work, if there is a leak in the gas circuit, not only directly lead to instrument instability or sensitivity loss, but also the risk of leakage, especially hydrogen should be taken more seriously. Gas chromatograph gas tightness check is generally check the carrier gas flow path, the focus is on the pipeline joints, for hydrogen and air flow path should also do the corresponding check.
5. Inlet port
Based on practical experience, most of the instruments in the injection of 50 to 100 times after the spacer will be damaged, peak retention time will sometimes change, and even ghost peaks, this time the need to update the spacer. The integrity of the spacer should be regularly checked, if found that the spacer cracks or more spacer debris must be replaced. At the same time, the glass liner in the inlet should be cleaned regularly.
6. Columns
When installing the capillary tube, make sure that both ends of the column are cut flat. When the capillary column is not used for a long time, the ends should be cut off about 2cm before connecting the capillary column to the sample port and detector.
7. Detectors
Detectors that are not in operation should not be turned on, but should be kept off. For the ECD detector, it should be equipped with a conduit when discharging the air to discharge the air outside. Care should also be taken not to introduce air into the ECD detector during normal use.
Conclusion
Gas chromatograph as a modern mainstream testing equipment, its working principle is not difficult to understand, but the use of equipment has a strict operating procedures. Each part of the use of certain requirements, from the gas source, equipment to the external environmental conditions have special requirements. Only understand these requirements to efficiently and effectively use the gas chromatograph.