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Gas chromatography use precautions
Injection should pay attention to the problem
Do not take the needle of the syringe and the sample part, do not have air bubbles (slow when sucking the sample, quickly discharge and then slowly suck, repeated several times, the volume of the metal needle part of the 10ul syringe is 0.6ul, there is no bubble, can not see, suck more 1 -2ul push the tip of the syringe up to the top of the bubble and push the needle bar to remove the air bubbles (refer to a 10μl syringe with a flat syringe). The injection speed is fast (but not very fast), and the same speed is maintained for each injection. The tip of the needle is injected into the middle of the vaporization chamber to start the injection of the sample.
Install the column
1. Installation and removal of the column must be at room temperature.
2. The packed column has ferrule seal and gasket seal. The card sleeve is divided into three types, metal ferrule, plastic ferrule, and graphite ferrule. It is not easy to screw when installed. The gasket seal is replaced with a new gasket each time the column is installed (Shimadzu chromatography is a gasket seal).
3. Whether the two ends of the column are packed with glass wool. Prevent the glass wool and filler from being blown into the detector by the carrier gas.
4. Capillary Columns The length of the installation and insertion depends on the instrument's instructions. Different chromatographic vaporization chambers have different structures, so the length of insertion is also different. It should be noted that if you use a capillary column with splitless flow, the vaporization chamber uses a packed column interface. The capillary column connected to the vaporization chamber cannot be probed too much, slightly beyond the ferrule.
Effect of hydrogen to air ratio on FID detector
The ratio of hydrogen to air should be 1:10. When the proportion of hydrogen is too large, the sensitivity of the FID detector drops sharply. When the other conditions are the same when using the chromatogram, the sensitivity is lowered to check the hydrogen and air flow rates. Hydrogen and air have a kind of gas that emits a "beep" when it is ignited, and then extinguishes it. Generally, when you ignite it, it will be extinguished, and then it will be turned off and then extinguished.
Use TCD detector
1. When hydrogen is used as a carrier gas, the exhaust gas must be discharged to the outside.
2. Nitrogen can not be set up as a carrier gas bridge, which is much smaller than when using hydrogen.
3. If the carrier gas is not passed, the bridge flow cannot be given. The bridge flow should be given before the sample temperature is stabilized.
How to determine if the FID detector is on fire
Different instruments have different judgment methods. The base flow shows the size of the base flow. Without the base flow display, the wrench with the polished surface is close to the detector outlet, and the surface is observed to have no condensation.
How to determine if the inlet seal should be replaced
It feels particularly easy when injecting. When the TCD detector is not injecting, there are regular small peaks on the recorder, indicating that the gasket is leaking. Replace the gasket without tightening too tightly. Generally, it will be at normal temperature when it is replaced. It will be tighter after the temperature rises. If the gasket is tightened too tightly, it will cause difficulty in injection, and the syringe needle will often be bent.
How to choose the right gasket
The gasket is divided into a general gasket and a high temperature resistant gasket. When the temperature of the vaporization chamber exceeds 300 ° C, a high temperature gasket is used, and one side of the high temperature gasket has a film, and the surface of the membrane is facing downward when used.
How to prevent the needle from bending
Many newcomers to chromatographic work often bend the needle and syringe shaft of a syringe for:
1. The injection port is too tight, and the screw is too tight at room temperature. When the temperature of the vaporization chamber rises, the silicone gasket will expand more tightly, and the syringe is difficult to get stuck.
2. Position is not good to find the needle stuck in the metal part of the inlet.
3. The syringe shaft bend is too strong when injecting, and the inlet chromatograph has an injector frame. When the sample is loaded with the injector frame, the syringe rod will not be bent.
4. Because the inner wall of the syringe is contaminated, the needle bar is pushed at the time of injection. When the syringe is used for a while, it is found that there is a small black piece near the top of the needle tube. At this time, the injection of the sample is difficult. The cleaning method pulls out the needle bar, injects a little water, inserts the needle bar into the polluted position and pushes and pulls it repeatedly. Once again, it can not inject water until the contaminants are removed. At this time, you will see the water in the syringe become cloudy. Pull the needle bar out and wipe it with filter paper, then wash it several times with alcohol. When the sample to be analyzed is a solvent-dissolved solid sample, the syringe should be washed with a solvent in time.
5. Be sure to be steady when injecting. If you are eager to hurry, you will bend the syringe. As long as you are proficient in the injection, it will be faster.
Several ways to improve the resolution
1. Increasing the length of the column increases the resolution.
2. Reduce the amount of injection (solid sample to increase the amount of solvent).
3. Improve the injection technology to prevent two injections.
4. Reduce the carrier gas flow rate.
5. Reduce column temperature.
6. Increase the temperature of the vaporization chamber.
7. To reduce the dead volume of the system, the column connection should be inserted in place, and the split-free injection chamber should be selected without splitting the injection.
8. Capillary column To split, choose the appropriate split ratio.
In summary, the system should be explored according to the specific conditions, such as reducing the carrier gas flow rate and lowering the temperature of the column, which will widen the peak of the chromatogram. Therefore, it is necessary to look at the chromatographic peak shape to change the condition. The ultimate goal is to achieve separation and fast peak times.
Proper column installation is guaranteed to perform at its best and last for life. Please refer to the following steps for proper installation:
Step 1. Check the gas filter, carrier gas, sample pad and liner to check the gas filter and the injection pad to ensure that the gas for the auxiliary gas and detector is unobstructed. If you have previously done a dirty sample or a compound with higher activity, you need to clean or replace the liner of the inlet.
Step 2. Place the nut and seal on the column and carefully cut the ends of the column
Step 3. Attach the column to the inlet. Insert the column into the inlet depending on the GC instrument used. Proper and appropriate insertion will ensure the reproducibility of the test results to the greatest extent possible. In general, the column inlet should be kept in the middle and lower part of the inlet. This is ideal if the needle tip is 1-2 cm from the column inlet after the needle has been inserted through the septum completely into the inlet. (Refer to the random manual of the GC used for the specific degree of insertion and method.) Avoid bending the capillary column with force and be careful not to let the sharp-edged items such as the label contact the capillary column to prevent the column from being damaged. After inserting the column correctly into the inlet, screw the coupling nut by hand. After tightening (do not twist by hand), use a wrench to tighten 1/4-1/2 turn to ensure the sealing degree of the installation. Because it is not tightly mounted, it not only causes leakage of the device, but also causes permanent damage to the column.
Step 4. Switch on the carrier gas. When the column is connected to the inlet, pass the carrier gas and adjust the column pressure to get the proper carrier gas flow rate (see table below).
The column front pressure is set to Psi
15m 25m 30m 50m 100m
0.20mm 10-15 20-30 18-30 40-60 80-120
0.25mm 8-12 13-22 15-25 28-45 55-90
0.32mm 5-10 8-15 10-20 16-30 32-60
0.53mm 1-2 2-3 2-4 4-8 6-14
(The above is only the recommended initial setting, the specific value depends on the actual carrier gas flow rate.) Insert the outlet end of the column into the sample bottle containing hexane. Under normal circumstances, we can see the stable and continuous bubbles in the bottle. If there are no air bubbles, check whether the download gas device and flow controller are correctly set and check the entire gas path for leaks. After all the problems have been solved, remove the column outlet from the bottle to ensure that there is no solvent residue in the column port before proceeding to the next step.
Step 5. Attach the column to the detector. The installation and required considerations are approximately the same as the column and inlet connections. If the system is using ECD or NPD in the application, the column should be disconnected from the detector when aging the column, so the detector may be more stable.
Step 6. Determine the carrier gas flow and check the column installation. Note: If the column is heated without a carrier gas, the column will be quickly and permanently damaged.
Step 7. Column aging After the column installation and system leak detection are complete, the column can be aged.
Raise the column to a constant temperature, usually at its upper temperature limit. In special cases, it can be heated to about 10-20 °C above the maximum operating temperature, but it must not exceed the upper temperature limit of the column, which will easily damage the column. When the aging temperature is reached, the baseline is recorded and observed. The baseline should continue to rise during the initial phase and begin to decline 5-10 minutes after reaching the aging temperature and will last for 30-90 minutes. It will stabilize when it reaches a fixed value. If the baseline is still not stable after 2-3 hours or there is no significant downward trend after 15-20 minutes, there may be leakage or contamination of the system device. In such a situation, the temperature of the column should be immediately lowered to below 40 °C, and the system should be checked as soon as possible and the related problems should be solved. If you continue to age, not only will the column be damaged, but you will never get a normal, stable baseline.
In general, columns coated with a polar stationary phase and a thicker coating have a longer aging time, while a weakly polar stationary phase and a thinner coated column require less time. The aging methods for PLOT columns vary. PLOT column aging step: HLZ Pora series 250 ° C, 8 hours or more Molesieve (molecular sieve) 300 ° C 12 hours Alumina (alumina) 200 ° C 8 hours or more due to irreversible adsorption of water in alumina and molecular sieve PLOT column, making these two The column is prone to retention behavior drift.
After the column has been separated from the sample containing high moisture, the column needs to be re-aged to remove the moisture adsorbed in the stationary phase.
Step 8. Set to confirm the carrier gas flow rate For capillary columns, the type of carrier gas is preferably high purity nitrogen or hydrogen. The purity of the carrier gas is preferably greater than 99.995%, and the less oxygen content therein, the better. If you are using a capillary column, evaluate the carrier gas based on the average linear velocity of the carrier gas (cm/sec) instead of the carrier gas flow (ml/min). Because the efficiency of the column is calculated using the average linear velocity of the carrier gas. Recommended average line speed value: Nitrogen: 10-12cm/sec Hydrogen: 20-25cm/sec Carrier gas impurity filter Adding a gas filter to the carrier gas line not only extends the life of the column, but also greatly reduces the background. noise. It is recommended to install a high capacity deoxygenation tube and a carrier gas purifier. When using an ECD system, it is best to install a deoxygenation tube in its auxiliary gas path.
Step 9. Column Loss Detection After the aging process of the column is completed, a temperature test is performed using a temperature rise (no injection). It is generally raised from 50 ° C to the maximum use temperature at 10 ° C / min, and maintained for 10 min after reaching the maximum use temperature. This way we will get to a loss map. These values ??may be useful for future comparison experiments and solving experimental problems. In the chromatogram of the blank test, no chromatographic peak should appear. If a peak appears, it can usually be a contaminant from the inlet. If the performance of the column begins to decrease under normal conditions of use, the baseline signal value will increase. In addition, if the baseline signal value is significantly greater than the initial value at very low temperatures, there may be contamination of the column and GC system. Other: Preservation of the column The sample pad is sealed with the sample pad and placed back in the original packaging. When installing, cut off both ends of the column to ensure that no debris from the injection pad remains in the column.
Note: There is a danger of explosion when the amount of hydrogen in the air is 4-10%. So be sure to have a good ventilation system in the lab.
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