Gas Chromatography Columns | The correct installation of meteorological chromatographic columns can ensure the best performance and extended service life. Please refer to the following steps for correct installation:
Step 1. Check the gas filter, carrier gas, sample pad and liner, etc. Check the gas filter and sample pad to ensure that the auxiliary gas and detector gas flow is smooth and effective. If you have done a dirty sample or a compound with high activity before, you need to clean or replace the inlet liner.
Step 2. Install the nut and gasket on the column, and carefully cut the two ends of the column
Step 3. Connect the GC column | Meteorological column to the inlet. The insertion depth of the GC column | Meteorological column in the inlet depends on the GC instrument used. Correct and proper insertion can ensure the reproducibility of test results to the greatest extent possible. Generally speaking, the inlet of the chromatographic column should be kept in the middle and lower part of the inlet. When the needle is fully inserted into the inlet through the septum, if the needle tip is 1-2cm away from the inlet of the column, this is the ideal state. (Refer to the random manual of the GC used for the specific insertion degree and method.) Avoid squeezing and squeezing the capillary column hard, and be careful not to allow objects with sharp edges such as label plates to rub against the capillary column to prevent the column body from being damaged. After inserting the chromatographic column into the inlet correctly, tighten the connecting nut by hand. After tightening (cannot be tightened by hand), use a wrench to turn an additional 1 / 4-1 / 2 turn to ensure the sealing degree of the installation. Not tight installation will not only cause leakage of the device, but also may cause permanent damage to the column.
Step 4. Turn on the carrier gas. When connected to the inlet, pass the carrier gas and adjust the pre-column pressure to obtain the proper carrier gas flow rate (see table below).
The pre-column 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, and 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 stable and continuous bubbles in the bottle. If there are no air bubbles, it is necessary to re-check whether the gas download device and flow controller are correctly set, and check the entire gas circuit for leaks. After all problems are resolved, remove the column outlet from the bottle to ensure that no solvent remains in the column port, and then proceed to the next installation.
Step 5. Connect the chromatographic column to the detector. The installation and precautions required are the same as those for connecting the column to the inlet. If the system uses ECD or NPD in the application, the column should be disconnected from the detector when the column is conditioned, so that the detector may reach stability more quickly.
Step 6. Determine the carrier gas flow rate, and then check the installation of the column. Note: If the column is heated without carrier gas, the column will be quickly and permanently damaged.
Step 7. Aging of the chromatographic column After the installation of the chromatographic column and leak detection of the system are completed, the chromatographic column can be conditioned.
Raise the column to a constant temperature, usually its upper temperature limit. Under special circumstances, it can be heated to about 10-20 ℃ higher than the maximum operating temperature, but it must not exceed the upper temperature limit of the column, which is very easy to damage the column. When reaching the aging temperature, record and observe the baseline. In the initial stage, the baseline should continue to rise, and it will begin to decrease 5-10 minutes after reaching the aging temperature, and will continue for 30-90 minutes. It will stabilize when it reaches a fixed value. If the baseline is still unstable after 2-3 hours or there is no obvious downward trend after 15-20 minutes, then there may be a leak or contamination of the system equipment. In such a situation, the column temperature should be immediately reduced to below 40 ° C, and the system should be checked as soon as possible and related problems resolved. If the aging continues, not only will the column be damaged, but the normal stable baseline will never be obtained.
In general, columns with polar stationary phases and thicker coatings have a longer aging time, while columns with weak polar stationary phases and thinner coatings require less time. The aging methods of PLOT columns are different. Aging step of PLOT column: HLZPora series 250 ℃, more than 8 hours Molesieve molecular sieve 300 ℃ 12 hours Alumina alumina 200 ℃ more than 8 hours Due to the irreversible adsorption of water in alumina and molecular sieve PLOT columns, these two columns are prone to occur Retention behavior drift.
After the column has separated the sample with high moisture content, the chromatographic column needs to be reconditioned to remove the moisture adsorbed in the stationary phase.
Step 8. Set 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 lower the oxygen content, the better. If you are using a capillary column, the carrier gas should be evaluated based on the average linear velocity of the carrier gas (cm / sec) instead of the carrier gas flow rate (ml / min). Because the calculation of column efficiency uses the average linear velocity of the carrier gas. Recommended average linear velocity value: Nitrogen: 10-12cm / sec Hydrogen: 20-25cm / sec Carrier gas impurity filter Adding a gas filtration device to the carrier gas pipeline can not only extend the life of the column, but also greatly reduce the background noise. It is recommended to install a high-capacity deaerator and a carrier gas purifier. When using the ECD system, it is best to install a deoxygenation tube in the auxiliary gas circuit.
Step 9. Column bleed detection After the aging process of the chromatographic column is completed, use a temperature program to perform a blank test (without injection). Generally, the temperature is increased from 50 ° C to the highest service temperature at 10 ° C / min, and it is maintained for 10 minutes after reaching the highest service temperature. So we will get a loss map. These values ​​may be helpful for comparative tests and experimental problem solving in the future. In the blank test chromatogram, there should be no chromatographic peaks. If a chromatographic peak appears, it is usually a contaminant from the inlet. If the performance of the column begins to decline under normal use, the baseline signal value will increase. In addition, if the baseline signal value is significantly larger than the initial value at a very low temperature, it is possible that the column and GC system are contaminated. Others: The column is sealed with a sample pad to seal the two ends of the column, and put it back in the original packaging. During installation, part of the column should be cut off to ensure that no debris from the sample pad remains in the column.
Note: When the content of hydrogen in the air is 4-10%, there is a danger of explosion. Therefore, we must ensure that the laboratory has a good ventilation system.
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