| Capillary Column Installation Click here for information about column text mixes. Proper column installation is essential in obtaining the highest performance and lifetime from a capillary column. Also, having the proper tools and information makes column installation easier and more consistent. 1. System Inspection: 2. Column Nut and Ferrule Placement: Table 11. Ferrule Sizes
3. Cutting the Column: Stainless steel columns: Stainless steel columns: Using a ceramic wafer or syringe needle file, scribe a continuous line around the circumference of the column. Greater scribing force is required than with fused silica tubing. Do not cut through the tubing. While firmly holding the column, grasp the tubing end and move this section in a circular path of gradually increasing diameter. The tubing should break at the scribed line. Wipe the end (away from the opening) with a methanol soaked Kimwipe. Inspect the cut edge with a 10-20X magnifier. The cut end should be have a uniform, inward taper or at least be at a 90° angle relative to the tubing wall. The opening should be circular and free from burrs or jagged areas. If necessary, re-cut the column until a proper end is obtained. 4. Installation in the Injector:Unwind a sufficient length of column from the cage and secure the column on the column hanger. Consult the GC instruction manual for the proper distance to insert the column into the injector. In most cases, the column will protrude 0.5-2 cm above the top of the ferrule. Insert the column into the injector and finger tighten the column nut. Using a wrench, tighten the column nut 1/4-1/2 turn when using graphite ferrules or 1/2-3/4 turn for graphite/vespel ferrules. If the column can be moved within the fitting, the column needs further tightening. Make sure the column is back in the proper location, and tighten the column nut an additional 1/4 turn or until the column is securely held in the fitting. Be careful not to overtighten. Unwind a sufficient length of column from the cage and secure the column on the column hanger. Consult the GC instruction manual for the proper distance to insert the column into the injector. In most cases, the column will protrude 0.5-2 cm above the top of the ferrule. Insert the column into the injector and finger tighten the column nut. Using a wrench, tighten the column nut 1/4-1/2 turn when using graphite ferrules or 1/2-3/4 turn for graphite/vespel ferrules. If the column can be moved within the fitting, the column needs further tightening. Make sure the column is back in the proper location, and tighten the column nut an additional 1/4 turn or until the column is securely held in the fitting. Be careful not to over tighten. Note: Graphite/vespel ferrules have a tendency to leak after heating. After the ferrules have been heated for 5-10 minutes, re-check the fitting to ensure a leak has not developed. This is especially important if the fitting was cold when the ferrule was tightened. 5. Turn on the Carrier Gas:Adjust the column head pressure within the appropriate range in Table 12. If desired, verify carrier gas flow through the column by inserting the detector end of the column into a vial containing hexane or other light solvent. A steady stream of bubbles is visible if there is carrier gas flow. Table 12. Approximate Head Pressures (psig)
Following the same precautions as for the injector, install the column into the detector. Turn on the detector as specified in the GC manual. 7. Confirm Carrier Gas Flow and Column Installation: Set the GC oven to 40-50°C. Using a split injector, inject 1-2 µL of a suitable non-retained compound. The split vent flow should be above 25 mL/min. A Megabore direct injector may require dilution of the non-retained compound. If a sharp, symmetric peak is obtained, the column installation and carrier gas flow are satisfactory. If the peak exhibits a tail or is very broad, check the GC parameters and re-install the column. If no peak is obtained, check the syringe for a plug or leak, and verify that the detector is on. 8. Condition the Column: Heat the column at its isothermal temperature limit. Alternatively, a temperature 20-30°C above the highest operating temperature (without exceeding the column’s temperature limit) can be used. Using a temperature program is not necessary. Plot the baseline after the column has reached the conditioning temperature. Adjust the baseline as necessary to keep it on scale. Upon reaching the conditioning temperature, the baseline will continue to rise for 5-30 minutes then drop for another 30-90 minutes. A flat baseline should be obtained 1-3 hours after reaching the conditioning temperature. If the baseline does not stabilize after 2-3 hours or does not remain constant, stop the conditioning process. Usually a leak in the carrier gas line or injector area, or system contamination, is responsible for unstable baseline. Either problems, needs to be fixed before proceeding with column installation. Make sure the drop in the baseline is not caused by detector stabilization. Some detectors require hours to a day before a completely stable background signal is obtained. Heat the column at its isothermal temperature limit. Alternatively, a temperature 20-30°C above the highest operating temperature (without exceeding the column’s temperature limit) can be used. Using a temperature program is not necessary. Plot the baseline after the column has reached the conditioning temperature. Adjust the baseline as necessary to keep it on scale. Upon reaching the conditioning temperature, the baseline will continue to rise for 5-30 minutes then drop for another 30-90 minutes. A flat baseline should be obtained 1-3 hours after reaching the conditioning temperature. If the baseline does not stabilize after 2-3 hours or does not remain constant, stop the conditioning process. Usually a leak in the carrier gas line or injector area, or system contamination, is responsible for unstable baseline. Either problems, needs to be fixed before proceeding with column installation. Make sure the drop in the baseline is not caused by detector stabilization. Some detectors require hours to a day before a completely stable background signal is obtained. 9. Setting the Linear Velocity: Set the oven to the initial temperature of the temperature program or to the isothermal value. Inject 1-2 µL of the appropriate non-retained compound. Adjust the head pressure until the desired average linear velocity is obtained. Details on calculating and setting the average linear velocity can be found on the carrier gas page of this section. 10. Background or Bleed Test: Acquire a blank run (no injection) using a temperature program. Starting at 40-50°C, ramp at 10-20°/min, then hold for 10-15 minutes at the conditioning temperature. Click here for more information on column bleed. 11. Test Mix: If desired, inject a test mixture to measure system performance. The column manufacturer’s test mixture or your own sample can be used to determine system performance. If desired, inject a test mixture to measure system performance. The column manufacturer’s test mixture or your own sample can be used to determine system performance. One test mixture is the same one used by the column manufacturer to measure column quality and performance. This test mixture is useful as a comparative and diagnostic sample. If the same test conditions are used, the test chromatogram can be compared to the one included with the column. The compounds in the test mixture provide specific information about the performance of the column. Manufacturer column test mixtures are usually analyzed using split injectors at a very high split ratio, a FID, and isothermal temperature conditions. Hydrocarbons: Hydrocarbons are used to measure efficiency (N) and retention (k). Hydrocarbons exhibit the best peak shapes. Only very severe problems cause tailing or irregular hydrocarbon peaks. Alcohols: The alcohols are used to measure column activity or contamination. Alcohol peaks tail if the column is active or contaminated; however, it is not possible to distinguish between the two problems based on the shape of the alcohol peak. Acid compound: A phenol or carboxylic acid is used to measure the acidity of the column. Acid peaks exhibit tailing if the column is too alkaline. The acid peak may exhibit minor tailing for an active or contaminated column. Some carboxylic acid peaks always exhibit some tailing. Base compound: An aniline or alkyl amine is used to measure the basicity of the column. Base peaks exhibit tailing if the column is too acidic. The base peak may exhibit minor tailing for an active or contaminated column. Some amine peaks always exhibit some tailing. Other compounds: Usually fatty acid methyl esters (FAMEs) or PAHs are used as additional measures of retention. Retention indices (I) are often calculated for these compounds to determine stationary phase structure. Your own test mix: Another type of test mixture is a standard or sample that is representative of the analyses performed with the specific GC system. This mixture should contain the same compounds as in the analyses performed with the system. Their concentrations should be similar to those usually analyzed. Including only the most difficult to chromatograph compounds in the test mixture is satisfactory in most cases. Active compounds and those that are difficult to resolve from one another should always be included as test compounds. By using your own test mixture, the GC is tested using the conditions used for the actual analyses. A change in the GC conditions or hardware may be necessary to properly recreate the test conditions used by the column manufacturer. When using a selective detector, a column manufacturer’s test mix may contain compounds that do not generate a response.
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