How to Troubleshoot a Gas Chromatograph
A gas chromatogram, or GC, is used to separate different chemicals based on their chemical structure. The machinery is quite sensitive, with high pressure and electricity needed for proper machine function. When troubleshooting your GC, first determine what type of problem is occurring. GC problems can be divided into several categories: ghost peaks, baseline disturbances, quantification difficulties, irregular peak sizes or shapes, loss of resolution, retention time shifts, rapid column deterioration and broad solvent front. Many of the maintenance steps can solve more than one problem.
1. Check for any loose or dirty cable connections if you notice many baseline disturbances in the chromatogram. Another possible cause of baseline disturbance include a dirty detector. Clean the detector following the procedure detailed in the GC manual. Ensure that the GC column was properly installed and the end is not inserted into the flame of the detector. Make sure that the flow rates are correct for your detector and that there is no leak in the column. A leak detector kit is very helpful to find leaks.
2. Check that the column injector has not been plugged, as this can result in irregular or wrong-sized peaks. Irregular peak shape or size can also be caused from using the wrong detector, a broken column, detector gases that are not on, low carrier gas flow, or the setting for the injector split ratio could be too high. Check all settings to ensure they are on and functioning at the proper level. Change the injector or fiber, check the levels in the gas supply you are using and that the valves are open. Run a sample with a method you have used successfully in the past. If the resulting peaks are fine, then the settings for the new method are not correct.
3. Ensure that the carrier gas flow, linear velocity and column temperature match what is set in the GC program. If the actual settings are different from the program, then there is an electrical problem and all of your chemicals will be eluting at a different time, resulting in a retention time shift. A common cause of retention time shift is a leaking septa. Injector septa can only be used so many times. Check how many injections the septa you are using can do and if it is near this number, change the septa.
4. Rinse the column with solvent if you notice a loss in resolution. Compounds that are stuck and contaminate the column will cause chemicals to stick in the column and not elute at all. If the baseline is very high or you notice a lot of bleeding in the chromatogram, then the column stationary phase is damaged and the column must be replaced. Be aware that large concentration changes between samples can be an issue.
5. Make sure all of your injection techniques are consistent if quantification difficulties occur. Inconsistencies with the split ratio or the activation used for splitless injection are the main issues with quantification problems. Other difficulties may be due to the integrator settings used on the software. Verify all of the settings and make sure you are integrating each peak the same on every sample.6. Find and repair any leaks if the column is rapidly deteriorating. Air exposure at high temperatures quickly damages columns. Having the temperature exceed the column specifications will also damage the column. The types of chemicals you inject in the column can also cause serious damage. If you have injected organic bases or acids or high molecular weight compounds, then the column may be damaged beyond repair. Replace the column and avoid using these damaging compounds in the future.
7. Change the septum to a higher temperature brand if ghost peaks are occurring. A low-temperature septum will retain some compounds that can appear in subsequent samples. Clean the liner and injector as well, as compounds may stick and then be released when higher temperatures are reached.
8. Set the split ration and injector temperature higher if there is a broad solvent front. Higher temperature and pressure can eliminate this problem. You also may be injecting too much sample into the injector. Use several different injection volumes to eliminate any solvent front.