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FLEXO Magazine : October 2009
www.flexography.org OCTOBER 2009 FLEXO 59 Editor's Note: This is the fifth and final part in this series of FLEXO articles focusing on gas chromatography appli- cations for flexible packaging printing/converting. Previous articles appeared in FLEXO's Feb. (page 44), April (page 30), June (page 26) and Sept. (page 78) issues. The septum. The manual injection port of a GC by its very nature needs to be serviced regularly. If you are using a syringe to make your injections, the septum in the injection port cap will need to be replaced on a regular basis. This, of course, depends on the number of injections made. The septum should be replaced after every 50 to 100 injections or a loss of sample can occur, with subsequent reporting of incorrect low sample results. The Septum Cap. The cap that holds the septum must be tightened securely but not over-tightened or it will crush the soft silicone rubber septum, resulting in a shortened life. The Glass Liner. Most GC injection ports have a remov- able glass liner installed in them. The glass liner should al- ways have a small plug of glass wool in the center to trap any septum particles that are generated by the syringe needle during injections. This small plug of glass wool will prevent solid particles from reaching the GC column where they may plug the column and either stop or reduce the flow. COLUMN The column is the heart of the gas chromatograph. It is doing the work of separating the compounds of interest in the sample. It can be damaged thermally and chemically if the GC method is not set up correctly. Peak shape is a good indication of when the column is damaged or worn out. Though modern capillary columns are very robust, even the best column will wear out after years of use. Comparison of the current peak shape with that at the time of the new column installation will show how the column has degraded with time and use. DETECTOR The flame ionization detector (FID) is commonly used for residual solvent analysis. This detector uses air and hydrogen to produce a small, carefully controlled flame to ionize the sol- vent peaks as they elute from the GC column. If either the air or hydrogen is contaminated, the GC baseline signal will be unstable, high or both. Filters installed on these gas supplies will eliminate contamination problems. Always replace these filters when they become exhausted. The FID is a robust and trouble-free detector that should need little maintenance. However, over time the jet and the collector assembly can become contaminated if the detector is not operated at a high enough temperature relative to the column. The FID detector should always be operated at a temperature at least 20 C to 50 C hotter than the maximum column temperature to prevent condensation of any contami- nates in the detector. An electronic flow meter is essential to determine proper GC operation and is used to set and confirm all column, detector and split gas flow settings. THE DATA SYSTEM The data system acquisition parameters must be set correctly or the final results will be inaccurate at best. An important parameter is the data sampling rate. The data ac- quisition rate should be set based on the actual peak width of the GC peaks so that from 15 to 40 data points are collected for each peak. This will allow the data system to produce ac- curate and reproducible results. GC TROUBLESHOOTING This can be very time consuming and frustrating. However, using a few simple rules can help to eliminate most common GC problems before they occur. A short list of GC trouble- shooting guidelines is as follows. 1. Always do the easiest things first. For example, if there is no signal from the detector, has the FID flame gone out? Has one or more of the GC gas supply tanks become empty? Are the GC method parameters set up correctly for this assay? Leaks, however, are the single biggest problem in gas chro- matography and usually the easiest to resolve. Don't forget to leak-check every time a column is installed or replaced. Air will be drawn into a leak at the septum or at the column injec- tor fitting and can quickly degrade the column liquid phase through oxidation. Some import rules to remember here are: • Use electronic leak detectors • Avoid using liquid leak detectors of any kind • Leak-check after initial instrument setup and also after any type of maintenance • Check all critical seal areas as well as all gas supply line connections 2. Recheck the last thing changed or replaced on the system. If a problem appears it could be a result of the last change on the GC. For instance, the column injector fitting may not have been tightened enough. Also, don't assume that the system was checked after a change was made. 3. Always analyze a known standard to check that the GC system is operating correctly. All GC results are based on running a known standard mixture of com- pounds. Always inject and analyze a known standard solution to confirm the proper operation of your gas chromatograph. 4. Bad peak shape. This is usually the result of an aging or worn out column or contaminated injection port liner. Always remember that the column is doing all the work of component separation. One of the main purposes of the glass liner in the injector is to prevent contaminates from reaching the column and to allow for easy removal when it is contaminated. SUMMARY Gas chromatography is similar to other analytical tech- niques in that attention to the important details of the initial analysis setup and routine maintenance will prevent most normal problems. Attention to the details of routine mainte- nance on your GC system will help to eliminate problems in advance, give you accurate and reproducible data and years of trouble-free operation. ABOUT THE AUTHOR: Burton Todd is the technical director of ChromLab. He has provided expert on-site GC training and support for the analysis of residual solvents in flexible pack- aging materials and for the analysis of printing ink solvents for more than 25 years. He can be reached by email at bur- firstname.lastname@example.org or by phone at 610-644-2260. TECHNOLOGIES & TECHNIQUES
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