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FLEXO Magazine : September 2009
80 FLEXO SEPTEMBER 2009 www.flexography.org TECHNOLOGIES & TECHNIQUES A manual headspace vial crimper. Editor's Note: This is part four in a series of FLEXO articles focusing on gas chromatography applications for flexible packaging printing/converting. Previous articles appeared in FLEXO February 2009 (page 44), April 2009 (page 30), and June (page 26). All quantitative gas chroma- tography sample results are based on the analysis of a known calibration standard. The Calibration Standard DISCUSSION You may ask why the GC data system cannot be calibrated in advance at the factory. This is because the GC can be used to identify and determine the amount of literally thousands of different volatile organic compounds. Additionally, the GC has many parameters that affect the results, such as sample size, use of different column types, oven temperatures, and injector and detector setup. These all control how the GC responds to each compound and will affect how much of each residual solvent is detected by the GC. WHAT IS GC CALIBRATION? In its simplest form, calibration of a GC entails injection of a mixture of known compounds of interest (volatile solvents). This mixture must be carefully prepared with known amounts that are all near the concentration for the compounds of interest in the sample. In practice, a small known amount of the standard is sealed into a headspace vial, heated for a fixed amount of time and then analyzed using the GC method parameters. This is saved into the GC data system as the calibration data. This calibration standard can be prepared in the user 's laboratory. The solvents used to prepare the calibration stan- dard must have a known purity of greater than 99 percent and must be prepared by accurately weighing exact amounts of each solvent of interest into a known volume. If they are not stored in sealed glass ampules, the volatile standards prepared in this way will need to be remade on a regular basis to ensure that they are accurate. This can be time consuming and assumes that users have high-purity sol- vents, calibrated analytical balances and accurate glassware available in their laboratories. The more common approach is to purchase a custom prepared calibration standard mixture that is pre-made to the user's solvent mixture specifications. These standards are sealed in individual, 1mL glass ampules, which allow the standard solution to be kept for one to two years without any loss of accuracy. WHEN TO CALIBRATE Calibration is performed at initial setup of the GC and the calibration results are stored in the GC data system. After the initial calibration of the GC, the standard is analyzed as a sample on a regular basis. This is called a check sample and is preferably done each time unknown samples are analyzed by the GC. This is done to ensure that the GC instrumenta- tion is stable and capable of producing accurate results for known, as well as unknown samples. Typically, a range of results for the stan- dard analysis is +/- 20 percent of its actual value before reca- libration of the GC is performed. Larger changes in response to the standard, or standards results that continue to change require that a qualified analyst determine the cause of the changing results being produced by the GC instrumentation. COMMON PROBLEMS Headspace crimp. The single biggest challenge in residual sol- vent analysis is to ensure that the headspace vial seal is properly crimped. The vial is pressurized before analysis when using some automated headspace systems, and poorly sealed vials will allow the volatilized sample to be forced out of the sample vial before analysis. The headspace hand-crimper typically needs to be readjusted on a regular basis and should always be replaced after 3 to 5 years. Outdated standards. Old calibration standard solutions create additional problems in GC analysis. These standards will have incorrectly low and high concentrations as the more highly volatile solvents in the standard evaporate out of the standard solution, with the result that these compounds will be reported incorrectly. Shifts in retention times. Component peaks may also be reported incorrectly as a result of shifts in retention times as the column ages or instrumentation conditions change. In this case, the data system method will need to be checked for proper solvent identifications and the system should be recalibrated with a new standard solution. Poor maintenance. Lack of proper maintenance of the GC, particularly the GC injector, column and detector, can cause major changes in the relative concentration values of the solvents in the standard. The area of some compounds will be affected more than others. A written and readily available calibration and maintenance procedure is critical. SUMMARY The identification and quantization of the compounds of interest in printed film sample by gas chromatography is a direct result of the analysis of a properly prepared new calibration standard. Failure to perform the GC data system calibration procedure or check the calibration results with a good, recently prepared or opened standard will result in erroneous residual solvent data. 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.