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FLEXO Magazine : September 2009
78 FLEXO SEPTEMBER 2009 www.flexography.org TECHNOLOGIES & TECHNIQUES FLEXIBLE PACKAGING • Calibration of a GC entails injection of a mixture of known compounds carefully prepared with amounts near the concentration for the compounds of interest in the sample. • Calibration is performed at initial setup of the GC and the calibration results are stored in the GC data system. • Lack of proper maintenance of the GC can cause major changes in the relative concentration values of the solvents in the standard. Gas chromatography is used to measure the amount of residual solvents that are retained in printed flexible packaging. To do this mea- surement, the gas chromatograph (GC), as well as the computer, must be set up correctly. This setup is done by analyzing a known mixture of solvents that is called the calibration standard or standard for short. The GC method is then checked by analyzing the calibration standard as if it were an unknown sample. Following is a true story that vividly illustrates the point of this article that proper calibration of the GC is very important for the residual solvent analysis of printed films: Everyone, including all of the plant managers, was watching as I went about preparing the new calibra- tion standard. The GC results of an important customer's samples were reporting a very high presence of one of the volatile solvents that was well out of spec for the recently produced printed film samples. The entire production line for this product had been shut down with large hourly losses building up and product shipment deadlines not being met. They had called me in a panic and requested that I come out to their plant immediately to help resolve the problem of " why the GC results all of a sudden were showing high, out-of-spec" residual solvents for all of the printed samples. Arriving early the next day, I prepared a new solvent standard mixture. I then showed the assembled group the new bottle of calibration standard that had just been prepared, along with the old standard the technicians had been using to calibrate the instrumentation. The bottle with the newly prepared standard was almost full, while the original bottle of standard that they had been using was less than one-half full and had been prepared over six months previously! I knew that the technicians had been using the standard regularly; the septum seal was punctured with many holes. As a result, most of the highly volatile solvents in this mixture had evaporated out of the bottle over the past six months leaving behind the less volatile solvents. As the more volatile solvents had evaporated away and decreased in volume, the less volatile solvents were now at an incorrectly higher relative volume than they were in the originally prepared standards solution. This meant the results for the more volatile solvents were reported as too low and the less volatile solvents were reported incorrectly as too high. Together we recalibrated the GC method using the newly prepared standard and began running the current film samples. These samples were all well below the ac- ceptance levels for all of the solvents and the plant was back in operation. I suggested to the managers at this point that they start using pre-made custom calibration standards that are sealed in glass ampules to prevent any loss of the volatile solvents. The lab personnel could open one each time he or she needed to check the GC's calibration or to re-calibrate the GC's data system. This customer has had no problems since using the pre-made, ampulized standards for its GC assay. Calibration of Your Gas Chromatograph What this Means for Your Residual Solvent Data By Burton S. Todd Standards in sealed glass ampules. A headspace vial heater.