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FLEXO Magazine : April 2009
TECHNOLOGIES & TECHNIQUES FIGURE 1. Component retention time and area. How to Review and Analyze Gas Chromatographic Data By Burton S. Todd verters. Its primary use is to test for residual solvents in printed fl exible packaging materials. Gas chromatography data analysis is performed by compar- ing the sample data with a standard of known component identities and amounts. Before the advent of digital electronics, the GC operator had to compare manually the known standard and sample chromatograms in order to identify the component peaks present in the unknown sample. If quantitative informa- tion was required for the sample, the analyst had to manually measure each peak in the standard and sample chromatograms. This data was then used to calculate the component amounts in the sample. It was a tedious procedure at best and accuracy was always a challenge. Today, we have fast and powerful personal computers to do I this for us. However, the PC still must be told how to analyze the GC data it receives and stores. We must remember that the data system in reality knows nothing about the raw data that is sent to it by the gas chromatograph. The GC data system also can never know that it has identifi ed and calculated peak amounts correctly. Therefore, before the GC data is used it must be reviewed and analyzed. 30 FLEXO APRIL 2009 n this series on the fundamentals of gas chromatography, we are looking at the use of the gas chromatography (GC) in qual- ity control of printed fl exible packaging by printers and con- BASIC GC ANALYSIS During initial setup, the GC data system must be told what solvent components to look for, where to look for them in the chromatogram and what their peak size represents. This is ac- complished by fi rst analyzing a known mixture of components called the calibration standard or the standard for short. The standard is a mixture of known solvents that is very ac- curately prepared with known amounts of each solvent. This in- formation about the standard is stored in the PC data system and used to automatically identify and calculate the amount of the residual solvents in your fi lm samples. This standard also is used as a reference or check standard for ongoing performance checks of the GC instrumentation. COMPONENT IDENTITY AND AMOUNT In GC analysis, the retention time of a component is the time it takes for that component to travel through the GC column and enter the detector. The component retention times are used to establish the identity of components in the standard. Low-boiling compounds are retained for less time in the col- umn and come out earlier, while heavier components are re- tained in the column longer. A signal, or peak, is created as a component passes through the detector. The size of this peak is relative to the amount www. f le xography. org FLEXIBLE PACKAGING