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FLEXO Magazine : April 2009
TECHNOLOGIES & TECHNIQUES It is a good practice to check the detector signal by running a blank before running any samples if your GC has not been used in the past day. This is simply done by setting up the GC for anal- ysis and starting a run with no injection. OFF-SCALE DETECTOR RESPONSE The detector signal of all GCs has an upper limit of response. If a component in a sample is too large, it may overload the detec- tor and the peak will go off -scale at the maximum detection limit. The data for the portion of the peak that went off -scale will be lost, with the result that these overly large peaks will be reported with lower than actual amounts. Always check that the largest peaks of interest in your chro- matogram are within the maximum detector response limit for your GC and data system. REPORTED PEAK AREAS/AMOUNTS Up to this point in this discussion we have focused on review- ing the chromatogram itself. Equally important is a careful review of the analysis report that is generated by the data system of your check standard results. Review the printed report and see that all check standard com- ponents are present and that they are listed in the correct order. GC problems or changing conditions can cause the data system to identify peaks incorrectly. From a quantitative standpoint, the individual areas and re- ported amounts for each component in your check standard injections should remain within reasonable limits. Compare your current check standard results with those of a reference injection. If there is a discrepancy, locate the cause of the problem and re- inject the check standard before analyzing your samples, or their results may be reported incorrectly. An important point to remember is that it is relatively easy to lose sensitivity in a GC analysis, resulting in low reported areas. Low area counts for all components in the check standard might indicate a major leak in your headspace sample vial seal or at the column connection to the GC inlet. Low area counts for the most volatile components in the check standard, e.g. ethyl acetate, indicate that the check standard itself is suspect and that a new standard should be prepared. On the other hand, a GC can not generate signal by itself. High reported results usually mean that the components themselves FIGURE 3A. Example of negative baseline drift. FIGURE 3B. Example of positive baseline drift. 34 FLEXO APRIL 2009 www. f le xography. org