Question

1. In this experiment we injected the sample to be analyzed by Gas Chromatograph equipped with an FID (Flame Ionization Detector). The detector ionizes the sample as it reaches it, and the peak is proportional to the number of ions with a live flame. Explain in detail, why we need to run a standard when the GC is equipped with an FID detector to identify the component in a sample?

Answer 1

The voltage from the detector is proportional to the number of molecules passing through the detector at any given time. For well-separated peaks, the total number of molecules of each component reaching the detector is then proportional to the area under the peak. The recorder determines the area of each peak by integration and reports this in the results table. The proportionality factor between area and amount must be determined by a calibration experiment. Note that the integrator will also determine areas for peaks that are not well-separated by dropping a vertical line where the slope changes sign. the detector were equally sensitive to each component in a mixture, the peak areas could be used directly to give the percentage composition of the mixture by dividing the area of each peak by the total area under all of the peaks. Since the detector is not equally sensitive to the different components, each peak area must be multiplied by a suitable factor (called the response factor, k) to correct for this difference. The corrected areas are then used for the calculation of the percentage composition of the mixture. We will use something called a relative response factor, f, which ratios each response factor to that of a chosen component. The relative response factors are determined by measuring the peak areas for a mixture of known composition. These relative response factors can then be used to determine the percent composition of an unknown mixture of the same components.