<P> Another important disadvantage is that the FID flame oxidizes all oxidizable compounds that pass through it; all hydrocarbons and oxygenates are oxidized to carbon dioxide and water and other heteroatoms are oxidized according to thermodynamics . For this reason, FIDs tend to be the last in a detector train and also cannot be used for preparatory work . </P> <P> An improvement to the methanizer is the Polyarc reactor, which is a sequential reactor that oxidizes compounds before reducing them to methane . This method can be used to improve the response of the FID and allow for the detection of many more carbon - containing compounds . The complete conversion of compounds to methane and the now equivalent response in the detector also eliminates the need for calibrations and standards because response factors are all equivalent to those of methane . This allows for the rapid analysis of complex mixtures that contain molecules where standards are not available . </P> <P> To detect these ions, two electrodes are used to provide a potential difference . The positive electrode doubles as the nozzle head where the flame is produced . The other, negative electrode is positioned above the flame . When first designed, the negative electrode was either tear - drop shaped or angular piece of platinum . Today, the design has been modified into a tubular electrode, commonly referred to as a collector plate . The ions thus are attracted to the collector plate and upon hitting the plate, induce a current . This current is measured with a high - impedance picoammeter and fed into an integrator . The manner in which the final data is displayed is based on the computer and software . In general, a graph is displayed that has time on the x-axis and total ion on the y - axis . </P> <P> The current measured corresponds roughly to the proportion of reduced carbon atoms in the flame . Specifically how the ions are produced is not necessarily understood, but the response of the detector is determined by the number of carbon atoms (ions) hitting the detector per unit time . This makes the detector sensitive to the mass rather than the concentration, which is useful because the response of the detector is not greatly affected by changes in the carrier gas flow rate . </P>

Which of the following is not used for detection in gc