The Spark Discharge Detector
Lovelock noted that the voltage at which a spark will occur between two electrodes situated in a gas will depend on the composition of the gas between the electrode tips and suggested that this could form the basis for a GC detector. The system suggested by Lovelock is shown in figure 49.
The sensor consists of a glass tube in which two electrodes are sealed. The electrodes are connected in the circuit depicted in figure 49. The voltage across the electrodes is adjusted to a value that is just less than that required to produce a spark. When a solvent vapor enters the sensor, the sparking voltage is reduced and a spark discharge occurs. This discharges the capacitor until its voltage falls below that which will maintain the spark discharge. The capacitor is then charged up through the charging resistor until the breakdown voltage is again reached and another spark is initiated. Thus the spark frequency will be proportional to (or at least be a monotonic function of) the vapor concentration. The total counts in a peak will be proportional to the peak area and, if a digital–to–analog converter is also employed, the output will be proportional to the concentration in the detector and thus, plotted against time, will provide the normal chromatogram. This detector does not appear to have been developed further but is an interesting example of a sensor that, in effect, produces a digital output.
The Radio Frequency Discharge Detector
When an RF discharge occurs across two electrodes between which the field is diverging (i.e. within a coaxial electrode orientation) a DC potential appears across the electrodes, the magnitude of which depends on the composition of the gas through which the discharge is passing. Karman and Bowman developed a detector based on this principle. A diagram of their detector is shown in figure 50.
