Gas Chromatography Detectors - Ionization Detectors > The Electron Capture Detector > Page 74

The sensor is designed to operate with oxygen–free nitrogen or argon/methane mixtures. The radioactive source is 63Ni. This source has a long life and is stable up to 450oC. The detector can be used for the analysis of aqueous samples as steam has no effect on the source in the sensor. The sensor is thermostatted in a separate oven which can be operated at temperatures ranging from 100oC to 450oC. The column is connected to the sensor at the base and make–up gas can be introduced into the base of the detector if open tubular columns are employed as these columns are usually operated with hydrogen or helium as the carrier gas. The electron capture detector is extremely sensitive, probably one of the most sensitive GC detectors available (minimum detectable concentration ca. 10-13 g/ml) and is widely used in analysis of pesticides. Unfortunately, its sensitivity is often given in terms of the minimum mass of solute eluted, which can be misleading. The detector is concentration sensitive and thus the concentration of the solute for a given mass will vary with the position it is eluted in the chromatogram (for a given mass of solute, an early peak would be narrow and have a small volume and a high concentration at the peak maximum: however, if eluted as a late peak it would be broad, have a relatively large volume and a lower concentration at the peak maximum). Consequently, a mass of solute just identifiable (signal to noise = 2) when eluted as an early peak may not be detected or discerned when eluted as a late peak

 

The linear dynamic range of the electron capture detector is again ill-defined by many manufacturers. In the DC mode the linear dynamic range is usually relatively small, perhaps two orders of magnitude, with the response index lying between 0.97 and 1.03. The pulsed mode has a much wider linear dynamic range and values up to 5 orders of magnitude have been reported. The linear dynamic range will depend on the strength of the radioactive source and the detector geometry. If a response index lying between 0.98 and 1.02 is assumed, then a linear dynamic range of at least three orders of magnitude should be obtainable from most electron capture detectors. An example of a pesticide analysis employing an electron capture detector to monitor the separation is shown in figure 42.