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nbsp; The Pulsed Discharge Electron Capture Detector The pulsed discharge electron capture detector is an extension of the previously discussed pulsed discharge helium ionization detector, a diagram of which is shown in figure 44. The detector functions in exactly the same as that of the traditional electron capture detector but differs in the method of electron production. The sensor consists of two sections: the upper section has a relatively small diameter and is where the discharge takes place. The lower section has a much wider diameter and in this part of the sensor, the column eluent is sensed and electron capturing occurs. As with the pulsed discharge helium ionization detector, the potential across the electrodes is pulsed at about 3 kHz with a discharge pulse width of about 45 msec. The discharge produces electrons and high energy

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

1 x 10-9 to 5 x 10-9 amp, the noise level is about 1.2 x 10-13 amp and the ionization efficiency is about  0.07%. It is claimed to be about 10 times more sensitive than the flame ionization detector and to have a linear dynamic range of 105. An example of the use of a pulsed helium discharge detector for monitoring the separation of some aromatics on a capillary column is shown in figure 37. The pulsed helium discharge detector appears to be an attractive alternative to the flame ionization detector and would eliminate the need for three different gas supplies. It does, however, require equipment to provide specially purified helium, which diminishes the advantage of using a single gas.   The Electron Capture Detector Lovelock's work on ionization detectors culminated in the invention of the electron capture detector (25).  However, the electron capture detector operates on an entirely different principle from that of the argon detector. A low energy b-ray source is used

8 Mirex 486 pg   Courtesy of Valco Instruments Company Inc.   Figure 45 The Separation of Some Pesticides Monitored by the Valco Pulsed Discharge Electron Capture Detector   The linear dynamic range is also not precisely clear from the original publication but appears to be at least three orders of magnitude for a response index of (r) where 0.97 < r < 1.03, but this is an estimate from the data published. The modified form of the electron capture detector,devoid ofaradioactivesource,is obviously an attractive alternative to the conventional device and appears to have similar, if not better, performance characteristics. An example of the use of the  pulsed discharge electron capture detector to monitor the separation of a mixture of pesticides is shown in figure 45.   In general, the electron capture detector is used extensively in forensic analyses and in environmental chemistry. It is very simple to use and is one of the least

a small standing current. If an electron capturing molecule (for example a molecule containing an halogen atom which has only seven electrons in its outer shell) enters the cell, the electrons are captured by the molecule and the molecules become charged. The mobility of the captured electrons is much smaller than the free electrons and the electrode current falls dramatically. The DC mode of detection, however, has some distinct disadvantages. The most serious objection is that the electron energy varies with the applied potential. The electron capturing properties of a molecule varies with the electron energy, so the specific response of the detector will depend on the applied potential Operating in the pulsed mode, a mixture of 10% methane in argon is employed which changes the nature of the electron capturing environment. The electrons generated by the radioactive source rapidly assume only thermal energy and, in the absence of a collecting

GC Detectors A large number of GC detectors have been developed and made commercially available. In general, GC detectors are 4 to 5 orders of magnitude more sensitive than LC detectors and, thus, are ideal for trace analysis and environmental monitoring. The detectors with the highest sensitivity tend to be specific and sense specific types of sample (e.g., halogenated substances by the electron capture detector). Conversely, those detectors with a catholic response, although highly sensitive compared to LC detectors (e.g. the flame ionization detector) are significantly less sensitive than the specific detectors. The detectors with a catholic response are the most popular and the majority of GC separations are monitored by the flame ionization detector (FID). The most commonly used specific detectors are the nitrogen phosphorus detector (NPD) and the electron capture detector (ECD) The