Electron capture detector background current

Electron capture detector. Most ionisation detectors are based on measurement of the increase in current (above that due to the background ionisation of the carrier gas) which occurs when a more readily ionised molecule appears in the gas stream. The electron capture detector differs from other ionisation detectors in that it exploits the recombination phenomenon, being based on electron capture by compounds having an affinity for free electrons the detector thus measures a decrease rather than an increase in current. 

The heated alkali bead emits electrons by thermionic emission which are collected at the anode and provides background current through the electrode system. When a solute that contains nitrogen or phosphorus is eluted, the partially combusted nitrogen and phosphorus materials are adsorbed on the surface of the bead. This adsorbed material reduces the work function of the surface and, as a consequence, the emission of electrons is increased, which raises the current collected at the anode. The sensitivity of the NPD is very high and about an order of magnitude less than that of the electron capture detector (ca. 10 g/ml for phosphorus and lO g/ml for nitrogen). 

The electrolytic conductivity detector has been used to determine organic iodine with excellent success by Westlake (20), as well as chlorine in organochlorine pesticides, operating in the reducing mode to yield HI or HCl as the detected product. The minimum detectability for chlorine is approximately equal to that of the current microcoulometric detection system. Coulson (4, 15) compared the responses of the electrolytic conductivity, microcoulometric, and electron capture detectors for organochlorine compounds in various extractives and found the first two approximately equal and the electron capture detector unsatisfactory because of high background. 

Principle of operation a Ni source emits a (3 electron beam. A current between two electrodes is generated. When electrophilic species enter the detector, a decrease in the detector background current is observed due to the capture of the electrons by the electrophilic species. 

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