Non-radioactive electron-capture detector operating in the pulsed mode

Abstract

This paper describes an electron-capture detector (ECD) which does not require a radioactive source for the generation of electrons and which can be operated in the pulsed mode. Other researchers have developed non-radioactive versions of the ECD; however, all of these have required the application of a constant potential for electron collection. The pulsed mode allows electron attachment to occur under field-free conditions, and is the more commonly used mode of ECD operation. In our detector, electrons are produced by energetic species derived from a microwave-induced discharge in helium. The identity of these energy carriers has not been established definitively, but various possibilities are discussed. The remainder of the apparatus is identical to that used with a normal pulsed-mode ECD with the exception that our detector operates at a pressure of 10-20 Torr. In this paper we report characteristic data for the significant operational parameters (pulse period and pulse width) and measured quantities (rate constants for loss of electrons and for production of electrons) associated with our detector. These are compared to corresponding data typical of a conventional pulsed-mode, radioactive-foil ECD. The detection limit (signal-to-noise ratio  2) of our detector for carbon tetrachloride. which captures electrons dissociatively with an extremely high capture coefficient, is on the order of 70–80 fg. For methylene chloride, which also captures dissociatively but with a much lower capture coefficient, the detection limit is 8 ng. For hexafluorobenzene, which captures non-dissociatively, the detection limit is 530 fg. These limits were obtained using a prototype detector. We anticipate that improvements in design will result in even better performance.