| 摘要 |
<p>Electron capture detector for use with an effluent stream from a gas chromatograph includes an ionization cell (256) and a sample inlet system (250) connected to the ionization cell (256) for providing an effluent fluid having a sample concentration therein. In the preferred constant-current, variable frequency mode of operation, a cell current is measured and compared to the reference current. The pulse rate is then adjusted to maintain a constant cell current. When a sample compound that captures electrons is present in the ionization cell, the pulse rate varies. The pulse rate is converted to a voltage, processed according to a novel linearization formula, and recorded. The ionization cell (256) includes a radioactive ionization source (258) and the ionization cell (256) defines an optimized cell volume in which the electron capture reaction is forced to occur according to a concentration mode of operation. Improved sensitivity in the electron capture detector may be achieved by restricting the effective radioactivity in the ionization cell to be in the range of 0.5 to 3 milliCuries, and preferably in the range of 1 to 2 milliCuries. The ionization cell volume is substantially reduced while the aforementioned effective level of radioactivity is nonetheless maintained. An optimum cell volume was found to be in the range of 100-150 microliters. Linearization of the electron capture detector is obtained by overcoming and/or compensating for several sources of nonlinearity by: (1) forcing the detector to operate in only the concentration mode of operation, and (2) processing the signal derived from the detector response so as to compensate for a heretofore unrealized logarithmic decay of the response factor at higher sample rates. An electron capture detector constructed according to the present invention has been observed to operate with improved sensitivity and linearity over a very wide dynamic range (i.e., six decades of sample concentration). <IMAGE></p> |
