Electron-capture detector constant current

Gas Chromatograph -- A Varian 6000 equipped with two constant-current/pulsed-frequency electron capture detectors, a 30-m x 0.53-mm ID DB-5 fused-silica open-tubular column (1.5-/xm film thickness), and a 30-m x 0.53-mm ID DB-1701 fused-silica open-tubular column (1.0-/im film thickness), both connected to a press-fit Y-shaped fused-silica inlet splitter (Restek Corporation, Bellefonte, Pennsylvania), was used to analyze for the nitroaromatic compounds. The columns were temperature-programmed from 120°C (1.0-min hold) to 200°C (1-min hold) at 3°C/min, then to 250°C (4-min hold) at 8°C/min injector temperature 250°C detector temperature 320°C helium carrier gas 6 mL/min nitrogen makeup gas 20 mL/min. 

Assuming that the detector signals are proportional to the masses of the products, the yield of DFBP is proportional to the absorbed dose however, the yields of the major products are less than proportional above 10-15 kGy. Some of the minor products show yields that are more than proportional at this range, and become more prominent at larger doses. If the detector sensitivity is assumed to be constant per monomer unit, i.e. the sensitivity of the tetramers is twice that of the dimer and 4/3 times that of the trimer, then the yield of all the products can be calculated. This total yield was found to be linear with the dose for the whole range studied (up to 25 kGy). Absolute yield can be measured only for DFBP, for which a standard exists, and it is 0.0465 molecule/100 eV. Measuring the formation of total polymer by the total ion current in the GC/MS gave a total yield of 1.7 molecules/100 eV, and similar yields are obtained by the electron capture detector. 

Figure 3.22. Cross-sectional view of a pulsed discharge electron-capture detector operating in the constant current mode. (From ref. [308] American Chemical Society).

Virtually all modern electron capture detectors operate according to the pulse-modulated constant-current technique (PMCC). The ionization source is a Ni foil (B -emitter), which is attached to the inside of the... 

The detector can be made to function in two ways either a constant potential is applied across the sensor electrodes (the DC mode) or a pulsed potential is used (the pulsed mode). In the DC mode, a constant electrode potential of a few volts is employed that is just sufficient to collect all the electrons that are produced and provide a small standing current. If an electron capturing molecule (for example a molecule containing a halogen atom which has only seven electrons in its outer shell) enters the sensor, the electrons are captured by the molecules and the molecules become charged. The mobility of the captured electrons are much reduced compared with the free electrons and, furthermore, are more likely to be neutralized by collision with any positive ions that are also generated. As a consequence, the electrode current falls dramatically. 

Recent EC detectors have pulsed-voltage power supplies that maintain a constant current. With no sample, the pulse frequency is very low as the sample enters the detector, the frequency increases to offset the current loss due to the electron-capturing species. The pulse frequency is proportional to the sample concentration and can be used for quantitative analysis. 

The beta-rays emitted from the cathode ionize the carrier gas, thereby liberating electrons. If a pulsed voltage is applied to the electrode in the cell, these electrons are captured, so producing an electric current. If electrophilic molecules are introduced into the cell, these absorb electrons and become negatively ionized. The electron density in the detector therefore decreases, so that a smaller number of electrons are captured at each pulse. The total number of electrons captured per unit of time (i.e. the current) can be kept constant by increasing the pulse frequency when the number of electrons decreases. The pulse frequency is then proportional to the concentration of the electrophilic molecules passing through the detector [8]. 

Pulsed variable frequency with constant current mode is considered the superior method and is used by most manufacturers of ECDs. A preselected level of current is required. When an electron-capturing solute enters the detector, the standing current decreases the electronic circuit is adjusted to a frequency of pulsing to maintain a constant current. The detector response is in the frequency... 

In Equation 6.16, Is is the standing current, I the current measured when electronabsorbing species are in the detector at concentration c, and A is a constant characteristic to the cell and the species present (also known as the electron-capture coefficient). 

Electron capture is more effective, the slower the electrons move. For this reason, sensitive ECDs are operated using a pulsed DC voltage. By changing the pulse frequency, the current generated by the electrons is kept constant. The pulse frequency thus becomes the actual detector signal. 

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