The ECD works by directing the gas phase output from the column across an electrical field applied across two electrodes, either using a constant DC potential or a pulsed potential. The electrical field is produced using a thermally stable 63Ni source that ionizes some of the carrier gas or auxiliary detector gas (usually nitrogen or a mixture of argon 95%/methane 5%) and produces a current between a biased pair of electrodes. In the DC mode, the electrical field collects any electrons produced, providing a standing current. When electron-capturing molecules enters the detector field(e.g., a halogenated molecule) they capture electrons and become charged. The mobility of the electrons captured by the halogenated material is much less than that of free electrons and the captured electrons are also more likely to be neutralized by collision with positive ions. As a consequence, the detector current falls dramatically and is measured as the analyte signal.
In the pulsed mode, the period of the pulsed potential is adjusted so that relatively few of the slow negatively charged molecules reach the anode, but the faster moving electrons are all collected. During the off period of the pulse the electrons re-establish equilibrium with the gas. The operating variables are pulse duration, the pulse frequency and the pulse amplitude. By appropriate adjustment of these pulse characteristics, the current can be made to indicate the mobilities of the different charged species in the cell and thus, provide some discrimination between different electron capturing materials. The ECD is one of the most sensitive gas chromatography detectors available. The sensitivity of the ECD enables it to provide unmatched performance for extremely tough applications. It is the first choice for certain environmental chromatography applications due to its extreme sensitivity to halogenated compounds like PCBs, organochlorine pesticides, herbicides, and halogenated hydrocarbons. The ECD is 10-1000 time more sensitive than the FID (Flame Ionization Detector), but has a limited dynamic range and finds its greatest application in analysis of halogenated compounds.
- Baseline Resolution of EPA Method 608 Pesticides
Achieving Baseline Resolution of EPA Method 608 Pesticides with Alltech Pesticide Capillary, Alltech Application Note ANE005, 1997.
To demonstrate baseline resolution of all EPA Method 608 pesticides for the EPA Contract Laboratory Program (CLP), a capillary column, Alltech Pesticide Capillary (part No. 13662) 30m x 0.25mm x 0.25µm was installed on a GC equipped with an electron capture detector (ECD).
A mixture of pesticides were injected: a-BHC, g-BHC (lindane), b-BHC, d-BHC, heptachlor, aldrin, heptachlor epoxide, endosulfan I, p,p’-DDE, dieldrin, endrin, p,p’-DDD, Endosulfan II, p,p’-DDT (DDT), endrin aldehyde and endosulfan sulfate.
The column temperature was increased from 190°C to 240°C at 6°C/min. All 16 pesticides eluted within 17 minutes with baseline separation, including four typically difficult to resolve compounds baseline separated: beta-BHC resolved from gamma-BHC and p,p’-DDD resolved from Endosulfan II.