Electron-capture detector operation

Figure 5.15. anomalous responses that can occur with an electron capture detector operated in the d-c mode (Ref.182, with permission). The chromatograms depicted are illustrative of observed phenomena ... 

The purpose of the carrier is to transport the sample through the column to the detector. The selection of the proper carrier gas is very important because it affects both column and detector performance. Unfortunately, the carrier gas that gives the optimum column performance is not always ideal for the particular detector. The detector that is employed usually dictates the carrier to be used. For instance, an electron capture detector operating in the pulsed mode requires an argon-methane mixture a thermal conductivity detector works best with hydrogen or helium. The most common carrier gases are listed in Table 6.1. 

An Electron Capture Detector operates as follows a CECD is essentially a tube thru which a stream of inert gas flows. A weak radioactive source on the walls of the tube irradiates the gas and generates within it a population of free electrons. These electrons are extracted from the gas stream by a positive electrode. The number extracted per second is measured as a current flowing thru this electrode... 

The development of the ionization detectors by Lovelock that evolved from the original argon detector culminated in the invention of the electron capture detector [2]. However, the electron capture detector operates on a different principle from that of the argon detector. A low energy 3-ray source is used in the sensor to produce electrons and ions. The first source to be used was tritium absorbed into a silver foil but, due to its relative instability at high temperatures, this was quickly replaced by the far more thermally stable Ni source. 

Common Pulse Pattern for the Electron Capture Detector Operated in the Pulsed Mode... 

The direct method uses a 4 foot, 4 mm, column at 230°C packed with 3% QV-17 on 60/80 mesh Gas Chrom Q. The detector was a Ni 3 electron capture detector operated at 310°. The use of stable high temperature phases and the high temperature Ni63 detector has enabled the quantitation of intact diazepam and its metabolites in blood and urine extracts at the following sensitivities. 

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 method of choice for the determination of a- and P-endosulfan in blood, urine, liver, kidney, brain, and adipose tissue is gas chromatography equipped with an electron capture detector (GC/ECD) (Coutselinis et al. 1976 Demeter and Heyndrickx 1979 Demeter et al. 1977 Le Bel and Williams 1986). This is because GC/ECD is relatively inexpensive, simple to operate, and offers a high sensitivity for halogens (Griffith and Blanke 1974). After fractionation of adipose tissue extracts using gel permeation chromatography, detection limits of low-ppb (1.2 ng/g) were achieved for endosulfan and other chlorinated pesticides using GC/ECD (Le Bel and Williams 1986). 

An ion mobility spectrometer offers to prospective users an attractive detector for a GC, from the perspective of detection limits and specificity. A mobility spectrometer, even with low resolution, allows interrogation of compound identities and imparts better specificity than the electron-capture detector. When gaseous analytes are delivered individually to IMS, the mobility spectrum contains information for identification, provided that operating conditions are kept constant for the unknown and reference spectra. The connection of a GC column to an ion mobility spectrometer is... 

The electron-capture detector was originally found to be a sensitive detector for the methylarsines [716]. After improvements of the atomic absorption detectors had been made (especially concerning adsorptive losses and peak shapes of the methylarsines), it was found that this detector could be used to replace the electron-capture detector, which because of its lack of specificity and its sensitivity to contamination and changes in operating conditions was very inconvenient to work with. 

Chromatographic Conditions. GC-ECD analyses were performed in an HP 5890 series II GC equipped with an electron capture detector and a split/splitless injector, operated by an HP Chemstation software. PCBs were separated on a 25 m length X 0.32 mm i.d., HP-1 column coated with a 0.17 pm film. The GC oven temperature program was as follows 90 °C hold 2 min, rate 20 °C/min to 170 °C, hold for 7.5 min, rate 3 °C/min, to final temperature 280 °C, and hold for 5 min. Nj was employed as carrier and makeup gas, with a column flow of 1.2 mL/min at 90 °C. Split flow was set at 50 mL/min. 

Health and safety requirements. Special licenses are required to operate an instrument that uses radioactive substances (e.g., the radioactive sources for the electron capture detector)... 

Some altitude effects on the operation of chromatographic instruments are anticipated. To achieve reproducible retention times for identifying compounds, mobile-phase flows need to be controlled so that they are independent of ambient pressure. Detectors may also respond to changes in pressure. For example, the electron capture detector is a concentration-sensitive sensor and exhibits diminished signal as the pressure decreases. Other detectors, such as the flame ionization detector, respond to the mass of the sample and are insensitive to altitude as long as the mass flow is controlled. 

The type and amount of the sample injected can affect the operation and response of the detector.(A 1-yl injection of carbon tetrachloride into an electron capture detector will yield an inoperable detector for several minutes). Overloading must... 

The capture process of the electron capture detector can be very temperature-sensitive. The sensitivity may either increase or decrease with an increase in temperature, depending on the compound involved, as illustrated in Figure 6.24 for three benzene derivatives. Since detector temperature may affect sensitivity it is sometimes possible to improve the analysis by operating at a different temperature. The radioactive source determines the maximum temperature limit for the detector which is listed in Table 6.6. Exact values vary with manufacturer. 

For the analysis of sulfur hexafluoride an electron capture detector must be used. Analysis time for halothane, acetone, ether, cyclopropane, acetylenes, ethane, methane, and sulfur hexafluoride, under normal conditions, will average 8 min. Since the two detectors operate under different conditions samples containing both the hydrocarbons and sulfur hexafluoride are run through the gas chromatograph twice. By simultaneous use of separate ovens, the FID and ECD measurements can be made concurrently. Otherwise, the separate in jections must be made after switching the detectors. 

The best method for determining pentachlorophenol is conversion into the methyl ether followed by analysis using gas chromatography with an electron-capture detector, or gas chromatography coupled with mass spectrometry [2], Both of these methods require an extensive amount of pretreatment and highly-trained personnel f or the operation of the equipment. 

AW-DCMS (80-100 mesh) and operated at 200°C with nitrogen (90ml min-1) as carrier gas and an electron capture detector. 

---