Pulsed helium detector

The pulsed helium discharge detector [7,8] is an extension of the helium detector, a diagram of which is shown in figure 11. 

The Separation of Some Aromatic Hydrocarbons Monitored by the Pulsed Helium Discharge Detector... 

The Simple or Macro Argon Detector Sensor The Micro Argon Detector The Triode Detector The Thermal Argon Detector The Helium Detector The Pulsed Helium Discharge Detector The Electron Capture Detector The Pulsed Discharge Electron Capture Detector References Chapter 7... 

Fig.l The discharge ionization detector (courtesy of GOW-M AC Instruments) and the pulsed helium discharge detector (courtesy of Valeo Instruments). 

The base current ranges from 1 x 10 to 5 X 10 A, the noise level is about 1.2 x 10 A, and the ionization efficiency is about 0.07%. It is claimed to be about 10 times more sensitive than the flame ionization detector and to have a linear dynamic range of 10. The pulsed helium discharge detector appears to be an attractive alternative to the flame ionization detector and would eliminate the need for three different gas supplies. It does, however, require equipment to provide specially purified helium, which diminishes the advantage of using a single gas. 

The pulsed discharge detector (PDD) is a universal and highly sensitive nonra-dioactive and non-destructive detector, also known as a helium photoionization detector. It is based on the principle of the photoionization by radiation... 

The general operation of the pilot scale reactor has be previously described by Pareek et. al. [3]. However, modifications were required to allow the injection of the gas and liquid tracers, and their subsequent detection at the outlets. The liquid tracer, 5mL Methyl blue solution (lOgL" ), was injected via a syringe inserted into the liquid feed line. Outlet samples were measured with a Shimadzu 1601 UV-Vis Spectrophotometer at a wavelength of 635nm. A pulse (20mL) of helium gas tracer was introduced using an automated control system, with the outlet concentration monitored in real-time with a thermal conductivity detector. Runs were carried out based on a two-level... 

Carbon dioxide chemisorptions were carried out on a pulse-flow microreactor system with on-line gas chromatography using a thermal conductivity detector. The catalyst (0.4 g) was heated in flowing helium (40 cm3min ) to 723 K at 10 Kmin"1. The samples were held at this temperature for 2 hours before being cooled to room temperature and maintained in a helium flow. Pulses of gas (—1.53 x 10"5 moles) were introduced to the carrier gas from the sample loop. After passage through the catalyst bed the total contents of the pulse were analysed by GC and mass spectroscopy (ESS MS). 

The pulsed operation of the gas-filled detector illustrates the principles of basic radiation detection. Gases are used in radiation detectors since their ionized particles can travel more freely than those of a liquid or a solid. Typical gases used in detectors are argon and helium, although boron-triflouride is utilized when the detector is to be used to measure neutrons. Figure 5 shows a schematic diagram of a gas-filled chamber with a central electrode. 

The CGC analysis of the volatile degradation products were performed using a Perkin-Elmer Sigma 2000 capillary gas chromatograph. The column used was either a fused silica 0.25 micron, bonded methyl silicone (10 m, 0.25 mm I.D.) or a methyl/5% phenyl silicone (15 m 0.25 mm I.D.) bonded phase. The carrier gas was helium and the capillary column head pressure was maintained at 20 psi. The make-up gas for the pulsed electron capture detector (ECD) was 95% Ar/5% methane supplied at a flow rate of 60 ml/min. 

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. 

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. 

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