Thermoionization

FTD Flame thermoionic detector HEN High-efficiency nebuliser... 

TSD (1) Thermoionic specific detector (2) Thermally stimulated discharge... 

Measurement yields both the differences between the outer potentials and the work functions (real potentials). If two phases oc an / with a common species (index i) come into contact, at equilibrium /, (< ) = (/ ), that is at(a) - <, (/ ) = ZiFApty. These quantities are mostly measured using the vibrating condenser, thermoionic, calorimetric, and photoelectric methods. 

The most important apphcation of this metal is as control rod material for shielding in nuclear power reactors. Its thermal neutron absorption cross section is 46,000 bams. Other uses are in thermoelectric generating devices, as a thermoionic emitter, in yttrium-iron garnets in microwave filters to detect low intensity signals, as an activator in many phosphors, for deoxidation of molten titanium, and as a catalyst. Catalytic apphcations include decarboxylation of oxaloacetic acid conversion of ortho- to para-hydrogen and polymerization of ethylene. 

Rubidium metal and its salts bave very few commercial apphcations. They are used in research involving magnetohydrodynamics and thermoionic experiments. Rubidium is used in pbotocells. The metal also is a getter of oxygen in vacuum tubes. The beta-emitter rubidium -87 is used to determine age of some rocks and minerals. Radioisotopes of rubidium have been used as radioactive tracers to trace the flow of blood in the body. The iodide salt treats goiters. Rubidium salts are in pharmaceuticals as soporifics, sedatives, and for treating epilepsy. 

Samarium salts are used in optical glass, capacitors, thermoionic generating devices, and in sensitizers of phosphors. The metal is doped with calcium fluoride crystals for use in lasers. It also is used along with other rare earths for carbon-arc lighting. Its alloys are used in permanent magnets. 

Electron ionisation is still the most widely used technique for the analysis of volatile molecules. It is considered to be a hard ionisation process, which leads to reproducible spectra that can be compared to a library of mass spectra for compound identification. In this technique, ionisation occurs in the ion source by the collision of the sample molecules with electrons that are emitted from a filament by a thermoionic process (Fig. 16.15). 

The low level comparator will reject pulses of low intensity which are generated by thermoionic emission in the dynodes. It is obvious that an electron emitted by dynode 5 for instance will produce a much smaller avalanche at the anode than an electron emitted by the photocathode but of course thermoionic emission from the photocathode itself would still appear as a genuine pulse, and for this reason PM tubes used in photon counting applications must be cooled down. 

The sample extract is analyzed by GC-NPD and/or GC/MS. Other GC detectors, as mentioned earlier, may be used instead of NPD. If low detection level is desired, quantitation should be done from the GC analysis. The presence of ary analyte found in the sample must be confirmed on an alternate GC column or preferably by GC/MS. If a thermoionic detector is used and if the sample extract is to be analyzed without any cleanup, it is necessary to exchange the solvent from methylene chloride to methanol. 

U.S. EPA Method TO7 describes the determination of N-nitrosodimethylamine in ambient air (U.S. EPA, 1986). The method is similar to the NIOSH method discussed above and uses Thermosorb/N as adsorbent. The air flow is 2 L/min and the sample volume recommended is 300 L air. The analyte is desorbed with methylene chloride and determined by GC/MS or an alternate selective GC system, such as TEA, HECD, or thermoionic nitrogen-selective detector. The latter detector and the TEA are more sensitive and selective than the other detectors. Therefore, the interference from other substances is minimal. Other nitrosamines in air may be determined in the same way. 

WF Work Function is an electron work function of the elements a quantity (eV) that determines the extent to which emission will occur. The experimental method thermoionic, field emission, photoelectric, and contact potential difference at the experimental conditions (e.g., vacuum of 10 8 Pa, clean surfaces, and identifi-cation of crystal-face distribution). 

The vacuum-tube diode, invented by Fleming24 in 1904 [2,3], works because of the relative geometrical shapes of the two concentric electrodes, the cathode and the anode. It consists of a cylindrical glass enclosure that is partially evacuated, bonded, and sealed to a metal base. It contains an inner metallic thin-wire "cathode" (negative electrode, consisting of W, oxide-covered W, or a Th-W alloy), placed along the cylinder axis. This cathode is electrically heated to 900 K or above, using an auxiliary filament circuit, typically driven by a 6.3-V power supply, to foster thermoionic emission of electrons from the cathode. This cathode is cylindrically surrounded by a metallic outer electrode, the anode or "positive electrode" or "plate," which is a hollow metallic cylinder, whose axis coincides with that of the cathode. The... 

The NPD is a destructive detector that can be used in series only after non-destructive detectors (e.g. ECD). The NPD is sensitive to water that affects the condition of the thermoionic bead. The active element of the bead eventually will become depleted (especially when using halogenated solvents like dichloromethane, chloroform etc.) and requires replacement. 

To illustrate the importance of emission source, synthetic goethite particles deposited on 0.1 -pm pore-size polycarbonate filters (150,000 x mag.) were imaged using both a JEOL 6310 and a JEOL 6320F microscope equipped with thermoionic LaB6 and field-emission (EE) electron sources, respectively (Figure 11.8). The... 

Two general mechanisms are usually advanced to explain ionization of molecules in flames direct ionization by thermoionization, photoionization, or chemiionization and indirect ionization by charge transfer with other ions. The assessment of both mechanisms requires knowledge of the ionization potential of molecules. In the following discussion, computations developed in the Appendix are used to estimate approximate ionization potentials of polynuclear aromatic hydrocarbons. 

Thermoionization. A fraction of the large hydrocarbon molecules and soot particles, both represented by R, can be ionized directly via the reaction ... 

The mass spectrometric analysis starts with an ionization process (see also Section 3.5). This ionization takes place in the ion source of the MS instrument, where the analyte is introduced as gas phase. There are two common ionization procedures used for GC/MS electron ionization (El) and chemical Ionization (Cl). Other ionization procedures are also used in mass spectroscopy (see below and Section 5.4). The El process consists of an electron bombardment, which is commonly done with electrons having an energy of 70 eV. The electrons are usually generated by thermoionic effect from a heated filament and accelerated to the required energy. A schematic diagram of an El source is shown in Figure 5.3.1. 

The theory of a simple thermoionic process [64] has been adapted to find the number of desorbed particles from a given surface covered with an organic layer that does not absorb the laser energy. However, the heat flux in the substrate, which absorbs the energy, heats the sample to the same temperature as the substrate. This number N is given by relation ... 

Many detectors have been used to detect pesticides and herbicides in SFC. Among these detectors, the flame ionization detector (FID) is most commonly used for detection of a wide range of pesticides and herbicides, with a detection limit ranging from 1 ppm (for carbonfuran) to 80 ppm (for Karmex, Harmony, Glean, and Oust herbicides). The UV detector has frequently been used for the detection of compounds with chromophores. The detection limit was as low as 10 ppt when solid-phase extraction (SPE) was on-line coupled to SFC. The mass spectrometric detector (MSD) has also been used in many applications as a universal detector. The MSD detection limit reached 10 ppb with on-line SFE (supercritical fluid extraction)-SFC. Selective detection of chlorinated pesticides and herbicides has been achieved by an electron-capture detector (ECD). The limit of detection for triazole fungicide metabolite was reported to be 35 ppb. Other detectors used for detection of pesticides and herbicides include thermoionic, infrared, photometric, and atomic emission detectors. 

The most popular detector for PAHs is the UV detector. The detection limit was 0.2-2.5 ppb for 16 PAHs. A diode-array detector was also used for PAHs in SFC, and the detection limit was reported to be as low as 0.4 ppb. Other detections used for PAHs include mass spetrometric, thermoionic, infrared, photoionization, sulfur chemiluminescence, and fluorescence detectors. 

Detectors may be classified on the basis of selectivity. A universal detector responds to all compounds in the mobile phase except carrier gas. A selective detector responds only to a related group of substances, and a specific detector responds to a single chemical compound. Most common GC detectors fall into the selective designation. Examples include flame ionization detector (FID), ECD, flame photometric detector (FPD), and thermoionic ionization detector. The common GC detector that has a truly universal response is the thermal conductivity detector (TCD). Mass spectrometer is another commercial detector with either universal or quasi-universal response capabilities. 

Detectors can also be grouped into concentration-dependent detectors and mass-flow-dependent detectors. Detectors whose responses are related to the concentration of solute in the detector cell, and do not destroy the sample, are called concentration-dependent detectors, whereas detectors whose response is related to the rate at which solute molecules enter the detector are called mass-flow-dependent detectors. Typical concentration-dependent detectors are TCD and GC-FTIR. Important mass-flow-dependent detectors are the FID, thermoionic detector for N and P (N-, P-FID), flame photometric detector for S and P (FPD), ECD, and selected ion monitoring MS detector. 

Soil Samples Analysis. Standard soil samples from various locations were used for this study. Aliquots (2 g) were extracted in 20 ml of methanol/water (80/20 v/v). For the competitive ELISA, soil extracts were routinely diluted 1 40 in PBS supplemented with 0.1% Tween-20. The HPLC determination of hydroxyatrazine was done after cleanup of the methanol-extract (17). The samples were injected in a Lichrospher column, SI 60, and the hydroxy-s-triazines were detected at 240 nm (17). The GLC determination of atrazine was performed using a thermoionic (P-N) detector (18). GC-MS for atrazine determination was carried out as described previously (19). 

---