Types of electrothermal devices

In most cases the furnace is made of graphite, which has good thermal and corrosion resistance. As a result of its porosity, graphite can take up the sample without formation of appreciable salt deposits at the surface. However, apart from graphite, atomizers made of refractory metals such as tungsten have also been used (Fig. 57) (see e.g. Refs. [175, 175a]). 

In the case of graphite, tubes with internal diameters of around 4 mm, a wall thickness of 1 mm and a length of up to 30-40 mm are usually used. However, filaments enabling the analysis of very small sample volumes and mini-cups of 

In order to bring the sample rapidly into a hot environment, use is often made of the platform technique, as was first introduced in atomic absorption spectrometry by L vov [179]. Here the very rapid heating may enable the formation of double peaks to be avoided, which are a result of various subsequent thermochemical reactions, all of which have their own kinetics. Also the high temperature avoids the presence of any remaining molecular species, which are especially troublesome in the case of atomic absorption spectrometry. Thin platforms can be made of graphite, which have a very low heat capacity, or from refractory metals. In the latter case wire loops, on which a drop can easily be previously dried, are often used. 

When using electrothermal evaporation for sample introduction, the development of a suitable temperature program for the elements to be determined in a well-defined type of sample is of prime importance. In the case of liquid samples a small sample aliquot (10-50 pL) is brought into the electrothermal device with a syringe or with the aid of an automated sampler and several steps are performed. 

Direct solids sampling with electrothermal evaporation can be performed by dispensing an aliquot of a slurry prepared from the sample into the furnace. The analytical procedure is then completely analoguous with the one with solutions (see e.g. Ref. [181]). However, powders can also be sampled with special dispensers, 

This even has the advantage that no pressure jumps occur, which makes this approach feasable for coupling with low-power sources, as shown by the case of the MIP in Ref. [180] (Fig. 58). 

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This section is devoted to the types of devices most frequently used for both liquid and solid sampling prior to introduction into atomic spectrometers [12-14]. Atomic techniques and mass spectrometry make massive use of electrothermal devices, the maturity of which has been endorsed by lUPAC, which has included it in its Nomenclature, Symbols, Units and their Usage in Spectrochemical Analysis. XII. Terms Related to Electrothermal Atomization , published in 1992 and subsequently reprinted in Spectro-chimica Acta [1]. 

Various types of GD have proved highly suitable for use as atom reservoirs for AAS however, this solid sampling approach has been less frequently used in atomic absorption than in atomic emission and mass spectrometry, possibly as a result of the wider commercial availability of electrothermal devices. 

Conventional flame techniques present problems when dealing with either small or solid samples and in order to overcome these problems the electrothermal atomization technique was developed. Electrothermal, or flameless, atomizers are electrically heated devices which produce an atomic vapour (Figure 2.36). One type of cuvette consists of a graphite tube which has a small injection port drilled in the top surface. The tube is held between electrodes, which supply the current for heating and are also water-cooled to return the tube rapidly to an ambient temperature after atomization. 

The dead-space above the sample in the dart-like tip is much reduced by the use of the type of micropipette which uses fine capillary tips. The Oxford Ultramicro-sampler is an example. Use of this device completely overcomes the solvent expulsion problem. The principal disadvantage from the point of view of electrothermal atomisation with a graphite furnace is that its maximum capacity is 5 pi. This may be too little for the sensitivity of some elements. The tip material provided with this syringe is still prone to droplet formation, but this can be replaced by PTFE capillary tubing of the correct dimensions, e.g. Polypenco size TW 24, which appears to overcome the problem completely. Good precision should be attainable with tips made from this material, provided the tips are cut across at 90° to the tube axis. Chamfered tips give rise to a variable position of the meniscus with consequent loss of reproducibility. 

There are three basic types of electric propulsion systems electrothermal, electrostatic, and electromagnetic. In electrothermal propulsion, the propellant is heated either by an electric arc or a resistance heater. The hot propellant is then exhausted through a conventional rocket nozzle to produce thrust. Electrostatic propulsion uses electric fields to accelerate charged particles through a nozzle. In electromagnetic propulsion, an ionized plasma is accelerated by magnetic fields. In all three types, electricity from a nuclear source, such as a fission reactor, is used to power the propulsion device (Allen et ak, 2000 Bennett et al., 1994). The power flow for a typical nuclear-electric propulsion scheme is shown in Figure 1. 

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