Electrothermal device

Torsi et al. [395] have carried out a systematic investigation to establish the potential value of such an apparatus. The apparatus is basically an electrothermal device in which the furnace (or the rod) is replaced by a small crucible made of glassy carbon. Figure 5.10 provides an overall view of the apparatus. Figure 5.11 shows a block diagram of the electrolysis circuit the crucible (6) acts a cathode, while the anode is a platinum foil dipped into either the sample solution reservoir (1) or the washing solution reservoir (2). Pre-elecrolysis was performed at constant current with a 500 V dc variable power supply (5). Under these conditions, the cathode potential is not controlled, so that other metals can be codeposited with lead. 

Concerning AFS, the atomiser can be a flame, plasma, electrothermal device or a special-purpose atomiser e.g. a heated quartz cell). Nowadays, commercially available equipment in AFS is simple and compact, specifically configured for particular applications e.g. determination of mercury, arsenic, selenium, tellurium, antimony and bismuth). Therefore, particular details about the components of the instrumentation used in AFS will not be given in this chapter. 

Electrothermal atomisers come in many shapes and sizes, ranging from tube furnaces to metal ribbons. At present, the most popular form is the graphite tube furnace of which a number of designs are commercially available. The electrothermal devices vary markedly in their atomisation characteristics and a method suitable for one design will not necessarily work on another without some modification. 

Standards for the analysis may be prepared from organometallic standards, analysed samples or the NBS (GM-5) Heavy Oil Standard. The most satisfactory results are likely to be obtained using the second or third options. The sensitivity available is critically dependent on the electrothermal device to be used. This and the size of aliquot chosen for injection into the atomiser (normally 5—100 pi) will determine the selection of the concentration ranges chosen for the standards. Refer to the manufacturers information on Ni and V sensitivity and linear range and prepare calibration standards accordingly. Always prepare a blank solution and at least three standards to cover the chosen range. 

Devices for solid sample treatment prior to introduction into atomic spectrometers electrothermal devices and glow-discharge sources... 

In essence, glow-discharge sampling is more closely related to the techniques described in Chapter 9 than to electrothermal devices however, it is discussed here both to facilitate comparisons and to balance the contents of the two chapters. 

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. 

Therefore, in electrothermal devices, transient signals are obtained. They increase sharply and have a more or less exponential decay lasting 1-2 s. Their form has been studied from the point of view of the volatilization processes. The real signal form (see Fig. 56) is also influenced by adsorption and then subsequent desorption of the analyte inside the electrothermal device at the cooler parts. 

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. 

Recombination processes, minimized by transferring Ihe sample into the electrothermal device on a carrier platform with low heat capacity [109]... 

Interference in electrothermal evaporation may stem from differences in the physical properties of the sample liquids from one sample to another. This may influence the wetting of the graphite or the metal of the electrothermal device. When the latter is temperature dependent, it leads to differences in volatilization. Differences in the anions present may severely influence evaporation (chemical matrix effect). The boiling points of the compounds formed dictate the volatilization. Accordingly. the occuiTence of double peaks is easily understood. In the case of Cd. this is documented for a sample rich in chloride (CdCh evaporates at... 

Electrothermal atom cells have changed radically since their inception in the late 1950s. The majority of electrothermal devices have been based on graphite tubes that are heated electrically (resistively) from either end. Modifications such as the West Rod Atomizer (a carbon filament) were also devised but were later abandoned. Tubes and filaments made from highly refractory metals such as tungsten and tantalum have also been made, but they tend to become brittle and distorted after extended use and have poor resistance to some acids. Their use continues, however, in some laboratories that need to determine carbide-forming elements. For example, silicon reacts with the graphite tube to form silicon carbide, which is both very refractory and very stable. The silicon is therefore not atomized and is lost analytically. Use of a metal vaporizer prevents this. 

---