Graphite cuvettes

Evenson, M. A. and Warren, B. L. "Determination of Serum Copper by Atomic Absorption, With Use of the Graphite Cuvette". Clin. Chem. (1975), 21, 619-625. 

Platform Cuvettes. The concept of platform atomization was first described by L vov in 1978. A platform is a thin rectangular piece of pyrolytic graphite. The platform is positioned within a conventional graphite cuvette as shown in Figure 47. Samples are injected on the platform. Contact... 

Figure 61 Various designs of graphite cuvettes (Perkin Elmer Corp. and Unicam Analytical Systems Ltd.)...

Platform Atomization. A platform is a thin, rectangular shaped piece of graphite (normally prepared from pyrolytic graphite), which is mounted inside the conventional graphite tube. The platform is positioned in the centre of the cuvette. The platform is heated by cuvette wall radiation since it fits into a slot in the wall of the graphite cuvette and therefore does not come into direct contact with the hot centre portion of the tube. 

Instrumentation Laboratories has recently modified their tantalum boat unit to also accommodate a graphite cuvette. The cell is enclosed so atomization of the sample can occur in an inert atmosphere. A temperature-sensing device is included to permit accurate temperature measurements and temperature control. 

The unit, with pyrolytic graphite cuvettes, can be programmed to provide an atomization temperature up to approximately 3500°C. The tantalum boats are limited to temperatures below the melting point of tantalum (2995°C). A photograph of the Instrumentation Laboratories flameless atomizer is shown in Figure 10-28. 

Andreae [712] used four different detectors in his investigations the electron capture detector (for the methylarsines), the quartz cuvette atomic absorption detector (for arsenic and antimony species), the graphite furnace atomic... 

The graphite furnace system was originally developed by Andreae [712] when he found that the quartz cuvette gave only very poor sensitivity for germanium. This was attributed to the formation of GeO, a very stable diatomic species, at the relatively low temperatures of the quartz cuvette. At the higher temperatures available with the graphite furnace (2600 °C for the determination of Ge), a sensitivity could be obtained for germanium comparable to that of the other hydride elements. 

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. 

When the graphite furnace is used for extreme-trace analysis, relative standard deviations are in general above 10 relative percent. The solid sample analysis described in section II.B.l, using a steel chip and the rectangular cuvette, leads to an RSD of 13 relative percent in the determination of a... 

A series of additional alterations of the graphite tube atomizer intended to improve performance (e.g. the cup and cup cuvette atomizer, boat and microboat platforms, carbon rods, graphite probes, ring chambers and surface atomizers) have also been reported [23]. Interested readers are referred to specific sources [2-7] for more details. 

Fig. 84. Spatially isothermal graphite furnace for atomic absorption spectrometry using side-heated cuvettes with integrated contacts, (a) Cuvette contact area clamped in terminal blocks, (b) injection port, (c) aperture for fiber optics. (Reprinted with permission from Ref. [272].)...

The lifetime of TPCs is much longer than pyrolytically coated or normal graphite tubes. Figure 60 shows peak shapes for vanadium and platinum using totally pyrographite cuvettes, pyrolytically coated cuvettes, and electrographite cuvettes. The TPC peak is narrower and appears earlier than the peaks of the other cuvette types. 

The mechanism by which free atoms are produced in the graphite furnace depends on a number of factors, such as the compounds still present in the cuvette at the atomization temperature, the cuvette material, the atmosphere inside the cuvette, the heating rate of the cuvette, and the temperature of operation of the cuvette. 

Organic solvents have low surface tensions and easily wet and soak into the graphite. As they become warm, they tend to spread out over the cuvette. Because of these phenomena, smaller sample volumes and pyrolytic-ally coated cuvettes are generally used with organic solvents. 

Atomization. Atomization in the graphite furnace can emanate from either molecules or atoms depending on the nature of the sample and behaviour of the analyte. If atomization emanates from molecules, it can be a thermal decomposition or dissociation of a compound, or the reduction of a metal oxide on the hot graphite surface. The difference between these two mechanisms is the active participation of the cuvette material (carbon) in the dissociation of the sample molecules. If atomization emanates from the metal, it can be classified either as desorption or volatilization. 

The space required for the analyte is less than 10 mm. This area is significantly less than the surface area of the graphite tube. If the analyte element is distributed evenly over the area wetted by the sample solution, then monoatomic or monomolecular layers should exist. Although small heaps of atoms or molecules (crystals) are formed, there is always good contact of the analyte with the tube surface. However, this may not be true when solid samples or highly concentrated salt solutions are introduced into the cuvette. 

Electrolyte and trace element determinations in the urine were varried out by flame atom absorption spectroscopy. Only by this method and by using a long suction time of 6-8 ml is it possible to determine the very low copper concentrations. This allows calculation of mean values with low mean standard deviations (RSD) from numerous single values of the digital multimeter. An attenuation of at least 3 seconds is recommended, as copper concentrations in the urine are slightly above the sensitivity limit of flame spectroscopy. Under these technical conditions it is advantageous to use the flame rather than the graphite tube cuvette. 

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