Graphite tube pretreatment

Arsenic, Hg, Sb, Se, and Sn were determined in untreated beer samples by direct HG-ET-AAS using a batch system [119]. Elements were preconcentrated in situ onto the Pd- (for As, Sb, Se, and Sn) or Au-pretreated (for Hg) interior wall surface of a graphite furnace. The authors considered beneficial to degas beer samples by filtration and to generate the hydrides in the presence of an antifoam agent. An extra gas-liquid separator was necessary to minimize the amount of moisture from the reaction vessel reaching the graphite tube. With the combination of HG and ET-AAS LoDs of 28, 90, 21, 10, and 50 ng l-1 were reached for As, Hg, Sb, Se, and Sn, respectively. 

In a number of cases the efficiency of the trapping can be increased still further by a pretreatment of the graphite tubes. Indeed Zhang et al. [162, 163] and Sturgeon et al. [164] have shown that Pd can be used for the efficient trapping of hydrides and they explained the mechanism of preconcentration on the basis of the catalytic reactivity of Pd, which promotes the decomposition of hydrides at relatively low temperatures (200-300 °C). Normally Pd(NC>3)2 is used for this purpose. After trapping the elements they can subsequently be released by heating up the furnace. 

This method was developed by Stoeppler et al. (1978). It has the advantage of a simple and fast sample pretreatment. Since lead is determined in an acidic aqueous solution obtained from blood by acid deproteinization, the matrix influences are relatively low and there are practically no salt residues left in the graphite tube. The method. 

In 1977 Dittrich and in 1978 Fuwa proposed, independently, that non-metals could be determined by molecular spectrometry at high temperatures with electrothermal vaporization (ETV-MAS). The method is relatively simple and allows determinations at sub p.p.m.-levels. Sample and reagent solutions are introduced into the graphite tube as a mixture or one after another. The reproducibility is usually better when the sample and reagent solutions are mixed together before introduction. However, if precipitation occurs by mixing the solutions, then they must be introduced separately into the atomizer. The sample is dried and ashed like in GF-AAS. After the thermal pretreatment steps, the conditions of the furnace are chosen so that... 

Electrothermal atomizers for AES share many of the advantages associated with their use for AAS. The argon inert gas that prevents oxidation of the graphite tube ensures that minimal quenching occurs, although a number of other interferences may occur. By and large, the interferences are very similar to those experienced in traditional electrothermal AAS, such as carbide formation (for some analytes), scatter by particulate matter, and losses during thermal pretreatment. A more comprehensive overview of interferences may be found in the sections on AAS. The use of matrix modifiers to assist in the separation of the analyte from the matrix is still often necessary. 

As was found in Ref. [13], the method of catalytic decomposition of acetylene on graphite-supported catalysts provides the formation of very long (50 fim) tubes. We also observed the formation of filaments up to 60 fim length on Fe- and Co-graphite. In all cases these long tubules were rather thick. The thickness varied from 40 to 100 nm. Note that the dispersion of metal particles varied in the same range. Some metal aggregates of around 500 nm in diameter were also found after the procedure of catalyst pretreatment (Fig. 2). Only a very small amount of thin (20-40 nm diameter) tubules was observed. 

The second analytical method uses a combustion system (O Neil et al. 1994) in place of reaction with BrF,. This method was used for the crocodiles because they were represented by very thin caps of enamel. The enamel was powdered and sieved (20 mg), pretreated in NaOCl to oxidize organic material and then precipitated as silver phosphate. Approximately 10-20 mg of silver phosphate were mixed with powdered graphite in quartz tubes, evacuated and sealed. Combustion at 1,200°C was followed by rapid cooling in water which prevents isotopic fractionation between the CO2 produced and the residual solid in the tube. Analyses of separate aliquots from the same sample typically showed precisions of 0.1%o to 0.4%o with 2 to 4 repetitive analyses even though yields are on the order of 25%. 

A dispersion of graphite in ceresin wax has been suggested as a way to prepare an electrode suitable for use in nonaqueous solvents. The hot paste is tamped into a Teflon tube and allowed to solidify. No pretreatment is necessary, and the surface can be renewed by wiping with cellulose tissue. 

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