Graphite, tube atomiser

Moffett [179] determined chromium in seawater by Zeeman corrected graphite tube atomisation atomic absorption spectrometry. The chromium is first complexed with a pentan-2,4 dione solution of ammonium 1 pyrrolidine carbodithioc acid, then this complex extracted from the water with a ketonic solvent such as methyl isobutyl ketone, 4-methylpentan-2-one or diisobutyl ketone. 

Moffett JH (1987) Varian Atomic Absorption No. AA69, Measurement of Varain Techitron Pty, Ltd., Mulgrave, Victoria, Australia. Chromium in environmental waters by Zeeman corrected graphite tube atomisation... 

GC-AAS has found late acceptance because of the relatively low sensitivity of the flame graphite furnaces have also been proposed as detectors. The quartz tube atomiser (QTA) [186], in particular the version heated with a hydrogen-oxygen flame (QF), is particularly effective [187] and is used nowadays almost exclusively for GC-AAS. The major problem associated with coupling of GC with AAS is the limited volume of measurement solution that can be injected on to the column (about 100 xL). Virtually no GC-AAS applications have been reported. As for GC-plasma source techniques for element-selective detection, GC-ICP-MS and GC-MIP-AES dominate for organometallic analysis and are complementary to PDA, FTIR and MS analysis for structural elucidation of unknowns. Only a few industrial laboratories are active in this field for the purpose of polymer/additive analysis. GC-AES is generally the most helpful for the identification of additives on the basis of elemental detection, but applications are limited mainly to tin compounds as PVC stabilisers. 

Bishop [75] determined barium in seawater by direct injection Zeeman-modulated graphite furnace atomic absorption spectrometry. The V203/Si modifier added to undiluted seawater samples promotes injection, sample drying, graphite tube life, and the elimination of most seawater components in a slow char at 1150-1200 °C. Atomisation is at 2600 °C. Detection is at 553.6 nm and calibration is by peak area. Sensitivity is 0.8 absorbance s/ng (Mo = 5.6 pg 0.0044 absorbance s) at an internal argon flow of 60 ml/min. The detection limit is 2.5 pg barium in a 25 ml sample or 0.5 pg using a 135 ml sample. Precision is 1.2% and accuracy is 23% for natural seawater (5.6-28 xg/l). The method works well in organic-rich seawater matrices and sediment porewaters. 

Halliday et al. [396] have described a simple rapid graphite furnace method for the determination of lead in amounts down to 1 xg/l in polluted seawater. The filtered seawater is diluted with an equal volume of deionised water, ammonium nitrate added as a matrix modifier, and aliquots of the solution injected into a tantalum-coated graphite tube in an HGA-2200 furnace atomiser. The method eliminates the interference normally attributable to the ions commonly present in seawater. The results obtained on samples from the Firth of Forth (Scotland, UK) were in good agreement with values determined by anodic stripping voltammetry. 

Molybdenum is an element for which platform atomisation does not offer an advantage. Just the opposite is the case sensitivity is very poor and memory effects are very strong. The Zeeman detection limit for wall atomisation in a pyrocoated graphite tube using 100 til of reference solution is 0.03 xl (for both peak height and peak area evaluation) [709]. 

Batley and Matousek [390,778] examined the electrodeposition of the irreversibly reduced metals cobalt, nickel, and chromium on graphite tubes for measurement by electrothermal atomisation. This method offered considerable potential for contamination-free preconcentration of heavy metals from seawater. Although only labile metal species will electrodeposit, it is likely that this fraction of the total metal could yet prove to be the most biologically important at the natural pH [779]. 

An electrothermal atomiser, where a drop of the liquid sample is placed in an electrically heated graphite tube which consists of a cylinder (3-5 cm in length and a few millimetres in diameter) with a tiny hole in the centre of the tube wall for sample introduction (see Figure 1.7a). Both ends of the... 

Figure 1.7 Electrothermal atomisation atomic absorption spectrometry, (a) Photograph of a graphite tube, (b) Photograph of a L vov platform, (c) Schematic front and side-on views of a graphite tube with a L vov platform.

Furnace atomisation plasma emission spectrometry (FAPES) this consists of an atmospheric pressure source combining a capacitively coupled radiofrequency helium plasma formed inside a graphite tube which contains an axial powered electrode. This miniplasma has rarely been used in analytical atomic spectrometry, probably because of the small number of users and a lack of information about its applications and capabilities [7]. 

Ross et al. [6] analysed samples of soil leachates from laboratory columns and of soil pore water from field porous cup lysimeters for aluminium by atomic absorption spectrometry under two sets of instrumental conditions. Method 1 employed uncoated graphite tubes and wall atomisation method 2 employed a graphite furnace with a pyrolytically coated platform and tubes. Aluminium standards were prepared and calibration curves used for the colorimetric quantification of aluminium. Method 1 gave results which compared favourably with method 2 in terms of both sensitivity and interference reduction for samples containing 1-15 uM aluminium. 

Electro-active labile metal contents have also been measured by using a combination of electro-deposition and analysis by graphite furnace AAS (Batley and Matousek, 1977). Metals (e.g. Pb, Co, Ni, Cr from seawater) are plated on to a short graphite tube by application of a suitable potential. At the end of the electrolysis period, the graphite cell (plus pre-concentrated metal) is placed in an electro-thermal atomiser attached to an AAS spectrometer, and the element content determined. 

When a graphite tube is used, different interference effects appear. These can be caused by the sample as well as by the specific instrument [24], Interferences caused by the sample occur when the element is vaporised as a molecule, the chemical compound hinders atomisation, or when after vaporisation of the matrix the sample has poor contact with the graphite tube. Also, unspecific absorption can occur by simultaneous vaporisation of the matrix. Here, molecular absorption or light loss by scattering from solid particles are the most common effect. 

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. 

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