Heated Graphite Atomizers Atomic absorption spectrometry

Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

Lundgren, G., Lundmark, L., and Johansson, G. "Temperature Controlled Heating of the Graphite Tube Atomizer in Flameless Atomic Absorption Spectrometry . Anal. Chem. (1974), 46, 1028-1031. 

Nowka R, Muller H (1997) Direct analysis of solid samples by graphite furnace atomic absorption spectrometry with a transversely heated graphite atomizer and D2-background correction system (SS GF-AAS). Fresenius J Anal Chem 359 132-137. 

The recommended procedure for the determination of arsenic and antimony involves the addition of 1 g of potassium iodide and 1 g of ascorbic acid to a sample of 20 ml of concentrated hydrochloric acid. This solution should be kept at room temperature for at least five hours before initiation of the programmed MH 5-1 hydride generation system, i.e., before addition of ice-cold 10% sodium borohydride and 5% sodium hydroxide. In the hydride generation technique the evolved metal hydrides are decomposed in a heated quartz cell prior to determination by atomic absorption spectrometry. The hydride method offers improved sensitivity and lower detection limits compared to graphite furnace atomic absorption spectrometry. However, the most important advantage of hydride-generating techniques is the prevention of matrix interference, which is usually very important in the 200 nm area. 

Problems in the direct determination of cadmium in soil extracts by graphite furnace atomic absorption spectrometry are overcome by the use of a low atomisation temperature of 1200 °C (mini-furnace or high heating rate of > 2000 °C/s), the addition of molybdenum, hydrogen peroxide and nitric acid as a matrix modifier, and accurate optimisation of the instrumental parameters. 

Azzaria and Aftabi [ 149] showed that stepwise (as compared to continuous) heating of soil samples before determination of mercury by atomic absorption spectrometry gives increased resolution of the different phases of mercury. A gold-coated graphite furnace atomic absorption spectrometer has been used to determine mercury in soils [150]. 

Gawalko, E.J., Nowicki, T.W., Babb, J., Tkachuk, R., Wu, S. Comparison of closed-vessel and focused open-vessel microwave dissolution for determination of cadmium, copper, lead, and selenium in wheat, wheat products, com bran, and rice flour by transverse-heated graphite furnace atomic absorption spectrometry. J. AOAC Int. 80, 379-387 (1997)... 

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. 

The use of furnaces as atomizers for quantitative AAS goes back to the work of L vov and led to the breakthrough of atomic absorption spectrometry towards very low absolute detection limits. In electrothermal AAS graphite or metallic tube or cup furnaces are used, and through resistive heating temperatures are achieved at which samples can be completely atomized. For volatile elements this can be accomplished at temperatures of 1000 K whereas for more refractory elements the temperatures should be up to 3000 K. 

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].)...

Atomic absorption spectrometry (AAS) relies on the atomization of substances in an appropriate medium, like a flame, a plasma, or a graphite tube, and the capability of the free atoms to absorb light of a specific wavelength [89]. One of the usual techniques for the determination of trace elements is the heated graphite atomizer (HGA), which consists of a graphite tube connected as a resistor in a high electrical current circuit (Figure 6.22). 

Atomic Absorption Spectrometry (AAS) is a quantitative spectroscopic method based on the ability of free atoms, produced in an appropriate medium, like a flame, plasma, or a heated graphite tube, to absorb radiation of an atom-specific wavelengfh. 

Heated Graphite Atomizer (HGA) is a device used in atomic absorption spectrometry for atomization of compounds in graphite tube, which is connected as a resistor in a high electrical current circuit. 

Herzberg, J., et al., CARS Thermometry in a Transversely Heated Graphite Tube Atomizer Used in Atomic Absorption Spectrometry, Applied Physics, 61, 201, 1995. 

New flameless methods for sample presentation to the instrument, like the graphite rod and the. tantalum boat, are expanding the use and applications of atomic absorption spectrometry. In the first technique the burner is replaced by a graphite rod with a small well where a few microliters of sample are deposited and electrically heated by means of controlled power supply. Important advantages of this procedure are that a very small sample is needed and the dilution of viscous samples is not required. 

The introduction of atomic absorption spectrometry in 1955 by Walsh has brought about a preferred analytical technique among clinical chemists in the field of element determinations. Flame atomic absorption atomization techniques with solution aspiration is not sufficiently sensitive (detection limits varies from 0,05-3 mg Se/L for most clinical applications where sub-mg/L concentrations are encountered. The sensivity can, however, be improved by generation of volatile selenium hydride and subsequent atomization in argon-hydrogen flames or electrically heated quartz tubes. Electrothermal atomization of solutions in graphite tubes has developed rapidly since the analytical use was first proposed and studied by L vov (1961). For most clinical chemists this technique may be the most appropriate technique to analyze samples for low concentrations of selenium. 

Apostoli, P., Alessio, L, Dal Farra, M. and Fabbri, P.L (1988). Determination of vanadium in urine by electrothermal atomisation atomic absorption spectrometry with graphite tube pre-heating, J. Anal. Atomic Spectr., 2, 471. 

AAS is split into two types according to the method by which the sample is atomized. In flame atomic absorption spectrometry (FAAS) the sample is aspirated into a flame which is placed in the path ofthe light. In electrothermal atomic absorption (EAAS) the sample is placed in a graphite tube and heated in a brief pulse by passing an electric current through the tube. Generally, EAAS is more sensitive, giving better detection limits, but suffers from more matrix interference effects than FAAS. 

Electrothermal atomic absorption spectrometry A form of atomic absorption spectrometry in which an electrically heated graphite tube is used to excite the atoms in a sample. 

Atomic absorption spectrometry (AAS) was established as the most popular gas chromatography (GC) detection technique for lead speciation analysis in the first years of speciation studies. The increase of the residence time of the species in the flame using a ceramic tube inside the flame and, later, the use of electrically heated tubes, made out of graphite or quartz where electrothermal atomization was achieved, provided lower detection limits but still not sufficiently low. Later, the boom of plasma detectors, mainly microwave induced plasma atomic emission (MIP-AES) and, above all, inductively coupled plasma atomic emission and mass spectrometry (ICP-AES and ICP-MS, respectively) allowed the sensitivity requirements for reliable organolead speciation analysis in environmental and biological samples (typically subfemtogram levels) to be achieved. These sensitivity requirements makes speciation analysis of organolead compounds by molecular detection techniques such as electrospray mass spectrometry (ES-MS) a very difficult task and, therefore, the number of applications in the literature is very limited. 

Kumar, S.J., Meeravah, N.N., Arunachalam, J. (1998) Determination of trace impurities in high purity gaUium by inductively coupled plasma mass spectrometry and cross vaUdation of results by transverse heated graphite furnace atomic absorption spectrometry. Analytica Chimica Acta, 371,305-316. 

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