Heat graphite

Instead of employing the high temperature of a flame to bring about the production of atoms from the sample, it is possible in some cases to make use of either (a) non-flame methods involving the use of electrically heated graphite tubes or rods, or (b) vapour techniques. Procedures (a) and (b) both find applications in atomic absorption spectroscopy and in atomic fluorescence spectroscopy. 

Graphite bisulfates are formed by heating graphite with a mixture of sulfuric and nitric acids. In the reaction, the graphite planes are partially oxidized. There is approximately one positive charge for every 24 carbon atoms, and the HS04 anions are distributed between the planes, (a) What effect is this oxidation likely to have on the electrical conductivity (b) What effect would you expect it to have on the x-ray diffraction pattern observed for this material Refer to Major Technique 3 on x-ray diffraction, which follows this set of exercises. 

Evenson, M. A. and Anderson, C. T., Jr. Ultramlcro Analysis for Copper, Cadmium and Zinc In Human Liver Tissue by Use of Atomic Absorption Spectrophotometry and the Heated Graphite Tube Atomizer". Clin. Chem. (1975), 2, 537-543. 

Manning, D. C. and Fernandez, F. "Atomization for Atomic Absorption Using a Heated Graphite Tube". At. Absorp. 

Pekarek, R. S. and Hauer, E. C. "Direct Determination of Serum Chromium and Nickel by an Atomic Absorption Spectrophotometer with a Heated Graphite Furnace". Fed. Proc. 

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. 

Using a newly developed, transversely heated graphite atomizer and D2-back-ground correction (for details see Sections 2.2 and 4.3), Cd, Pb and Cr were determined in cement and river sediment samples. Of the various calibration approaches applied the best results, also in comparison with wet chemical procedures, were achieved with calibration curves constructed by means of different BCR CRMs with different analyte concentrations and usually n = to individual intakes (Nowka and Muller 1997). 

The sensitivity achieved should enable seawater samples to be analysed for molybdenum, because the concentration of molybdenum in seawater is usually 2.1 -18.8 pg/1. The selected temperature of 1700-1850 °C during the charring stage permits separation of the seawater matrix from the analyte prior to atomisation with the Perkin-Elmer Model 603 atomic absorption spectrometer equipped with a heated graphite atomiser (HGA-2100). 

Stein et al. [673] have described a simplified, sensitive, and rapid method for determining low concentrations of cadmium, lead, and chromium in estuarine waters. To minimise matrix interferences, nitric acid and ammonium nitrate are added for cadmium and lead only nitric acid is added for chromium. Then 10,20, or 50 pi of the sample or standard (the amount depending on the sensitivity required) is injected into a heated graphite atomiser, and specific atomic absorbance is measured. Analyte concentrations are calculated from calibration curves for standard solutions in demineralised water for chromium, or an artificial seawater medium for lead and cadmium. 

Heating graphite at the same time as compressing it under enormous pressure will yield diamond. The energy needed to convert 1 mol of graphite to diamond is 2.4 kJmol-1. We say the enthalpy of formation AHt for the diamond is +2.4 kJ mol-1 because graphite is the standard state of carbon. 

Chemistry (Brown et al. 1981). Direct aspiration into a flame and atomization in an electrically heated graphite furnace or carbon rod are the two variants of atomic absorption. The latter is sometimes referred to as electrothermal AAS. Typical detection limits for electrothermal AAS are <0.3 pg/L, while the limit for flame AAS and ICP-AES is 3. 0 pg/L (Stoeppler 1984). The precision of analytical techniques for elemental determinations in blood, muscles, and various biological materials has been investigated (Iyengar 1989). Good precision was obtained with flame AAS after preconcentration and separation, electrothermal AAS, and ICP-AES. 

CVD of SiC normally uses silane and a hydrocarbon as the precursor gases and a hydrogen carrier gas. The gases pass over a heated graphite susceptor that is coated by SiC or tantalum carbide (TaC). 

Several types of atom cell have been used for AAS. Of these, the most popular is still the flame, although a significant amount of analytical work is performed using various electrically heated graphite atomizers. This second type of atom cell is dealt with at length in Chapter 3, and the material here is confined to flames. 

In 1967, Massmann described a heated graphite furnace in which no auxiliary electrode was used, i.e. the graphite tube was both the resistance element and the furnace. The sample was micro-pipetted directly into a 55 mm long, 6.5 mm internal diameter, 1.5 mm wall thickness tube via a small 2 mm diameter orifice. The absorption tube device and a graphite... 

Temperature distribution in (a) longitudinally and (b) transversely heated graphite furnace. 

When heated, graphitic oxide decomp almost explosively (Ref 7)... 

Different mass spectrometric techniques can be classified according to the evaporation and ionization methods applied. Evaporation of solid samples can be performed, for example, by thermal (e.g., on a hot tantalum filament or in a heated graphite furnace) or laser-induced evaporation, and by electron or ion bombardment. Electron ionizaton (El), ionization during the sputtering process with a primary ion beam, resonant or non-resonant laser ionization or thermal surface ionization... 

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... 

Methylene and other species were detected by mass spectral analysis of the mixture obtained by heating graphite and hydrogen in a Knudsen cell.21... 

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