Wolfgang Wendl

Chemical reactions in the graphite tube for some carbide and oxide forming elements

Abstract The chemical reactions of V, Cr, Mo, Sn, Ge and Si in the graphite furnace are briefly summarized. A reaction diagram is presented which aids in the interpretation of the influence of the thermal program prior to atomization and of interfering elements on the absorption signal. It was found that the elements V, Cr and Mo form stable carbides in the graphite tube, which decompose into the elements during the atomization cycle. The atomic vapor of Si, Sn and Ge is produced mainly by reduction of their oxides to the metal. Losses of volatile suboxides of Sn and Ge and of nonatomizable SiC reduce the absorption signal of these elements.

Chemical reactions of lead in graphite furnace atomic absorption spectrometry

The chemical reactions of lead in graphite furnace atomic absorption spectrometry (AAS) are determined experimentally by absorbance measurements, X-ray diffraction and molecular absorption. Lead nitrate is decomposed into lead oxide during the drying cycle, which is reduced to metallic lead by the carbon of the tube.Interference from chloride ions is due to the formation of volatile lead chloride. The formation of this compound can be prevented by the addition of the matrix modifiers ammonium dihydrogenphosphate and ammonium dichromate. These modifiers, which must be applied in great excess, form stable lead phosphates and chromates. The influence of the palladium nitrate modifier on the reactions of lead is also discussed.

Chemical reactions of chromium and vanadium in graphite furnace atomic absorption spectrometry

Abstract The chemical reactions of V and Cr in the graphite furnace are determined by absorption measurements, X-ray diffraction and electron microscopy techniques. It is shown that both elements form carbides, VC and Cr 3 C 2 , respectively. An increase in absorption is obtained in tubes coated with pyrolytic graphite. Interferences of carbideforming elements on the absorption signal are studied. Reaction paths are proposed for both elements during their heat treatment in graphite furnace AAS.

Role of oxygen in the determination of oxide-forming elements by electrothermal atomic absorption spectrometry. Part 1. Effect of oxygen on the reactions of thallium

To study the effect of oxygen on the chemical reactions of oxide-forming elements in a graphite furnace a method was developed that allows a separation between the oxygen treatment and the subsequent measurement. This method demonstrated that the effects caused by oxygen are due to a change in the surface properties of the furnace. Thallium forms residues in untreated and oxygen-treated furnaces after the drying step as Tl2O3 on the surface. In untreated furnaces the reduction to the atomic state occurs via the volatile suboxide Tl2O, which is the reason for losses of atomizable material. Furnaces treated with oxygen avoid the formation of the volatile suboxide owing to the chemically modified graphite surface and enhance the absorbance considerably. The modification of the graphite surface is caused by chemisorption of oxygen on active sites of the graphite which are destroyed in a high-temperature step. Hence, the reduction process is shifted to higher temperatures with lower losses of atomizable material. Details of a pre-treatment that avoids the losses of material without the use of oxygen are also given.

Chemical reactions in the determination of germanium by graphite furnace atomic absorption spectrometry

Abstract The chemical reactions of Ge in the graphite furnace are determined by atomic absorption measurements, X-ray diffraction, electron microscopy and molecular absorption. The sodium germanate formed after the drying cycle is reduced by the carbon of the tube to elemental Ge. Volatile GeO is formed during this reduction process at temperatures higher than 1100 K leads to losses of atomizable Ge. Excess of NaOH enhances the absorbance value of Ge by a factor of two. The reason for this effect is an additional reduction process of GeO to Ge by metallic sodium at temperatures higher than 1500 K. Impregnation of the tube surfaces by carbide forming elements also leads to an enhancement of absorbance.

Role of oxygen in the determination of oxide forming elements by electrothermal atomic absorption spectrometry. Part 2. Effect of oxygen on the reactions of thallium, bismuth and lead in uncoated furnaces, pyrolytic coated furnaces and on platforms

The chemical processes occurring in the graphite furnaces of atomic absorption spectrometers were studied when the oxide forming elements, Tl, Bi and Pb were being determined. Furnaces fabricated from uncoated electrographite, pyrolytic graphite coated graphite furnaces, uncoated furnace coated platforms and uncoated furnace uncoated platforms were tested. Pyrolysis curves were measured in all the furnace types, with flowing argon and under gas stop. The reactions occurring for Tl and Bi are reduction reactions via volatile suboxides to the elements, whereas Pb is directly reduced to the metal. The volatile suboxides are responsible for material losses during pretreatment. Under gas stop conditions the losses may be reduced by further reduction of the suboxides by additional collisions with the graphite surface. Separate atomization from the furnace wall and from the platform after thermal pretreatment gave an indication of the location and the kinetics of the different reaction steps. The oxygen conditioning of the uncoated furnaces and the uncoated platforms resulted in the thermal stabilization of the elements under study. This can be explained by the intercalation of the metals into the graphite layers.

Role of oxygen in the determination of oxide forming elements by electrothermal atomic absorption spectrometry Part 3. Effect of oxygen on the reactions of tin in uncoated, pyrolytically coated and zirconium carbide coated graphite tube atomizers

Abstract The chemical reactions of tin have been investigated in uncoated, pyrolytic graphite coated, oxygen treated and zirconium carbide coated graphite tube atomizers. In all types of tubes SnO2 is reduced to the volatile tin monoxide, which is stable up to 1500 K. Above this temperature it is further reduced to the metal by collision with the atomizer surface. The rate of losses during thermal pretreatment is highest in coated tubes. SnO is formed in uncoated tubes also, but at a lower rate. The pyrolysis curve exhibits a significant plateau between 1500 and 1800 K. It is proposed that tin atoms are intercalated into the graphite lattice in that temperature range. Treatment of the tubes with oxygen does not principally change the processes, but reduces the rate of losses. Repeated coating of the tubes with zirconium carbide shifts the onset of losses to higher temperatures and reduces the rate of losses further. In addition, no plateau is observed as in the case of uncoated tubes.

Stabilization of the oxide forming elements, lead, thallium and tin, in graphite tubes for graphite furnace atomic absorption spectrometry by intercalation☆

Abstract Oxide forming elements such as Tl, Pb, or Sn show a stabilization to higher pyrolysis temperatures and a well known pulse shift in the atomization signal when the graphite tube is treated with oxygen. The same stabilization can be obtained by pretreatment under gas stop in uncoated tubes and to a lesser extent also in pyrocoated tubes due to a complete reduction to the metal. Intercalation of the metal atoms into the graphite layers is proposed as the mechanism for this stabilization. A method is described which makes it possible to distinguish between intercalated and surfacedeposited metal atoms by reoxidation.

Preflight Diffusion Experiments on Liquid Metals Under 1g Conditions for the FOTON-M2 Mission

Abstract:  We measured diffusion of up to 5% Ag in liquid Pb, using the Foton shear cell (designed and used for the FOTON‐M2 mission). Since the density change of the alloy with composition has been reported to be very small but is not reliably known, we performed two arrangements of layering: (a) with PbAg on top and Pb below and (b) with Pb on top and PbAg below. The Berlin group did interdiffusion experiments from a “thick layer” of PbAg into “semi‐infinite” Pb and the Karlsruhe group did the typical “interdiffusion” experiments with a diffusion couple of PbAg versus Pb. The results of both groups were equivalent, showing much lower D values for arrangement (b) than for (a). The lower values were even definitely lower than reference microgravity values from MIR/MIM and could be fitted by D=ATn with n about 2.2. We discuss the stability/instability by density layering depending on the arrangement.

Chemical reactions of tellurium in graphite tubes of atomic absorption spectrometry

Abstract The chemical reactions of tellurium were investigated in uncoated, pyrolytic graphite-coated, and zirconium-treated polycrystalline electrographite tubes. After drying, tellurium is present as TeO2, which is reduced by the graphite to the volatile suboxide TeO. This molecule was detected in the gas phase by molecular absorption spectroscopy. In coated tubes, this reduction process occurs at lower temperatures compared to uncoated and Zr-treated tubes, and shows a faster reaction rate. The suboxide is reduced to elemental Te on collision with the graphite tube wall. This transport process via the gas phase was observed by vaporization from L’vov platforms and subsequent separate atomization from wall and platform. In uncoated tubes, tellurium could be stabilized to temperatures above 1200°C due to intercalation, which was proven by re-oxidation experiments.