Electrothermal atomizers ashing

N1 and Zn from a graphite rod were significantly lower than from a tantalum filament, suggesting that these free metal atoms can be liberated by chemical reduction of their respective oxides, rather than by direct thermal dissociation. Findlay et al (19) emphasized the hazards of preatomlzatlon losses of trace met s In electrothermal atomic absorption spectrometry, when the ashing temperature Is permitted to exceed the minimum temperature for vaporization of the analyte. 

A typical measurement was performed as follows. The feeder was lowered into the crucible and the sample solution (seawater) was allowed to flow under an inert atmosphere with the suction on. A constant current was applied for a predetermined time. When the pre-electrolysis was over, the flow was changed from the sample to the ammonium acetate washing solution, while the deposited metals were maintained under cathodic protection. Ammonium acetate was selected for its low decomposition temperature, and a 0.2 ml 1 1 concentration was used to ensure sufficient conductivity. At this point the feeder tip was raised to the highest position and the usual steps for an electrothermal atomic absorption spectrometry measurement were followed drying for 30 s at 900 C, ashing for 30 s at 700 °C, and atomization for 8 s at 1700 °C, with measurement at 283.3 nm. The baseline increases smoothly with time as a consequence of an upward lift of the crucible caused by thermal expansion of the material. 

G. Cobo, M. Gomez, C. Camara, M.A. Palacios, Determination of fluoride in complex liquid matrices by electrothermal atomic absorption spectrometry with in-furnace oxygen-assisted ashing, Mikrochim. Acta 110 (1993) 103-110. 

Electrothermal atomization is particularly useful when the amount of sample is very small, when very low levels of detection are required and when the matrix is dilute or volatile. These criteria often apply to clinical samples (a pin-prick sample of blood produces only 50-100 mm of whole blood, but this is sufficient for analysis using an electrothermal atomizer, hence it is not essential for an intravenous sample to be taken). For such samples, often pretreatment is not required, and body fluids and biological tissues can be ashed in situ in the furnace. This also applies to some foods, although others may need some preliminary wet ashing. 

J. Moreda-Pineiro, P. Fopez-Mahia, S. Muniategui-Forenzo, E. Fernandez-Fernandez and D. Prada-Rodriguez, Direct As, Bi, Ge, Hg and Se(IV) cold vapor/hydride generation from coal fly ash slurry samples and determination by electrothermal atomic absorption spectrometry, Spectrochim. Acta, Part B, 57(5), 2002, 883-895. 

A procedure for the determination of molybdenum in serum, red blood cells, and urine is described. The low concentration of molybdenum in most unexposed individuals requires the sensitivity obtained using atomic absorption spectrophotometry and electrothermal atomization. Spike recovery tests indicate that low temperature ashing is required for accuracy. Severe matrix interferences preclude wet ashing or high-temperature ashing as sample pretreatments. Using the method described, it is possible to distinguish between industrially exposed and unexposed individuals. 

Kumpulainen, J., Lehto, J., Koivistoinen, P, Uusitupa, M. and Vuori, E. (1983). Determination of chromium in human milk, serum and urine by electrothermal atomic absorption spectrometry without preliminary ashing. Sci. Tot. Environ., 31, 71. 

FIGURE 9-7 Typical output for the determination of iead from a spectrophotometer equipped with an electrothermal atomizer. The sample was 2 pL of canned orange juice. The times for drying and ashing are 20 and 60 s. respectively. (Courtesy of Varlan Instrument Division, Palo Alto. CA.)... 

Electrothermal atomic absorption spectrometry is certainly a method for the determination of arsenic at trace concentration levels. Nevertheless, it has to be stated that this technique must be used carefully as arsenic might be lost during ashing and matrix interferences may occur. An ideal heating program cannot be given without knowledge of the sample composition. 

In electrothermal atomizers, a tew microliters of sample is first evaporated at a low temperature and then ashed at a somewhat higher temperature in an electrically heated graphite tube similar to the one in Figure 9-6 or in a graphite cup. After ashing, the current is rapidly increased to several hundred amperes, which causes the temperature to rise to 2000°C to 3000 C atomization of the sample occurs in a period of a few milliseconds to seconds. The absorption or fluorescence of the atomic vapor is then measured in the region immediately above the heated surface. 

One of the advantages of electrothermal atomization is that some materials can be atomized directly, thus avoiding the solution step. For example, liquid samples such as blood, petroleum products, and organic solvents can be pipetted directly into the furnace for ashing and alomization. Solid samples, such as plant leaves, animal tissues, and some inorganic substances. can be weighed directly into cup-type atomizers or into tantalum boats for introduction into tube-type furnaces. Calibration is. however, usually difRcult and requires standards that approximate the sample in composition. 

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