Vapor generation technique

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

The 71-page chapter by Ihnat (1982) on Application of Atomic Absorption Spectrometry to the Analysis of Foodstuffs, adheres strictly to the goal set out by editor Cantle of providing readers with a methods compendium. It concentrates on detailed recommended procedures for various food commodities, extracted from official , recommended and other sources judged reliable by the author, for the preparation (pretreatment and treatment, e.g., decomposition) of analytical samples and standards, and the determination of 21 elements by FAAS, including hydride and cold vapor generation techniques. Price and Whiteside... 

Vapor generation techniques The generation of gaseous analytes from the sample and their introduction into atomisation cells for subsequent absorption spectro-metric determination offers a number of advantages over the conventional sample introduction by pneumatic nebulisation of the sample solution. These include the elimination of the nebuliser, the enhancement of the transport efficiency, which approaches 100 %, and the presentation of a homogenous sample vapor to the atomiser. The most common and versatile techniques for the formation of volatile compounds are the hydride generation technique and the cold vapor technique. 

Only a few disadvantages are quoted for vapor generation techniques. These include the (chemical) interferences, notably in the presence of transition metals, a pronounced pH-effect on the reaction, and its dependence on the oxidation state of the element, which, however, can also be advantageously used for specia-tion analysis, and finally gas-phase atomisation interferences which may be caused by the presence of other volatile hydrides. 

Vapor-generation techniques comprise the following three steps (1) Transformation of the analyte into a volatile form, (2) its collection or preconcentration (if necessary) and transfer to the atomiser, and (3) decomposition of the volatile compounds to liberate the free analyte atom (not necessary for mercury) with subsequent measurement of the atomic absorption signal. 

Atomic Absorption Spectrometry (AAS) and Atomic Emission Spectrometry (AES) 451 Cold vapor generation technique... 

Once the amount of working fluid required is determined, the working fluid can be introduced into the heat pipe by an evacuation and backfilling technique, a liquid fill and vapor generation technique, a solid fill and subUmatimi... 

Applications of both vapor generation techniques have been widespread in that waters and effluents, metallurgical, clinical, biological, agricultural, geological, and environmental samples have all been analyzed at both the trace and ultratrace levels for these analytes. The reader is referred to the Further Reading section for an extensive compilation of specific applications. 

Currently, detection power is primarily hmited by reagent contamination. Progress in the widespread implementation of FI techniques, which feature online sample preparation and pretreatment capabilities as well as capabilities for rapid automation, should facilitate a further revolution in the use of vapor generation techniques in atomic spectroscopy. 

The specificity of the chemical vapor generation techniques for an inorganic, ionic form of the analyte can be used, if required, for speciation purposes. If conditions are chosen properly it may, for example, be possible to distinguish between the inorganic and organic constituents of an element in a sample. 

In the chemical vapor generation techniques, the analyte element passes into the atomizer as the gaseous hydride (HGAAS) or as the gaseous element (CVAAS), while concomitants normally remain in the reaction vessel. Consequently, due to the small number of components in the gas phase in the atomizer, spectral interferences can be virtually excluded. 

Several methods to generate controlled vapor concentrations that are currently in use are briefly described in this section. Each vapor generation technique serves a specific purpose. The most important difference among these techniques is the range of the vapor concentration generated. 

The atomic absorption spectrometer contrAA 300 from Analytik Jena AG (Jena, Germany) is the first commercially available instrument for HR-CS AAS, and it is closely related to the DEMON research spectrometers described in detail in Section 3.2.2. At the time of writing this book, the instrument was available only for flame atomization and chemical vapor generation techniques (refer to Figure 3.15). However, the manufacturer has already announced that a combined flame and graphite furnace version will be introduced in the near future. 

Guo X., Huang B., Sun Z., Ke R., Wang Q. and Gong Z. (2000) Preliminary study on a vapor generation technique for nickel without using carbon monoxide by inductively coupled plasma atomic emission spectrometry, Spearochim. Aaa, Part B 55 941-950. 

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