Cold 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... 

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

Two additional commercially available atomizers (really analysis techniques with unique atomizers) must be discussed, because they are extensively used in environmental and clinical analysis. They are the cold vapor-AAS technique (CVAAS) for determination of the element mercury, Hg, and the hydride generation technique (HGAAS) for several elements that form volatile hydrides, including As, Se, and Sb. These elements are toxic federal and state laws regulate their concentrations in drinking water, waste-water, and air, so their measurement at ppb concentrations is very important. Because... 

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. 

Hydride generation techniques are superior to direct solution analysis in several ways. However, the attraction offered by enhanced detection limits is offset by the relatively few elements to which the technique can be applied, potential interferences, as well as limitations imposed on the sample preparation procedures in that strict adherence to valence states and chemical form must be maintained. Cold-vapor generation of mercury currently provides the most desirable means of quantitation of this element, although detection limits lower than AAS can be achieved when it is coupled to other means of detection (e.g., nondispersive atomic fluorescence or micro-wave induced plasma atomic emission spectrometry). 

Cold vapor mercury Light absorbed by atoms of mercury generated by chemical reaction at room temperature is measured An excellent technique for mercury analysis... 

Spectrometric techniques based on atomic absorption or the emission of radiation flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma-optical emission spectrometry (ICP-OES), inductively coupled plasma-mass spectrometry (ICP-MS), and cold vapor (CV)/hydride generation (HG), mainly for trace and ultratrace metal determinations. 

Recommended conditions for flame and approximate values for ETA (graphite rod, etc.) atomizers are given in Table 2 for a number of elements important with regard to air pollution studies. Conditions are included in the table for the flame system used when hydrides of arsenic, antimony and selenium are generated and passed through the flame. Burrel [16] discusses generation of metal hydrides and cold-vapor mercury evolution techniques in great detail. 

Whatever instrument is used, provision must be made for using both air and nitrous oxide-supported flames. A fume exhaust must be provided. If arsenic, selenium, or mercury are to be determined, an apparatus for vapor generation should be used. Such apparatus is usually available from the instrument manufacturer. Mercury is usually determined by a flameless or cold vapor technique. 

Whatever the analytical method and the determinand may be, the greatest care should be devoted to the proper selection and use of internal standards, careful preparation of blanks and adequate calibration to avoid serious mistakes. Today the Antarctic investigator has access to a multitude of analytical techniques, the scope, detection power and robustness of which were simply unthinkable only two decades ago. For chemical elements they encompass Atomic Absorption Spectrometry (AAS) [with Flame (F) and Electrothermal Atomization (ETA) and Hydride or Cold Vapor (HG or CV) generation]. Atomic Emission Spectrometry (AES) [with Inductively Coupled Plasma (ICP), Spark (S), Flame (F) and Glow Discharge/Hollow Cathode (HC/GD) emission sources], Atomic Fluorescence Spectrometry (AFS) [with HC/GD, Electrodeless Discharge (ED) and Laser Excitation (LE) sources and with the possibility of resorting to the important Isotope... 

On-line solid-phase extraction (SPE) by ion-exchange and preconcentration using FIA techniques are becoming increasingly popular for trace analysis of heavy metals both by flame AAS and by hydride generation and cold vapor AAS45-49. 

Atomic absorption spectrometry, belonging to a class of techniques also defined as optical atomic spectrometry, has been for some four decades - and continues to be - one of the most important, dominant determinative techniques. It includes flame atomic absorption spectrometry (FAAS), electrothermal atomization atomic absorption spectrometry (ETAAS) (including graphite furnace AAS (GFAAS), carbon rod AAS, tantalum strip AAS), and gaseous generation (cold vapor AAS for Hg, hydride gener-... 

Chemical vapor generation is another important variant of AAS suitable for the determination of several elements forming elemental vapors (Hg) or volatile hydrides (As, Se, Bi, Sn, Ge, Te, Pd). The cold vapor technique generating the volatile element is almost exclusive to Hg, although there is one report of Cd. There is a voluminous literature on the determination of Hg by atomic absorption of Hg atoms in the gaseous phase beginning from the early days after development and continuing presently. 

Some interferences are removed by the hydride technique because the analyte is physically separated from the remainder of the matrix. But there are interferences in the process of generating the hydride and the variability in the rate of generation. There appears to be some black art in the quartz decomposition cell, and sometimes simple electrical furnaces are used to convert the hydride to the atomic vapor. In the opinion of this author, the furnace technique has fewer interference problems for the P block elements than the hydride technique but the furnace equipment is more expensive and, if the quantity of sample is not limited, the furnace is less sensitive. There is a great deal of historical and practical information on both the cold vapor method for Hg and the hydride method in the AAS book by Welz (1985), who is an experienced authority in these methods. 

The introduction of a gas phase sample into an atomizer has significant advantages over the introduction of solids or solutions. The transport efficiency may be close to 100%, compared to the 5-15% efficiency of a solution nebulizer. In addition, the gas phase sample is homogeneous, unlike many solids. There are two commercial analysis systems with unique atomizers that introduce gas phase sample into the atomizer. They are the cold vapor technique for mercury and the hydride generation technique. Both are used extensively in environmental and clinical chemistry laboratories. 

There are sample introduction systems that can handle slurries of particles suspended in hquids. Powders can be injected directly into the plasma for analysis. Lasers, sparks, and graphite furnaces (exactly the same as AAS graphite furnaces) are used to generate gaseous samples from sohds for introduction into the plasma. Hydride generation for As and Se and cold-vapor Hg introduction are used for ICP as for AAS these two techniques were discussed in Chapter 6. 

Fiydride generation (and cold-vapor) techniques significantly improve atomic absorption spectrometry (AAS) concentration detection limits while offering several advantages (1) separation of the analyte from the matrix is achieved which invariably leads to improved accuracy of determination (2) preconcentration is easily implemented (3) simple chemical speciation may be discerned in many cases and (4) the procedures are amenable to automation. Disadvantages with the approach that are frequently cited include interferences from concomitant elements (notably transition metals), pH effects, oxidation state influences (which may be advantageously used for speciation) and gas-phase atomization interferences (mutual effect from other hydrides). 

Hydride generation and cold-vapor techniques may be conveniently characterized by three steps (1) generation of the volatile analyte (2) its collection (if necessary) and transfer to the atomizer and (3) decomposition to the gaseous metal atoms (unnecessary for mercury) with measurement of the AA response. Each of these steps will be briefly reviewed prior to considering the analytical performance of these techniques. 

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