Hydride and cold-vapor techniques

For the determination of traces and ultratraces of Hg, As, Se, Te, As and Bi the formation of the volatile mercury vapor or of the volatile hydrides of the appropriate elements is often used, respectively. This allows a high sampling efficiency to be achieved and accordingly a high power of detection. The absorption measure- 

The cold-vapor technique for Hg allows detection limits of 1 ng to be obtained when using 50 mL of sample and they can be improved still further by trapping. With the hydride technique detection limits below the ng/mL level can be achieved for As, Se, Sb, Bi, Ge, Sn, etc. Accordingly, the levels required for analyses used to control the quality of drinking water can be reached. 

Hydride techniques, however, can suffer from many interferences (see Section 3.3). In AAS these interferences can not only occur as a result of influences on the hydride formation reaction but also as a result of influences of concomitants on the thermal dissociation of the hydride. Interferences from other volatile hydride forming elements can also occur [291]. Recently it has been found that still more elements can form volatile hydrides, as demonstrated e.g. by Cd. Here the hydride 

The cold-vapor technique for Hg allows detection limits of 1 ng to be obtained when using 50 mL of sample and they can be improved still further by trapping. 

For the determination of traces and ultratraces of Hg, As, Se, Te, As, and Bi, the formation of the volatile mercury vapor or of the volatile hydrides of the other respective elements is often used. This allows a high sampling efficiency to be achieved and accordingly a high power of detection. The absorption measurement is often performed in a quartz cuvette. Hg, for instance, can be reduced to the metal and then transported with a carrier gas into the cuvette. No heating is required for the absorption measurement. The other elements are reduced to the volatile hydrides, which are then transported with a carrier-gas flow (argon or nitro- 

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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. 

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... 

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. 

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. 

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... 

Another AA method applicable to volatile elements tmd compounds is the cold-vapor technique. Mercury is a volatile metal and can be determined by the method described in Feature 28-1. Other metals form volatile metal hydrides that can also be determined by the cold-vapor technique. 

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. 

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. 

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. 

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. 

Other techniques used in this context include graphite furnace analysis, hydride generation, and cold-vapor absorption. Other applications involve the indirect FIA-AAS determination of some drugs in pharmaceutical formulations based on a prior reaction or precipitation, or redox or complex formation. Examples are shown in Table 6. 

Most commonly used instruments use a flame (flame AAS (FAAS)) produced by combustion of an air/acetylene or dinitrogen oxide/acetylene mixture. The few interferences are easy to avoid, and the sensitivities that are reached are adequate for the metals of greatest interest to the food industry. Variants of this technique, such as the coupling of hydride generation (HG) systems (HG-AAS), increase its scope to higher-sensitivity determination of elements like selenium, arsenic, tin, and other elements that form hydrides. In a similar vein, the determination of mercury using the cold vapor technique should be highlighted. 

Atomic absorption spectrometry is commonly used to measure a wide range of elements as shown in Table 2. Such techniques as flame, graphite furnace, hydride generation, and cold vapor are employed. Measurements are made separately for each element of interest in turn to achieve a complete analysis these techniques are relatively slow to use. More sensitive, but also more expensive, multielement analytical techniques such as inductively coupled plasma-atomic emission spectrometry and inductively coupled plasma-mass spectrometry can be used if lower (pgl and below) detection limits are required. These detectors can also be coupled with separation systems if speciation data, e.g., Cr(III) and Cr(VI), are needed. 

These are among the most harmful pollutants in sewage. Essential elements (e.g., Fe) as well as toxic metals such as Cd, Hg, and Pb are included. Main sources of heavy metals are industrial wastes, mining, fuels, coal, metal plating, etc. Metal determinations in sewage are preferably carried out by atomic spectrometry (flame and electrothermal atomization), atomic emission spectrometry, inductively coupled plasma-mass spectrometry, stripping voltammetry, spectrophotometry, and kinetic methods. Hg is advantageously determined by the cold vapor technique and As by the hydride technique. 

Generally, the best detection limits are attained using ICP-MS or GFAA. For mercury and those elements that form hydrides, the cold vapor mercury or hydride generation techniques offer exceptional detection limits. Most manufacturers (e.g., Perkin-Elmer) define detection limits very conservatively with either a 95% or 98% confidence level, depending on established conventions for the analytic technique. This means that if a concentration at the detection limit were measured many times, it would be distinguished from a zero or baseline reading in 95% (or 98%) of the determinations. 

Solvent extraction, coprecipitation and ion-exchange techniques are the main concentration methods used for seawater analysis. Other interesting concentration techniques, such as electrodeposition, amalgam trap (for mercury), a cold trap-vaporization system for hydride generation, and recrystallization, are often used by marine and analytical chemists. The first three methods are briefly reviewed here. 

As more sophisticated metal hydrides are developed (nanocrystalline, multicomponent systems, composites and nanocomposites, graphite/metals or similar hybrid systems, clusters, etc.), it is important to be a vare that, for practical applications, a large volume of material should be processed in a fast, inexpensive and reliable vay, for example casting. Techniques such as cold vapor deposition may be impossible to scale up but this does not mean they should be discarded as a means of studying new metal hydrides. On the contrary, laboratory techniques allow much better control of the end product and permit the elaboration of new compounds. Once an attractive compound is found then another challenge w ill have to be faced scaling up the synthesis. In this respect, it is important for the community of metal hydrides researchers to also study large-scale production techniques in order to make the transition from laboratory to industrial scale easier. 

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