The hydride generation method

A hydride generation system in its simplest form (Fig. 2.5) consists of the hydride generator, in which the conversion to the volatile hydrides 

Schematic representation of hydride generation systems for the identification and determination of hydride-forming trace elements and trace element compounds with element-specific and general detectors. 

Common gas chromatographic detectors that are not element- or metal-specific, atomic absorption and atomic emission detectors that are element-specific, and mass spectrometric detectors have all been used with the hydride systems. Flame atomic absorption and emission spectrometers do not have sufficiently low detection limits to be useful for trace element work. Atomic fluorescence [37] and molecular flame emission [38-40] were used by a few investigators only. The most frequently employed detectors are based on microwave-induced plasma emission, helium glow discharges, and quartz tube atomizers with atomic absorption spectrometers. A review of such systems as applied to the determination of arsenic, associated with an extensive bibliography, is available in the literature [36]. In addition, a continuous hydride generation system was coupled to a direct-current plasma emission spectrometer for the determination of arsenite, arsenate, and total arsenic in water and tuna fish samples [41]. 

Additional features can be added to the basic hydride generation system to make it more versatile. When more than one hydride-forming 

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It should be noted that the hydride generation method may also be applied to the determination of other elements forming volatile covalent hydrides that are easily thermally dissociated. Thus, the hydride generation method has also been used for the determination of lead, bismuth, tin, and germanium. 

The use of the hydride generation method (Section 21.6) is far more sensitive for the determination of the listed elements. 

In summary, the hydride generation method cannot adequately differentiate between aquated SnIV and Sn11, which may coexist in certain, especially anaerobic, environments found in marine waters. Inorganic tin, speciated as tin (IV) , should probably be regarded as total reducible inorganic tin until more discriminatory techniques become available [578,580]. 

Cutter [18] has studied the application of the hydride generation method to the determination of selenium in saline waters. 

Arsenic and selenium can be determined using the hydride generation method. These metals in HC1 medium can be converted to their hydrides by... 

The ability to monitor trace levels of a number of heavy metals in a variety of samples is an important feature of modern environmental chemistry. Hence, sensitive analytical methods are required. When faced with the task of analyzing very low concentrations of antimony, bismuth and tin the hydride generation method is the first choice because of the improved sensitivity and lower detection limits as compared to many other techniques. The hydride generation technique includes the use of a reductant, such as a NaBH4 solution, to separate the volatile metal hydrides from the sample solution and the subsequent determination with atomic absorption after decomposition of the hydrides in a heated quartz cell. 

Acidification of water samples to a pH of 1.5 is recommended to preserve selenium compounds (Munoz Olivas et al. 1994). Nitric acid can be used, although it interferes with the hydride generation method of... 

Two modes of operation can be applied for the hydride generation technique (i) In the normal batch system, the whole sample is reduced in a hydride generator and the hydride formed transported in a carrier gas stream to an absorption tube (ii) In the flow injection (FIA) technique all stages of the hydride generation method take place in a fully automated closed system. The FIA system is discussed in section 6.3. 

Spectral Interferences. The separation of the analyte from the matrix is a major advantage of the hydride generation method. The analyte passes into the atomizer as a gaseous hydride, while concomitants normally remain in the reaction vessel. Spectral interferences can virtually be excluded since a relatively small number of components are present in the atomizer. 

Arsenic is both toxic and cai cinogenic element. It is necessary to have a fast, reliable and accurate method for determination of ai senic in water. The hydride-generation atomic fluorescence spectrometry (HG AFS) is one of the simple and sensitive techniques for the determination of this element in various types of waters. 

This apparatus may also be adapted for what are termed hydride generation methods (which are strictly speaking flame-assisted methods). Elements such as arsenic, antimony, and selenium are difficult to analyse by flame A AS because it is difficult to reduce compounds of these elements (especially those in the higher oxidation states) to the gaseous atomic state. 

Negative interferences by transition metal cations such as nickel and copper and nitrite were observed. However, these interferences have also been reported for the hydride generation atomic absorption method, and are due to... 

A standard UK hydride generation method [171] has been applied to the determination of selenium and arsenic in sludges and soils. 

In this method approximately 19 samples of marine sediment were oven dried at 110°C then digested with nitric acid-perchloric acid and hydrofluoric acid-hydrochloric acid. The digested solution is made up to 50ml of an equal volume mixture of 6M hydrochloric acid and 2M nitric acid. 0.1ml or less of the digest was pipetted into the hydride generator, followed by 1ml 2M acetic acid, diluted to 100ml with double distilled water and reacted with sodium borohydride. 

The hydride generation technique is a technique in which volatile metal hydrides are formed by chemical reaction of the analyte solutions with sodium borohydride. The hydrides are guided to the path of the light, heated to relatively low temperatures, and atomized. It is useful because it provides an improved method for arsenic, bismuth, germanium, lead, antimony, selenium, tin, and tellurium. 

ASTM D-4606. Standard Test Method for Determination of Arsenic and Selenium in Coal by the Hydride Generation/Atomic Absorption Method. 

The recommended procedure for the determination of arsenic and antimony involves the addition of 1 g of potassium iodide and 1 g of ascorbic acid to a sample of 20 ml of concentrated hydrochloric acid. This solution should be kept at room temperature for at least five hours before initiation of the programmed MH 5-1 hydride generation system, i.e., before addition of ice-cold 10% sodium borohydride and 5% sodium hydroxide. In the hydride generation technique the evolved metal hydrides are decomposed in a heated quartz cell prior to determination by atomic absorption spectrometry. The hydride method offers improved sensitivity and lower detection limits compared to graphite furnace atomic absorption spectrometry. However, the most important advantage of hydride-generating techniques is the prevention of matrix interference, which is usually very important in the 200 nm area. 

Azad et al. [ 186] used a similar technique for the determination of selenium in soil extracts using a nondispersive spectrometer, with which it was possible to observe fluorescence from the 196.1, 214.3 and 204.0 lines simultaneously, thus enabling a detection limit of 10 ng/ml to be observed using discrete sample introduction via the hydride generation technique. In this method, soil... 

Hydride generation AAS is a well-established technique for the determination of selenium because of its selectivity and sensitivity. The detection limits are improved by concentrating the hydride prior to the transfer to the spectrometer. Cryogenic condensation in a U-tube at liquid nitrogen temperature is a preferred method of pre-concentration. However, selenium hydride is formed essentially only from Se. The selectivity for Se has been combined with various chemical preparation steps to determine the sum of SeIV and Se and the total selenium content of samples. Se concentrations are measured by the difference between the (Se + Se ) and Se contents. The difference between total selenium and (Se + Se ) contents represents the concentrations of Se° and Se2- species. These methods have been used for determinations of selenium species in natural waters. Soluble organic matter in some groundwater samples interferes with the hydride generation determination of selenium. Isolation of humic substances by their adsorption on resins has overcome this problem. 

Dry ashing may also be applied to samples to be analyzed for the rather volatile elements As and Se. If suitable ashing aids, for example, MgO and/or Mg(NC>3)2, are used, no volatilization of these elements will occur. The increase in background caused by ashing aids is no problem when the detection is done by the hydride generation (HG) method. 

At the present time there are no ETA—AAS methods that can compete with the cold vapour technique for Hg or with hydride generation methods for Sb and Te. Another attractive method for Sb and Te is low pressure microwave induced plasma (MIP) emission spectroscopy [138]. Using low-temperature ashing and solvent extraction as preparation, physiological concentrations of both elements ([Pg.376]

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