Volatile hydride-forming element

There is great interest in the volatile hydride forming elements as mineral pathfinders , as pollutants and, for selenium at least, as an essential dietary element. There are no existing alternative methods for the elements which are both accurate and cost-effective and there is little doubt that hydride methods will be used extensively in the near future. 

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... 

Hydride generation technique. Hydride generation is an analytical technique to separate volatile hydride-forming elements from the main sample matrix before their introduction into the light path of the instrument, and to convert them into an atomic vapour once they are there. 

Table 32 Some detection limits reported in the literature for volatile hydride-forming elements by continuous hydride generation technique and solution nebulization...

The MIP is also useful for the excitation of volatile hydride-forming elements after stripping the hydrides of the excess of hydrogen. With different trapping techniques, detection limits down to the sub-ng level can easily be obtained [177]. A further usefid type of MIP is the so[Pg.261]

The above requirements limit the number of species that can be fractionated by this technique. However, hydride forming elements and those that can be converted into volatile alkyl derivatives can be used. Lead tin arsenic mer-... 

Chemical vaporisation. Some elements (such as arsenic, bismuth, tin and selenium) are difficult to reduce in a flame when they are in higher oxidation states. For these atoms, the sample is reacted with a reducing agent prior to analysis (sodium borohydride or tin chloride in acidic media) in a separate vessel. The volatile hydride formed is carried by a make-up gas into a quartz cell placed in the flame (Fig. 14.10). 

The species most often determined following hydride generation include Se, As, Sb, Bi, Pb, Te, Sn and Ge [1-3]. Although Hg is regarded as a hydride-forming element, some authors have expressed doubts about the nature of the resulting volatile species and... 

Shortening the collection time is recommended when other hydride-forming elements present in the sample react more slowly than the analyte forming the volatile species. This fixed-time kinetic approach is less sensitive than its steady-state counterpart but provides better selectivity and shorter analysis times [32,33]. 

The gas phase interferences are obviously limited to those species which can be transferred into the vapor phase under these conditions, i.e., the hydride-forming elements and mercury. And if tin(II) chloride is used for the determination of mercury, no other element is volatilized so that no such interferences can occur. The mumal interferences of hydride-forming elements, however, may be quite severe, particularly in batch systems [36]. FI systems have been shown one more time to be superior also in this case as these mutual interferences are one to two orders of magnitude less pronounced compared to batch systems [37]. They should therefore be no problem in the analysis of body fluids or tissues, even in the case of a severe intoxication. 

For some elements, sample introduction in the gaseous form presents an interesting alternative. This approach permits the separation of the element of interest from the concomitant matrix and provides (almost) quantitative analyte introduction. This approach has been used, for example, for Hg (reduction of Hg + to atomic Hg using SnCl2) [36], Os (oxidation to OSO4) [37-39], and hydride-forming elements, such as Se [40-42] (conversion into volatile hydrides using NaBHJ. 

In addition to the above widely used atomization techniques, mention must also be made of the hydride generation technique . The method is based on conversion of a hydride forming element (As, Bi, Ge, Sb, Se, Sn, Te) in an acidified sample to volatile hydride and transport of the released hydride to an atomizer (hydrogen diffuse flame, graphite furnace, heated quartz tube) where they are atomized to give free analyte atoms. Sodium borohydride is almost exclusively used as an agent for conversion of analyte to hydride ... 

Miscellaneous Atomization Methods A few elements may be atomized by a chemical reaction that produces a volatile product. Elements such as As, Se, Sb, Bi, Ge, Sn, Te, and Pb form volatile hydrides when reacted with NaBH4 in acid. An inert gas carries the volatile hydrides to either a flame or to a heated quartz observation tube situated in the optical path. Mercury is determined by the cold-vapor method in which it is reduced to elemental mercury with SnCb- The volatile Hg is carried by an inert gas to an unheated observation tube situated in the instrument s optical path. 

A number of elements form volatile hydrides, as shown in the table. Some elements form very unstable hydrides, and these have too transient an existence to exist long enough for analysis. Many elements do not form stable hydrides or do not form them at all. Some elements, such as sodium or calcium, form stable but very nonvolatile solid hydrides. The volatile hydrides listed in the table are gaseous and sufficiently stable to allow analysis, particularly as the hydrides are swept into the plasma flame within a few seconds of being produced. In the flame, the hydrides are decomposed into ions of their constituent elements. 

The elements listed in the table of Figure 15.2 are of importance as environmental contaminants, and their analysis in soils, water, seawater, foodstuffs and for forensic purposes is performed routinely. For these reasons, methods have been sought to analyze samples of these elements quickly and easily without significant prepreparation. One way to unlock these elements from their compounds or salts, in which form they are usually found, is to reduce them to their volatile hydrides through the use of acid and sodium tetrahydroborate (sodium borohydride), as shown in Equation 15.1 for sodium arsenite. 

Schematic representation of an AFS detection system for the determination of elements in the form of volatile hydrides (PMT = photo-multiplier tube). (With permission of PS Analytical Ltd)...

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