Hydride-forming elements

Nakashima et al. [719] detail a procedure for preliminary concentration of 16 elements from coastal waters and deep seawater, based on their reductive precipitation by sodium tetrahydroborate, prior to determination by graphite-furnace AAS. Results obtained on two reference materials are tabulated. This was a simple, rapid, and accurate technique for determination of a wide range of trace elements, including hydride-forming elements such as arsenic, selenium, tin, bismuth, antimony, and tellurium. The advantages of this procedure over other methods are indicated. 

All four dissolution procedures studied were found to be suitable for arsenic determinations in biological marine samples, but only one (potassium hydroxide fusion) yielded accurate results for antimony in marine sediments and only two (sodium hydroxide fusion or a nitricperchloric-hydrofluoric acid digestion in sealed Teflon vessels) were appropriate for determination of selenium in marine sediments. Thus, the development of a single procedure for the simultaneous determination of arsenic, antimony and selenium (and perhaps other hydride-forming elements) in marine materials by hydride generation inductively coupled plasma atomic emission spectrometry requires careful consideration not only of the oxidation-reduction chemistry of these elements and its influence on the hydride generation process but also of the chemistry of dissolution of these elements. 

Results presented by Stockwell [9] for some of the hydride-forming elements and for mercury illustrate the enormous increase in sensitivity achieved with automated analytical chemistry methods (Table 5.2). Earher developments centred on the batch approach. These methods have recently been dropped (in favour of continuous-flow techniques) because they were not easy to use, were very dependent on operator abihty, and were difficult to automate. 

However, a commercial instrument must have a broad appeal and the chemistry regime based on acid/sodium tetrahydroborate offers the most attractive approach. It has a rapid reaction, caters for all of the hydride-forming elements and can be very easily automated. To optimize the procedures, the use of standard Technicon AutoAnalyzer methodologies, i.e. matching blanks, standards and sample matrices, overcomes the difficulties due to matrix interference. Also, the detection levels achieved by a well-designed system allow the samples to be diluted, avoiding any problems still present due to matrix interference. Stockwell [9] has described a system which achieves detection levels that are an order of magnitude better than other systems. 

The hterature is full of papers describing interference effects of elements on the hydride forming elements, but almost all of these interferences can be overcome by a simple dilution. However, a commercial design has to cope with a broad range of apphcations and is not necessarily optimized either for sample types or for particular elements. 

Introduction into a DC plasma requires rather more care and attention owing to its inherent design features. As the hydride is being introduced into the plasma, it is necessary to provide a controlled sheath of argon to contain the hydride and direct it into the plasma. This chimney effect significantly improves the sensitivity for hydride-forming elements. This interface has also formed the basis of an introduction system for mercury vapour into an atomic-fluorescence spectrometer as described by Godden and Stockwell [12]. 

Typical cold vapour generation AAS system used for mercury determination. The same system can be used with a flame in place of the Pyrex tube to allow the determination of hydride -forming elements. 

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-... 

Hydride forming elements, Se, As, Hg, have been determined by transfering the hydride evolved after chemical pretreatment of the sample into a flame or onto a carbon furnace. Several authors have reviewed the application of both technique to the analysis and speciation of Se, As, Hg, and Pb. Attempts at using hydride formation as a way of achieving speciation without prior fractionation of the species have not been very successful, because of the difference in behaviour between the organic and inorganic forms of these elements. Consequently, for the present, attempts to differentiate the various oxidation states and species of these elements require a chromatographic step or other form of pretreatment before detection. 

Haring et al. [31] determined arsenic and antimony by a combination of hydride generation and atomic absorption spectrometry. These workers found that, compared to the spectrophotometric technique, the atomic absorption spectrophotometric technique with a heated quartz cell suffered from interferences by other hydride-forming elements. 

For some elements such as Hg, atoms (atomic vapor) can also be obtained by chemical reaction. The ions in solution (Hg2+) can be efficiently reduced to Hg, which is a vapor at room temperature. Similarly, other elements with high electronegativities (i.e., electronegativities close to that of hydrogen) can be efficiently converted into vapors by a reduction reaction similar to that used for mercury. This is done with the so-called covalent hydride-forming elements (As, Bi, Pb, Sb, Se, Sn, and Te), which are converted into gaseous hydrides at room temperature. These hydrides are then heated... 

AFS is based on the absorption of radiation of a certain frequency (the energy transition from the outermost electronic orbitals to a higher energy state) and the subsequent deactivation of the excited atoms with the release of radiation. The most useful type of fluorescence, resonance fluorescence, involves a fluorescence emission radiation of the same wavelength as that used for excitation. Because of the inherent sensitivity of the fluorescence emission process, AFS is one of the most sensitive atomic techniques. All the benefits of AFS are enhanced when this spectromet-ric technique is used in combination with vapor generation methods, especially for covalent-hydride-forming elements. 

Chaudhry, M.M., A.M. Ure, B.G. Cooksey, D. Littlejohn, and D.J. Halls. 1991. Investigation of in situ concentration of hydride forming elements in a graphite furnace atomizer. Anal. Proc. 28 44-46. 

Hydride reagents (e.g., NaBH4) for hydride-forming elements and compounds... 

It should be pointed out that few elements are present in most natural waters at concentrations where flame spectroscopic techniques are directly applicable. Those that are include calcium, magnesium, sodium, potassium, and, in some samples and if conditions are very carefully optimized, manganese, iron, and aluminium. Zinc, and sometimes cadmium, may be determined directly by AFS. Mercury and hydride-forming elements may be determined if cold vapour and hydride generation sample introduction techniques are employed, as discussed in... 

It will be clear from the above examples that hydride-forming elements especially are often subjected to such speciation studies. There are, of course, good reasons for this. These are the very elements which tend to form toxic organometallic compounds, and they are elements which may be determined with excellent sensitivity. Moreover, interferences are not usually a problem following a separation process. 

Conventional FAAS is chracterized by poor detection power. Serious interferences from hydride-forming elements such as As, Sb, and Se are well known. Hydride generation techniques may circumvent these problems, providing an excellent tool to determine those elements at trace and ultratrace levels this is particularly useful for the determination of Se in milk samples [54-56]. Other... 

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. 

As shown above, one possible strategy to lower the absolute value of reaction enthalpies in lightweight hydrides is to look for elements, which exhibit negative heats of mixing with the hydride-forming elements/compounds and thus stabilize the dehydrogenated state. 

The ABj intermetallics (vide supra) have three types of interstices that may be available for occupation by H atoms. In all three, the H atoms would be T j coordinated. There are 12 sites per formula unit in which the coordination is to two A and two B atoms, four sites with coordination to one A and three B atoms, and one with coordination to four B atoms. Because the A atoms are normally the hydride-forming elements, the H atoms should prefer the (2 A + 2 B) sites, for this would give maximum bonding to the A atoms in the ZrB compounds (B = Cr, Mn, V) the (2 Zr + 2 B) sites are the first to be occupied " and at higher concentrations of hydrogen the (1 Zr + 3 B) sites also become occupied. In ZrVj the number of atoms in the (1 Zr + 3 V) sites exceeds that in the (2 Zr + 2 V) sites . However, V is also a hydride former , although it... 

The interstices occupied by hydrogen in the hydrides of the AB compounds are those in which the coordination is predominantly to hydride-forming elements. In ZrNiHj and ZrCoHj " the H atoms are in two sites, T sites coordinated by three Zr and one Ni atom, and five-coordinated sites of four Zr and one Ni. In the monohydride of TiFe, the coordination is distorted 0, to four Ti and two Ni. However, in all three cases, the H atoms are 9-25% closer to the non-hydride-forming element (Ni or Co) than to the hydride-forming element (Zr or Ti). In the dihydride of FeTi there are four types of distorted sites in three the coordination is similar to that in the monohydride. However, in the fourth site, the H atoms are coordinated to four Ni and two Ti. The last type of site is difficult to fill (because of weak H—Fe bonding) and explains why stoichiometric TiFeHj cannot be attained. In the monohydride of ZrNi, the H atoms are T j coordinated to four Zr atoms . 

Direct analysis of solids for selenium by XRF has a detection limit of —0.5 mgkg and so is often insufficiently sensitive. Rock, sediment, and soil samples can be dissolved using wet chemical methods (HF-HCl-etc.) followed by La(OH)3 co-precipitation to separate hydride-forming elements including selenium. This is present as Se(IV) following acid dissolution (Hall and Pelchat, 1997). The methods described above for aqueous samples can then be used. 

A study to determine the interference by other hydride-forming elements was undertaken. Therefore series of test solutions were prepared with a fixed amount of antimony, bismuth or tin and increasing concentrations of the other hydride-formers. The results for the three elements are summarized in Tab. 1. 

The interfering effects of these ions were established, using the same procedure as described for the hydride-forming elements. The results are given in Tab. 2. The suppressive effect of nickel during bismuth, antimony [S] and tin determinations can be completely eliminated up to a 10000 fold excess when 23.S mg 1,10 phenanthroline is added per 10 ml aliquot. 

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]. 

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