Hydride and other volatile species generation

For elements with volatile hydrides or other volatile species, the sampling efficiency can be increased by volatilization of these species from the samples. 

This can be applied for the elements such as As, Se, Sb, Te, Bi. Sn as well as some others. Indeed, by in situ generation of the hydrides of these elements (AsH3, etc.) from the sample solutions the sampling efficiency can be increased from a few percent in the case of pneumatic nebulization to virtually 100%. 

In electrolytic hydride generation, a cell as shown schematically in Fig. 55 can be used [158]. It is made of PTFE rings and disks and can be easily disassembled for cleaning. The electrodes are platinum sheets with a surface of 10 cm2 each. The compartments of the anode (10 cm2) and the cathode (2 cm2) both have a solution inlet and outlet and are separated by a Nation membrane. The cell is held together with six screws. Solutions of the sample acidified with HC1 are used as the catho-lyte and dilute H2S04 as the anolyte. At a cell current of a few amperes, the solutions are continuously fed through the cell compartments, and on the cathodic 

In a number of cases the efficiency of the trapping can be increased still further by a pretreatment of the graphite tubes. Indeed Zhang et al. [162, 163] and Sturgeon et al. [164] have shown that Pd can be used for the efficient trapping of hydrides and they explained the mechanism of preconcentration on the basis of the catalytic reactivity of Pd, which promotes the decomposition of hydrides at relatively low temperatures (200-300 °C). Normally Pd(NC 3)2 is used for this purpose. After trapping the elements they can subsequently be released by heating up the furnace. 

This can be applied to many elements by appropriate choice of reactions. 

Modified flow-cell type hydride generator. (Reprinted with permission from Ref. [155].) 

Hydride generation can be performed efficiently by reduction with nascent hydrogen. This can be produced chemically by the reaction of zinc or NaBH4 with dilute acids. In the latter case, use can be made of a solid pellet of NaBH4 and placing the sample on it, which might be useful as a microtechnique (see, e.g., Ref. [173]). However, a flow of a solution of NaBH4 stabilized with NaOH can be joined by one of an acidifled sample. This can be done in a reaction vessel, as was first 

The procedure could be used for the determination of As in natural waters and in dissolved river sediments, provided that calibration by standard addition was applied. 

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For elements such as bromine, phosphorus, germanium, lead and others, reactions for the generation of hydrides or of similar volatile species can also be found. 

In the field of hydride generation, the use of membranes for gas-liquid separation has proved to be effective. An alternative to membranes are the pervaporation systems. Analytical pervaporation is defined as the integration of evaporation and gas diffusion into a single module. The volatile species present in a heated phase (donor) evaporate through a porous membrane and the vapour condenses on the surface of a cool stream (acceptor) on the other side of the membrane. The use of pervaporation devices may be more promising than membranes because the sample solution does not contact the membrane. This is possible because there exists an air volume separation between the sample and the membrane (Figure 5.14). 

The power of this MS technique has driven the development of methods to interface ICP/MS instruments with various sample introduction systems. Specialized sample introduction systems include ion chromatography (Seubert, 2001), gas chromatography (Vonderheide et al., 2002), and capillary electrophoresis (Costa-Fernandez et al., 2000). Other techniques are hydride generation (used to volatilize selected species and obtain some matrix/elemental separation) (Reyes et al., 2003) (Bings et al., 2002) laser ablation (Gonzalez et al., 2002 Heinrich et al., 2003 Russo et al., 2002), and electrothermal vaporization (Richardson, 2001 Vanhaecke and Moens, 1999). 

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