Hydride generator system, analytical

Hydride generation for analytical use was introduced at the end of the 1960s using arsine formation (Marshal Reaction) in flame atomic absorption spectrometry (FAAS). A simple experimental setup for a hydride generator is shown in Figure 5.18. Today, hydride generation,91,92 which is the most widely utilized gas phase sample introduction system in ICP-MS, has been developed into... 

Like most other FI analytical processes, the separations are also almost always performed under non-equiiibrated conditions, and the phase transfer factors P are rarely higher than 0.3. usuall> being in the range 0.05-0.2. While this sometimes may have some unfavourable effects on sensitivity, they may be compensated for whenever necessary, by preconcentration measures during the gas-liquid separation. On the other hand, the non-equilibrium conditions may be exploited favourably to improve selectivity through kinetic discrimination (cf. Sec. 5.5.1 Tolerance of interferences in FI hydride generation systems). 

The smaller sample volume introduces a smaller absolute amount of interferent into the hydride generation system. Since the absolute amount of interferent has often been shown to be more important than its relative concentration in competing with the analyte hydride for free radicals in the atomization process, the beneficial effiect of the smaller sample volume in FI hydride generation AAS is obvious. 

Initially hydride generation and cold vapour techniques were developed for the quantitative determination of the hydride-forming elements and mercury by atomic absorption spectrometry (Chapters, Sections 6.2 and 6.3), but nowadays these methods are also widely used in plasma atomic emission spectrometry. In the hydride generation technique, hydride-forming elements are more efficiently transported to the plasma than by conventional solution nebulization, and the production and excitation of free atoms and ions in the hot plasma is therefore more efficient. Spectral interferences are also reduced when the analyte is separated from the elements in the sample matrix. Both continuous (FIA) and batch approaches have been used for hydride generation. The continuous method is more frequently used in plasma AES than in AAS. Commercial hydride generation systems are available for various plasma spectrometers. 

Automating the sodium tetrahydroborate system based on continuous flow principles represents the most reliable approach in the design of commercial instrumentation. Thompson and co-workers [9] described a simple system for multi-element analysis using an ICP spectrometer, based on the sodium tetrahydroborate approach. PS Analytical Ltd developed a reliable and robust commercial analytical hydride generator system, along similar lines, but using different pumping principles from those discussed by Pahlavanpour and co-workers [9]. 

Analytical evaluation of an integrated ultrasonic nebuliser-hydride generator system for simultaneous determination of hydride and nonhydride forming elements by microwave induced plasma spectrometry. 

H. Matusiewicz and M. Slachcinski, Analytical evaluation of an integrated ultrasonic nebuliser-hydride generator system for simultaneous determination of hydride and non-hydride forming elements by microwave induced plasma spectrometry, Spectrosc. Lett., 2010, 43(6), 474M85. 

Table 5.3 sets out the advantages and disadvantages of the batch and continuous flow techniques. The introduction of continuous-flow hydride/vapour-generation has substantially advanced the value and acceptance of the technique for trace elemental analysis. Appfied Research Laboratories (now part of Fisons Elemental), P.S. Analytical and Varian have all introduced continuous-flow hydride/vapour-generation systems, whilst Perkin Ehner has used the flow injection modification to automate the techniques with their instrumentation. 

Figure S.2 shows a schematic diagram of the automatic hydride/vapour-generator system designed by P.S. Analytical. This has been widely used to determine hydrideforming elements, notably arsenic, selenium, bismuth, tellurium and antimony, in a wide range of sample types. To provide a wide range of analyses on a number of matrices the chemistry must be very well defined and consistent. Goulden and Brooksbank s automated continuous-flow system for the determination of selenium in waste water was improved by Dennis and Porter to lower the detection levels and increase relative precision [10, 11]. The system described by Stockwell [9] has been specifically developed in a commercial environment using the experience outlined by Dennis and Porter.

Chemical separation techniques can be used to reduce spectral interferences and concentrate the analyte. These techniques include solvent extraction(39) and hydride generation(39, 46, 47). At Imperial College, the hydride generation technique is being used on a daily basis(46) for the analysis of soils, sediments, waters, herbage, and animal tissue. The solvent extraction technique is ideally suited for automated systems where the increased manipulation is carried out automatically, and a labor intensive step and sources of contamination are avoided. 

Several elements (including As, Bi, Ge, Pb, Sb, Se, Sn, and Te) form volatile hydrides when reacted with sodium borohydride at room temperature. By introducing the analyte as a volatile hydride, high-transport efficiencies, and therefore improved detection limits, can be achieved. Often as importantly, much of the sample matrix is not introduced into the ICP because those species do not form volatile compounds. Commercial hydride generation sample introduction systems are available. 

Boron, Li, Mo, Pb, and Sb were determined in the standard mode, while Al, Cd, Co, Ni, Mn, Rb, Sb, Sn, and V were determined in the DRC mode. The determination of Ni was done with a gas flow of 0.15 ml min-1 of CH4, while for the other elements NH3 was used as cell gas at 0.4 ml min-1. The determination of Se by flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS) was carried out by means of the Perkin-Elmer FLAS 200 system, equipped with the Perkin-Elmer autosampler AS-90, and connected to an electrically heated quartz cell installed on a PerkinElmer absorption spectrometer AAS 4100. The analytical conditions are given in Table 10.3. 

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. 

In a flow injection system the sample flow and a reagent flow are continuously brought together so as to allow a chemical reaction to take place. This reaction produces a gaseous compound, which has to be separated off as in hydride generation, or forms a complex, which can be adsorbed onto a solid phase to be isolated and preconcentrated. In the latter case, elution with a suitable solvent is carried out and the analytes are led on-line into the AAS system. 

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