Hydride-generation Systems

Attempts were made to automate hydride generation determinations and decrease sample consumption using segmented continuous flow systems, but the improvements, if any, were only mai nal. 

Basic FI Hydride Generation Manifolds with Gas-expansion Separators 

The basic FI manifold configuration for hydride generation atomic spectrometry has varied little since the early publications, and a typical manifold using the gas-expansion separator in Fig. 5.4 b is shown in Fig. 5.9. The injected samples are usually preacidified to contain IM HCI and transponed by an IM HCl carrier stream to merge with the borohydride reductant flow at a confluence point. The reaction mixture passes through a length of reaction coil and merges with an inert carrier gas flow which carries the liquid-gas mixture into the gas expansion separator. The separation of the gas from the reaction mixture is achieved as described in Sec. 5.2.4. and the hydride is transported into the heated T-shaped quartz atomizer for atomization. 

The following points in the design of a FI hydride generation AAS system, based mainly on our own experiences using gas expansion separators, is worth mentioning 

The importance of the withdrawal rate of solution waste fliom the gas-expansion separator was stressed in Sec. 5.42. Forced withdrawal using a pump is strongly recommended in preference over free outflow of reaction waste for ensuring optimum precision and long term trouble-free operation. This aspect should not be overlooked, at least in the design of a FI hydride generation system using gas-expansion separators. When gas-diffiision separators are used, the requirements may be different. 

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Figure 5.16. Hydride generation system for photoluminescence. Source [658]... 

Hydride generation systems (a) batch mode operation (b) continuous mode operation. 

Fig. 3. Schematic diagram of a LC-ICP-MS interface with hydride generation system. Reprinted from Story et al. [49] by permission of Preston Publications, a division of Preston Industries.

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. 

Figure 7.7. Schematic diagram of the continuous flow hydride generation system, showing the two positions of the four-port valve (A) for analysis of the test solution and (B) for blank integrations used for changing the test solution. From [115]...

Hydride generation may result in a detection limit for arsenic of around 0.8 ng ml-1 by AAS under optimized conditions.2 Over recent years, this impressive detection power has resulted in the development of automated, flow-injection-based hydride generation systems for the determination of arsenic in plants3 and soil... 

Figure 18.6. Schematic of an HPLC flow injection-ICP-MS hydride generation system (a) and real-time measurement of As species via hydride formation (b).

Two Perkin Elmer hydride generation systems are used i.e. the MHS-1 (Mercury/Hy-dride System) electrothermal system and the MHS-10 flame-heated tube system, the latter being applied only for the tin determinations to avoid memory effects. 

Fig. m. Gas sampling glow discharge (A) and continuous-flow hydride generation system (dimensions in mm) (B), (a) reaction column, (b) fritted glass disk, (c) gas-.liquid separator and (d) condenser (Reprinted with permission from Ref. [488].)... 

R. R. Liversage, J. C. van Loon, and J. C. de Andrade, A Flow Injection/ Hydride Generation System for the Determination of Arsenic by Inductively-Coupled Plasma Atomic Emission Spectrometry. Anal. Chim. Acta, 161 (1984) 275. 

M. Burguera and J. L. Burguera, Flow Injection—Hydride Generation System for the Determination of Arsenic by Molecular Emission Cavity Analysis. Analyst, 111 (1986) 171. 

Milk (cow and human breast milk) plays an important role in nutrition, especially for infant food. All essential trace and mineral elements are under investigation. The analysis of the mineral elements Ca, K, Mg, Na and P presents no difficulties. Of the essential elements only Cu, Fe, Mn and Zn are detectable. Using a hydride generation system, one can also determine Se. A mineralization step is mostly necessary. An exception would be in applying an ultrasonic nebulizer, but further developments on it are necessary for routine applications (Table 3,4) (Schramel, 1979). 

As above described, Bu-P4 base was found to catalyse the coupling reaction of aryl fluorides with silyl ethers. The use of silyl ether was required for these reactions, and direct arylation of unsilylated alcohols using superbase catalysed coupling reactions has been a challenge. Therefore, the combination of triethylsilyl hydride (Et3SiH) and catalytic Bu-P4 is considered to represent an attractive hydride generating system with which to carry out sequential deprotonation and Sj-jAr reaction [55] (Figure 5.5). 

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). 

An interesting development of FI hydride generation systems is the use of a solid phase tetrahydroborate reagent in the reduction step reported by Tesfalidet and Irgum... 

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. 

Figure 90 A widely used hydride generation system in which a small oxygen-hydrogen Jlame is burning inside an unheated quartz tube. (Adapted from D. D. Siemer and L. Hagemann, Anal. Lett., 1975, 8, 323)...

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. 

Figure 136 Hydride generation system designed for a DCP-AES instrument. (Adapted from T. G. Gilbert, Anal. Lett., 1977, 10, 599)...

Mercury can be determined in plasma AES by reducing it first to elemental mercury and then transporting the mercury vapour into the plasma. The same reduction methods may be used as for AAS. Commercial hydride generation systems can be adopted to the cold vapour method. The detection limit is about 0.02 mgP ... 

Fig. 12.n Hydride generation systems, (a) Batch generation, (b) continuous generation, and (c) flow injection generation. 

Hgure 2 Hydride generation systems (A) movable bed generator (Reproduced with permission from Tian X-D, Zhuang Z-X, Chen B, and Wang W-R (1998) Movable reduction bed hydride generator coupled with ICP-OES for the determination of some hydride forming elements. Analysts 123 627-632.) (B) modified Meinhard concentric nebulizer and (C) cross flow nebulizer. 

A modified Soxhlet extractor is used to digest 1 mL heparinized blood with 1.5 mL cone. HNO3 and 0.5 mL cone. H2SO4, a method transferable to other biological materials. After 1 hr the flask is connected to a hydride generator system for measuring [74]. 

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