Atomization cell hydride

Coenzymes serve as recyclable shuttles—or group transfer reagents—that transport many substrates from their point of generation to their point of utilization. Association with the coenzyme also stabilizes substrates such as hydrogen atoms or hydride ions that are unstable in the aqueous environment of the cell. Other chemical moieties transported by coenzymes include methyl groups (folates), acyl groups (coenzyme A), and oligosaccharides (dolichol). 

The introduction of hydrides into plasma-based instmmentation has also been achieved. The sensitivity increases markedly when compared with conventional nebulization because of the improved transport efficiency of the analyte to the atom cell (close to 100%). Often, a membrane gas-liquid separator is usee ensure that aerosol droplets of liquid do not reach the plasma. 

Several types of atomization cell are available flame, graphite furnace, hydride generation and cold vapour. Flame is the most common. In the premixed laminar flame, the fuel and oxidant gases are mixed before they enter the burner (the ignition site) in an expansion chamber. The more commonly used flame in FAAS is the air-acetylene flame (temperature, 2500 K), while the nitrous oxide-acetylene flame (temperature, 3150K) is used for refractory elements, e.g. Al. Both are formed in a slot burner positioned in the light path of the HCL (Fig. 27.4). 

A specialized form of atomization cell is available for a limited number of elements that are capable of forming volatile hydrides (e.g. As, Bi, Sb, Se and Sn). In this situation, an acidified sample solution is reacted with a sodium tetraborohydride solution. After a short time, the gaseous hydride is liberated. 

Fig. lO Schematic figure of a FI hydride generation AAS system with segmented carrier stream and tubular membrane dual phase gas diffusion separator reponed in ref. 48. S. sample At, aigon flow T, microporous PTFE tubing G, dual-phase gas-diffusion separator, BH, borohydride reductant W, waste and AAS, quartz atomizer cell. 

The hydride is generated by first adding the sample to a HCl solution (0.5-5.0 moll ) and then NaBH4 (about 1% solution). The hydride formation by NaBH4 is very fast and the hydride vapour may be flushed immediately or after reaction times of 10 to 100 seconds into a silica tube atom cell at carrier gas flow rates of 20 to 100 mls. ... 

Mercury is the only metallic element that is liquid at room temperature and possesses a significant vapor pressure. As a result of these unique properties, mercury can be determined without an atomization cell simply by reducing it to the elemental state and transferring it to the vapor phase within the optical path of a suitable detection system. Absorption is usually measured at the 253.7run resonance fine. Similar to hydride generation, the majority of such... 

Detection systems employed with hydride generation approaches are conventional AA spectrometers, usually fitted with intense electrodeless discharge or hollow cathode lamp sources. Quartz tube cells are of suitable dimensions to be compatible with the optical systems of all modem spectrometers. Background correction is usually achieved in double-beam optics using deuterium sources, and Zeeman-effect background correction can be implemented when the graphite furnace is used as the atomization cell. 

The components of instruments for atomic fluorescence measurements are generally arranged as shown in Figure 7-Ib. The sample container is most commonly a flame but may also be an electrothermal atomization cell, a glow discharge, or an inductively coupled plasma, as described in Section lOA-1. Flow cells are often used in conjunction with vapor and hydride-based methods. 

Hydride generation is a common method for the detection of metalloids such as As, Bi, Ge, Pb, Sb, Se, Sn and Te, although other vapours, e.g. Hg or alkylated Cd, may also be determined. This technique improves the sensitivity of the analysis substantially. Since the sample is in the gas phase, the sample transport efficiency is close to 100%. The hydrides atomize readily in the flame, although this approach is usually used in conjunction with a quartz T-piece in the atom cell. Methods have been developed that trap the hydrides on the surface of a graphite tube for use with ETAAS. This leads to preconcentration and further improvements in detection limit. 

Atomic fluorescence has the advantage of being less prone to spectral interferences than either AES or AAS. Molecular fluorescence is less of a problem than molecular absorption is in AAS. Scatter from the light source and quenching from the gaseous species in the atom cell are often the major sources of interference. For many applications where the analyte is separated from the matrix (e.g. vapour generation) chemical interferences may exist for example, the presence of high concentrations of some transition metals may interfere in the hydride formation process. This will inevitably lead to errors in the measurement unless preventative steps are taken. 

A Perkin-Elmer 5000 AAS was used, with an electrically heated quartz tube atomizer. The electrolyte is continuously conveyed by peristaltic pump. The sample solution is introduced into the loop and transported to the electrochemical cell. A constant current is applied to the electrolytic cell. The gaseous reaction products, hydrides and hydrogen, fonued at the cathode, are flowed out of the cell with the carrier stream of argon and separated from the solution in a gas-liquid separator. The hydrides are transported to an electrically heated quartz tube with argon and determined under operating conditions for hydride fonuing elements by AAS. 

Willie et al. [17] used the hydride generation graphite furnace atomic absorption spectrometry technique to determine selenium in saline estuary waters and sea waters. A Pyrex cell was used to generate selenium hydride which was carried to a quartz tube and then a preheated furnace operated at 400 °C. Pyrolytic graphite tubes were used. Selenium could be determined down to 20 ng/1. No interference was found due to, iron copper, nickel, or arsenic. 

The most important coenzymes in synthetic organic chemistry [14] and industrially applied biotransformations [15] are the nicotinamide cofactors NAD/ H (3a/8a, Scheme 43.1) and NAD(P)/H (3b/8b, Scheme 43.1). These pyridine nucleotides are essential components of the cell [16]. In all the reactions where they are involved, they serve solely as hydride donors or acceptors. The oxidized and reduced form of the molecules are shown in Scheme 43.1, the redox reaction taking place at the C-4 atom of the nicotinamide moiety. 

Table 2.1 shows the crystal structure data of the phases existing in the Mg-H system. Pnre Mg has a hexagonal crystal structure and its hydride has a tetragonal lattice nnit cell (rutile type). The low-pressure MgH is commonly designated as P-MgH in order to differentiate it from its high-pressure polymorph, which will be discussed later. Figure 2.2 shows the crystal structure of p-MgH where the positions of Mg and H atoms are clearly discerned. Precise measurements of the lattice parameters of p-MgH by synchrotron X-ray diffraction yielded a = 0.45180(6) mn and c = 0.30211(4) nm [2]. The powder diffraction file JCPDS 12-0697 lists a = 0.4517 nm and c = 0.30205 nm. The density of MgH is 1.45 g/cm [3]. 

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