Atomization cell hydride generation

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). 

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. 

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. 

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. 

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. 

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). 

The ability to monitor trace levels of a number of heavy metals in a variety of samples is an important feature of modern environmental chemistry. Hence, sensitive analytical methods are required. When faced with the task of analyzing very low concentrations of antimony, bismuth and tin the hydride generation method is the first choice because of the improved sensitivity and lower detection limits as compared to many other techniques. The hydride generation technique includes the use of a reductant, such as a NaBH4 solution, to separate the volatile metal hydrides from the sample solution and the subsequent determination with atomic absorption after decomposition of the hydrides in a heated quartz cell. 

Broekaert J. A. C. Optimization of electrochemical hydride generation in a miniaturized flow cell coupled to microwave-induced plasma, atomic emission spectrometry for the determination of selenium, Fresenius Journal of Analytical Chemistry, in press. 

Hydride generation AAS (HGAAS) and cold vapour AAS (CVAAS) are special combinations of chemical separation and enrichment with AAS. In HGAAS the analyte is transformed to a volatile hydride, stripped off by an inert gas and atomized in a quartz tube, flame-in tube etc. About ten elements (As, Se, Bi, Sb etc.) can be determined by this technique. The accuracy and detection limits depend on the proper isolation of the hydride. CVAAS is the universally acknowledged most sensitive method for determination of Hg. The generation of elemental mercury vapour is similar to the hydride generation however the quartz cell may not be heated and this gives the name of the method. 

The hydride generation (HGAAS) technique is similar in many ways to CVAAS. The atomizer is a quartz tube cell sitting in the light path of the AA spectrometer. In the hydride generation technique, the cell must be heated. Having the cell clamped above the burner head and lighting an air-acetylene flame accomplishes this. The flame surrounds the cell and heats it. Alternatively, some systems have electrically heated cells. 

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. 

Fig.8.4 FI manifold and operational parameters of hydride generation AAS system (valve in sample fill position) for Se and As. V, injector valve L, sample loop SP, gas-liquid separator, S, acidified sample C, acid carrier. AAS. quartz cell atomizer W, waste flows a. b, reaction coils Ar, stabilized argon flow [3].

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. 

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