Hydride generation techniques

The 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 HGAAS 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. 

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Bertine and Lee [60] have described hydride generation techniques for determining total antimony, Sb (V), Sb (III), Sb-S species and organo-antimony species in frozen seawater samples. 

De Oliviera et al. [739] have described a technique for determining these elements based on the hydride generation technique. Detection limits are 1 xg/l for arsenic and antimony, and 0.5 pg/1 for selenium. 

Amankwah and Fasching [4] have discussed the determination of arsenic (V) and arsenic (III) in estuary water by solvent extraction and atomic absorption spectrometry using the hydride generation technique. 

Hydride generation techniques for the volatilization and separation of alkyl leads have been infrequently used as the organolead hydrides are not very stable. The commonest derivatization reagents are NaBtC IRR or Grignard reagents. The normal separation systems and methods of detection are used. 

The hydride generation technique is a technique in which volatile metal hydrides are formed by chemical reaction of the analyte solutions with sodium borohydride. The hydrides are guided to the path of the light, heated to relatively low temperatures, and atomized. It is useful because it provides an improved method for arsenic, bismuth, germanium, lead, antimony, selenium, tin, and tellurium. 

An automatic system using the hydride generation technique requires attention to (a) chemistry (b) automatic sample preparation and (c) introduction of the hydride into the... 

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. 

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. 

Azad et al. [ 186] used a similar technique for the determination of selenium in soil extracts using a nondispersive spectrometer, with which it was possible to observe fluorescence from the 196.1, 214.3 and 204.0 lines simultaneously, thus enabling a detection limit of 10 ng/ml to be observed using discrete sample introduction via the hydride generation technique. In this method, soil... 

The EPA has approved special methods for arsenic (EPA Method 7061) and selenium (EPA Method 7741) analysis by gaseous hydride generation technique, which offer a unique combination of selectivity and sensitivity. This technique has an advantage of being able to isolate these elements from complex matrices that may cause interferences in analysis with other techniques. 

We have already seen in Chapter 2 that choice of atomizer system to be used may have a dramatic effect upon sensitivity, and thus upon signal-to-noise ratio. It is necessary to choose not only between flames, electrothermal atomization (ETA), and cold vapour and hydride generation techniques (which are discussed in Chapter 6), but sometimes also between different flames. Those elements which tend to form thermally stable oxides, such as Al, Ti, Si, Zr, may only be determined in a hotter, reducing nitrous oxide-acetylene flame. They cannot be determined with useful sensitivity in the air-acetylene flame. Some elements, Ba and Cr for example, may be determined in air-acetylene, but are more efficiently atomized in nitrous oxide-acetylene. 

The element may be determined at 196.0 nm by A AS, using a nitrous oxide-acetylene flame (which is more transparent than air-acetylene at this low wavelength), or by AFS in a variety of flames.46,47 The detection limit of both techniques for selenium is around 1 mg 1 1, too low to be useful for environmental analyses. The element is therefore invariably determined by hydride generation techniques, coupled to AAS or AFS detection, as discussed in Chapter 6, section 2, or by furnace AAS, or occasionally by solution spectrofluorimetry using 2,3-diaminonaphthalene as a reagent. If direct flame AAS or AFS are to be used for some reason, then pre-concentration by solvent extraction is necessary.1 However, this approach is rarely used nowadays. 

Hydride generation techniques are applicable to tin, and the detection limit is then improved dramatically, generally to around 1 ng ml-1. For natural water samples, the element is still sometimes pre-concentrated prior to determination by hydride generation techniques.52,53... 

Conventional FAAS is chracterized by poor detection power. Serious interferences from hydride-forming elements such as As, Sb, and Se are well known. Hydride generation techniques may circumvent these problems, providing an excellent tool to determine those elements at trace and ultratrace levels this is particularly useful for the determination of Se in milk samples [54-56]. Other... 

Z. Slejkovec, J. T. van Elteren, U. D. Woroniecka, Underestimation of the total arsenic concentration by hydride generation techniques as a consequence of the incomplete mineralization of arsenobetaine in acid digestion procedures, Anal. Chim. 

The measurement of Bi in body fluids and tissues may be achieved using either ETA—AAS or hydride generation techniques. Rooney [101] compared these procedures and reported that the latter was the method of choice. The severe molecular absorption interferences at 213.2 nm necessitate some form of chemical pretreatment for ETA—AAS. Thomas et al. 

Mercury and those elements (antimony, arsenic, bismuth, germanium, lead, selenium, tellurium and tin) which form volatile covalent hydrides may be separated from the matrix by vapour generation. The use of tin(II) chloride to generate elemental mercury and its subsequent aeration into a long-path absorption cell with silica windows has been described elsewhere in this book, as has the use of sodium borohydride to produce hydrides which are swept to a flame or heated tube for atomisation. This approach is far more successful for mercury than for the other elements, as the hydride generation technique is subject to interference from a large number of transition metals and oxyanions. 

Nakazato, T., Tao, H., Taniguchi, T., Isshiki, K. Determination of arsenite, arsenate, and monomethylarsonic acid in seawater by ion-exclusion chromatography combined with inductively coupled plasma mass spectrometry using reaction cell and hydride generation techniques. Talanta 58, 121-132 (2002)... 

Odanaka and collaborators have reported that the combination of gas chromatography with a multiple ion detection system and hydride generation technique is useful for the quantitative determination of arsine, monomethyl-, dimethyl- and trimethylarsenic compounds and this approach is applicable to the analysis of environmental and biological samples including waters, crops, plants, biological materials and river and marine sediments. 

Fig. 4 Diagram of a typical unit for vapor or hydride generation technique. (Courtesy of Perkin-Elmer Instruments.)...

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. 

Hydride Generation [9]. The hydride generation technique is probably the most sensitive for direct ICP-AES measurement/detection (Figure 2.17). The sensitivity of this procedure is 50 to 300 times greater than that by direct nebulisation. The method is relatively free from interferences, as it involves separation of the metals as hydride gases from the sample solution after reaction with sodium borohydride in the presence of acid. The technique is limited to the elements As, Bi, Ge, Pb, Se, Sb, Sn and Te, which are known to form readily volatile covalent hydrides. The hydrides are purged directly into the plasma where they are atomised, excited and measured by ICP-AES in the normal way. 

Liu Y., Wang X., Yuan D., Yang P., Huang B. and Zhuang Z. (1992) Flow-injection-electrochemical hydride generation technique for atomic absorption spectrometry, J Anal At Spectrom 7 287-291. 

The validation has the objective to identify, during the method development process, all sources of error and eliminate them or to quantify their contribution to the total uncertainty of the determination. For hydride generation techniques particular attention must be given to the quantitative transformation of all As species into hydrides (arsenobetaine, arsenosugars etc). Several types of adapted materials must be prepared to test all steps of the process (from simple calibrant solutions or mixtures to spiked fish tissue samples). If they exist CRMs should be used for validating trueness. Laboratory RMs must be prepared for the establishment of control charts when the method is under statistical control [27, 28]. 

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