Hydride techniques

C, the rate of reaction tends to be self limiting at hydrogen pressures up to 10 torr. The hydriding technique is used to recover metallic plutonium residues clinging to the walls of ceramic crucibles, and can also be used to recover machining scrap if the feed is free of lubricants or oxides. Mulford and SturdyO4) have found the heat of formation for the reaction... 

Inorganic As(III) and As(V) were determined by atomic absorption spectrometry using the hydride technique. Total inorganic arsenic, As(III) + As(V), was measured after a prereduction reaction of As(V) to As(III) in acidic solution containing potassium iodide and ascorbic acid. For the selective hydride formation of As(III), samples were maintained at pH 5.0 during the hydride reaction (with 3% NaBH4, 1% NaOH) with a citrate-sodium hydroxide buffer solution (31). As(V) was determined by difference between total As and As(III). The detection limit of As(III) and As(V) was 0.1 nM. The selectivity of this method was checked by additions of As(III) and As(V) to lake water 95-100% recovery of As(III) and As(V) was found (32). 

Schlegel, D., J. Mattusch, and K. Dittrich. 1994. Speciation of arsenic and selenium compounds by ion chromatography with inductively coupled plasma atomic emission spectrometry detection using the hydride technique. J. Chromatogr. A 683 261-267. 

International Standard Organization. 1996. Water quality. Determination of arsenic. Atomic absorption spectrometric method (hydride technique). ISO 11969. International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland. 

The cold-vapor technique for Hg allows detection limits of <1 ng to be obtained when using 50 mL of sample and they can be improved still further by trapping. With the hydride technique detection limits below the ng/mL level can be achieved for As, Se, Sb, Bi, Ge, Sn, etc. Accordingly, the levels required for analyses used to control the quality of drinking water can be reached. 

Hydride techniques, however, can suffer from many interferences (see Section 3.3). In AAS these interferences can not only occur as a result of influences on the hydride formation reaction but also as a result of influences of concomitants on the thermal dissociation of the hydride. Interferences from other volatile hydride forming elements can also occur [291]. Recently it has been found that still more elements can form volatile hydrides, as demonstrated e.g. by Cd. Here the hydride... 

Broekaert J. A. C. and Leis F. (1980) Application of two different ICP-hydride techniques to the determination of arsenic, Fresenius Z Anal Chem 300 22-27. 

Welz B. and Melcher M. (1981) Mutual interactions of elements in the hydride technique in atomic absorption spectrometry, Anal Chim Acta 131 17-25. 

The determination of Ge by means of atomic absorption spectrometry (AAS) during the late 1960s was replaced by introduction of the hydride technique with sodium tetrahydroborate as a means of reduction. The detection limits indicated are 2xl0 g and 0.01 pg mL Abbasi et al. (2001) showed that there is a possibility of losing Ge in the presence of chloride when acid digestion procedures are carried out in open vessels. This is more pronounced if the Ge concentration is very low. In fact, 100 pg g can be considered as a critical value above which both open and closed vessels digestion methods can be used without any significant loss. 

Some interferences are removed by the hydride technique because the analyte is physically separated from the remainder of the matrix. But there are interferences in the process of generating the hydride and the variability in the rate of generation. There appears to be some black art in the quartz decomposition cell, and sometimes simple electrical furnaces are used to convert the hydride to the atomic vapor. In the opinion of this author, the furnace technique has fewer interference problems for the P block elements than the hydride technique but the furnace equipment is more expensive and, if the quantity of sample is not limited, the furnace is less sensitive. There is a great deal of historical and practical information on both the cold vapor method for Hg and the hydride method in the AAS book by Welz (1985), who is an experienced authority in these methods. 

Methods for elemental analyses For the determination of elements (e.g., lead, cadmium, etc.) the method of ICP-MS (EN Standard, 2004) should be preferred over AAS methods, whenever possible. The ICP-MS determination of iron could be subject to polyatomic interferences so the ICP-OES EN Standard, 1997b method should be applied. For mercury and selenium AAS hydride techniques (EN Standard, 1997c ISO, 2003b) should be applied due to the higher sensitivity of these techniques compared with the ICP-MS technique (EN Standard, 2004). All of the cited standards (see Table 1.5) can be used for the control of the parametric value. 

The main advantages of AAS are its high specificity and selectivity, the sensitivity being variable over broad ranges depending on the type of atomization selected (flame, graphite, cold vapour or hydride technique). 

Fig. 29. 1) = Spectrometer 2) = Data system 3) = Printer = Cold-vapour and hydride technique 5) = Graphite-tube furnace with automated...

Using sodium borohydride, arsenic ions are reduced to arsenic hydride, transferred to a heated quartz cuvette with the aid of a current of inert gas, decomposed thermally, and the absorption of the atoms is measured in the beam of an atomic-absorption spectrometer. In the hydride technique, the element which is to be determined is volatilized as a gaseous hydride and separated off from the matrix. Interferences may occur if there is a considerable excess of elements such as antimony, tin, bismuth, mercury, selenium or tellurium, which may also be volatilized using this technique. 

Since arsenic (III) and arsenic (V) are not equally sensitive when determined by the hydride technique, arsenic (V) should be reduced to arsenic (III) before determination (prereduction). 

The parameters quoted here for the hydride technique apply to a Varian AAS unit, model 775, in conjunction with a hydride system manufactured by Messrs. Berghoff, Tubingen, FRG. 

Since the hydride technique only permits quantitative detection of arsenic which is present ionogenically in solution, decomposition is required for organic or complexed arsenic. Organically loaded water samples must also be decomposed before measurement. Two methods of decomposition are described below. 

Distill the trivalent arsenic directly from the strongly hydrochloric water sample and carry out determination in the distillate using the AAS hydride technique. 

Since the hydride technique only permits quantitative detection of selenium (IV), selenium (VI) must be converted to selenium (IV) by prereduction (boiling in a strongly hydrochloric solution at the reflux). 

Determination of tin using the AAS hydride technique General remarks... 

These are among the most harmful pollutants in sewage. Essential elements (e.g., Fe) as well as toxic metals such as Cd, Hg, and Pb are included. Main sources of heavy metals are industrial wastes, mining, fuels, coal, metal plating, etc. Metal determinations in sewage are preferably carried out by atomic spectrometry (flame and electrothermal atomization), atomic emission spectrometry, inductively coupled plasma-mass spectrometry, stripping voltammetry, spectrophotometry, and kinetic methods. Hg is advantageously determined by the cold vapor technique and As by the hydride technique. 

The techniques in AAS have become manifold. Besides the flame technique, the flameless method is widely used. The hydride technique is proper for determination of volatile elements. All these techniques are used in different modifications. For details see the corresponding chapters of this handbook or other sources, e.g., Welz [31], Stoeppler [32], or Holcombe and Rettberg [33]. 

When determining antimony in human hair account should be taken of the points set down in Chap. 15 of this book when sampling. Before analysis the comminuted sample is decomposed in an oxygen-rich medium (hydrogen peroxide) until the decomposition solution is completely clear. The Sb determination is then carried out by the A AS hydride technique. 

Air is sampled at the place of work for antimony analysis using filter paper (Sartorius type SM 15400) that has been impregnated with 10% aqueous silver nitrate solution and dried [65]. After sampling the filter paper is extracted with 10% HNO3 and the extract analyzed by the AAS hydride technique as described in Sec. 4.3. 

The determination of germanium by means of atomic absorption spectrophotometry at the end of the 1960s was replaced by the introduction of the hydride technique with sodium tetrahydroborate as means of reduction. The detection limit is indicated with 2 x lOr g and 0.01 xg Ge/liter [44]. 

---