Batch Hydride Generation

Some detection limits reported in the literature for hydride generation techniques are presented in Table 32. With respect to the detection limits published in the literature, hydride-plasma-AES is comparable with hydride-AAS, except for lead. Relative to solution nebulization, detection 

3 Interferences. Chemical and spectral interferences are associated with the hydride generation technique. Molecular band interference caused by CO2, which is generated from contaminant in the NaBH4 solution, is 

Chemical interferences fall into two categories as in hydride—AAS (i) Interferences in the liquid phase that prohibit or limit the formation of the volatile hydride, and (ii) Interferences in the gas phase that diminish the analyte available for excitation. Interferences of the first type are caused mainly by the Group 8, 9, 10, and 11 transition elements. The degree and elimination of the interferences are the same as in AAS (Chapter 3, Section 6.2). 

---

R. B. Georgieva, P. K. Petrov, P. S. Dimitrov and D. L. Tsalev, Observations on toxicologically relevant arsenic in urine in adult offspring of families with Balkan endemic nephropathy and controls by batch hydride generation atomic absorption spectrometry, Int. J. Environ. Anal. Chem., 87(9), 2007, 673-685. 

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

Continuous, batch, and flow injection modes of hydride generation have been used successfully [39-41]. In the commonly used continuous mode the sample and sodium borohydride solutions are pumped by using a dual-channel peristaltic pump into a mixing chamber. The volatile hydride gas and hydrogen are carried into the plasma with a flowing argon gas and the excess liquid is directed to the drain. 

The reaction time for the hydride generation process in an FI system can be precisely controlled by the flow-rate and line lengths to favour the main reactions which are usually fast. The slower interfering reactions are often suppressed by using shorter reaction coil lengths. Such kinetic discriminations are not possible in batch procedures. 

Hydride generation methods involve three or four successive steps depending on the technique used (i) The hydride is generated by chemical reduction of the sample (ii) The formed hydride may be collected in the batch type methods (iii) The hydride is entrained in a gas stream into the atomizer (iv) The hydride is decomposed in the atomizer to form the atomic vapour, and the absorption signal is measured. A number of methods in use are based on this principle, but they differ in the means of reduction, atomization, and sample introduction. 

Two modes of operation can be applied for the hydride generation technique (i) In the normal batch system, the whole sample is reduced in a hydride generator and the hydride formed transported in a carrier gas stream to an absorption tube (ii) In the flow injection (FIA) technique all stages of the hydride generation method take place in a fully automated closed system. The FIA system is discussed in section 6.3. 

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. 

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

Figure 1 Hydride generation approaches (A) continuous generation (B) batch generation and (C) flow injection.

For the determination of the Hg species methylmercury, phenylmercury and Hg(ll), ozone was used successfully in a batch cold-vapor system [126]. The preconcentration and speciation of Cr(lll) and Cr(VI) in water sample can be performed using solid-phase extraction (SPE) [105]. An SPME (solid-phase microextraction) technique has been used as the sample preparation system for the innovative simultaneous multielement/multi-species determination of six different mercury, tin. and lead species in waters and urine with GC/MS-MS [141], [142], The determination of arsenic species [As(III), As(V), MMA. DMA, arsenocholine, arsenobetaine)] has also been shown to work with an on-line digestion step prior to hydride generation AAS [125] (see Fig. 7). 

Hydride mode, which allows for inline, batch-mode hydride generation for elements forming volatile hydrides such as arsenic, selenium, and antimony. This step is also performed and the analyte measured during the preconcentration loading/washing step, so it does not add time to the analysis. 

Figure 20.3 shows the seaFAST 3 timeline of the three modes of a seawater analysis. Let us take a closer look at this automated sampling procedure. At t = 0, the sample is loaded onto three separate loops and injected into the system. The ICP-MS read delay (A) is the time it takes to load the loops and for the direct mode signal to stabilize. During this time, both the batch-mode hydride generator and the preconcentration columns are loaded with sample from their respective sample loops. In direct mode (B), sample is diluted online approximately lOx, mixed with an internal standard, and introduced directly to the ICP-MS. 

When using the batch generation system, which involves producing the hydride compound from a fixed volume of sample, by the addition of the hydride-generating reagents in a reaction vessel, the analyte vapor is... 

Table 5.3 sets out the advantages and disadvantages of the batch and continuous flow techniques. The introduction of continuous-flow hydride/vapour-generation has substantially advanced the value and acceptance of the technique for trace elemental analysis. Appfied Research Laboratories (now part of Fisons Elemental), P.S. Analytical and Varian have all introduced continuous-flow hydride/vapour-generation systems, whilst Perkin Ehner has used the flow injection modification to automate the techniques with their instrumentation. 

Arsenic, Hg, Sb, Se, and Sn were determined in untreated beer samples by direct HG-ET-AAS using a batch system [119]. Elements were preconcentrated in situ onto the Pd- (for As, Sb, Se, and Sn) or Au-pretreated (for Hg) interior wall surface of a graphite furnace. The authors considered beneficial to degas beer samples by filtration and to generate the hydrides in the presence of an antifoam agent. An extra gas-liquid separator was necessary to minimize the amount of moisture from the reaction vessel reaching the graphite tube. With the combination of HG and ET-AAS LoDs of 28, 90, 21, 10, and 50 ng l-1 were reached for As, Hg, Sb, Se, and Sn, respectively. 

Volatile analytes cryogenically preconcentrated at —78°C on the U trap species identified by retention time matching, elemental isotopic fingerprints, and ion trap MSn analysis Batch HG to generate hydride derivatives cryogenic preconcentration at — 196°C on the U-trap thermally desorbed into an ICP-MS identification of species based on comparison with standards or calculation of boiling point... 

Although infrequently used, electrochemical generation of the hydrides is also possible and has been applied to the determination of arsenic and tin in a batch approach and to antimony, arsenic, germanium, selenium, and tin using a flow-through electrolytic cell. The hydride is generated in the cathodic space of an electrolytic cell, with concurrent oxidation of water in the anodic compartment, as illustrated by the reaction below. Here, Me-E represents the reduced analyte element on the metallic cathode surface (Me) ... 

---