Continuous flow hydride generation

Lab method using continuous flow or flow injection analysis hydride generation and atomic absorption spectrometry... 

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. 

Figure S.2 shows a schematic diagram of the automatic hydride/vapour-generator system designed by P.S. Analytical. This has been widely used to determine hydrideforming elements, notably arsenic, selenium, bismuth, tellurium and antimony, in a wide range of sample types. To provide a wide range of analyses on a number of matrices the chemistry must be very well defined and consistent. Goulden and Brooksbank s automated continuous-flow system for the determination of selenium in waste water was improved by Dennis and Porter to lower the detection levels and increase relative precision [10, 11]. The system described by Stockwell [9] has been specifically developed in a commercial environment using the experience outlined by Dennis and Porter.

Hydride/vapour generation techniques provide extremely good sensitivity. When coupled to continuous flow methodologies for use in routine analysis, simple and reliable analytical techniques are provided. TTie extension of chemistries and sample transfer systems to provide analytical protocols to cope with a wider range of elemental analyses should be pursued in the search for lower detection levels. While multi-element techniques offer very low levels of detection, the use of specific single element analytical instruments with detection capabihties similar to those described above may be the best route for routine laboratories with high sample throughput. 

Hydride generator PS Analytical continuous flow hydride generator. 

Reductant A solution of 1% w/v sodium borohydride (NaBHJ should be freshly prepared in 0.1 M sodium hydroxide (NaOH), and placed in the reductant reagent compartment of the PS Analytical continuous flow hydride generator. 

Figure 7.7. Schematic diagram of the continuous flow hydride generation system, showing the two positions of the four-port valve (A) for analysis of the test solution and (B) for blank integrations used for changing the test solution. From [115]...

Mohammad, B., Ure, A.M., Reglinski,J. and Littlejohn, D. (1990) Speciation of antimony in natural waters the determination of Sb(III) and Sb(V) by continuous flow hydride generation-atomic absorption spectrometry. Chem. Spec. Bioavail., 3, 117-122. 

A flame AAS (FAAS) detector can monitor the GC effluent continuously to provide on-line analysis. However, as the gas flow rates for the flame are quite high, the residence time in the flame is short, and this can adversely affect the detection limits. Detection limits in the microgram range are usually achieved. Improved detection limits can be obtained if the additional techniques of hydride generation or cold vapour mercury detection are used as described in Section 4.6. 

Cobo-Fernandez, M.G., Palacios, M.A. and Camara, C. (1993) Flow-injection and continuous-flow systems for the determination of Se (VI) and Se (IV) by hydride generation atomic absorption spectrometry with on-line prereduction of Se (IV) to Se (VI). Anal. Chim. Acta, 283, 386-392. 

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. 

D. Velez, N. Ybanez, R. Montoro, Analysis of geological materials for bismuth, antimony, selenium and tellurium by continuous flow hydride generation inductively coupled plasma mass spectrometry, J. Anal. Atom. Spectrom., 12 (1997), 91-97. 

Figure 2.17 Schematic diagram of continuous flow hydride generator. (Reproduced with kind permission from PSAnalytical, Orpington BR5 31 IP, UK)...

Figure 28-5 Continuous sample introduction methods. Samples are frequently introduced into plasmas or flames by means of a nebulizer, which produces a mist or spray. Samples can be introduced directly to the nebulizer or by means of flow injection (FIA) or high-performance liquid chromatography (HPLC). In some cases, samples are separately converted to a vapor by a vapor generator, such as a hydride generator or an electrothermal vaporizer.

In a flow injection system the sample flow and a reagent flow are continuously brought together so as to allow a chemical reaction to take place. This reaction produces a gaseous compound, which has to be separated off as in hydride generation, or forms a complex, which can be adsorbed onto a solid phase to be isolated and preconcentrated. In the latter case, elution with a suitable solvent is carried out and the analytes are led on-line into the AAS system. 

Fig. m. Gas sampling glow discharge (A) and continuous-flow hydride generation system (dimensions in mm) (B), (a) reaction column, (b) fritted glass disk, (c) gas-.liquid separator and (d) condenser (Reprinted with permission from Ref. [488].)... 

D. (1995) A flowing electrolytic hydride generator for continuous sample introduction in atomic spectrometry, Anal Chim Acta 316 129-44. 

Brumbaugh WG, Walther MJ. 1989. Determination of arsenic and selenium in whole fish by continuous-flow hydride generation atomic absorption spectrophotometry. J Assoc Off Anal Chem 72(3) 484-186. 

Schramel P and Xu L-Q (1991) Determination of arsenic, antimony, bismuth, selenium and tin in biological and environmental samples by continuous flow hydride generation ICP-AES without gas-liquid separator. Fresenius J Anal Chem 340 41-47. 

A different approach to the CV generation was utilized by Tao and Miyazaki (1991), who reduced the sample with borotetrahydride in a continuous flow system, and used porous PTFE tubing as a gas-liquid separator. Disturbing gaseous hydrogen and water vapour, diffusing together with Hg(0) and metal hydrides, were removed in a hollow fiber... 

Ndite, J. (1991) Continuous-flow hydride generation combined with conventional nebuliza-tion for ICP-AES determination. At. Spectroscopy, 12,199-203. 

Attempts were made to automate hydride generation determinations and decrease sample consumption using segmented continuous flow systems, but the improvements, if any, were only mai nal. 

The most widely used atomiser for hydride generation is the heated quartz T-tube atomiser with a typical diameter of 10 mm and a length of 100—150 mm, making it compatible with the optical path of most AA spectrometers. The quartz tube is electrically heated to 700—1000 °C which permits one to optimise the atomisation temperature for each element. The quartz tube may either have open ends, or these ends are sealed by removable quartz windows, and holes at the extreme ends of the quartz tube provide the gas flow outlets. This set-up increases the residence time of the atoms in the light path and thus improves sensitivity. With continued use the performance of the quartz tube atomiser invariably deteriorates in terms of sensitivity and precision. This is attributed both to devitrification of the inner surface of the quartz tube to a less inert modification, and to contamination of the inner atomiser surface by deposition of small particles and droplets that were not efficiently removed by the gas—liquid phase separator. 

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

Despite the better sensitivity of GF-AAS the coupling with HPLC has only found limited use, mainly due to the difficulties of coupling a continuous flow separation technique with the discrete nature of GF-AAS. Improvements may be expected by coupling HPLG and GF-AAS after on-line hydride generation as outlined for HPLC-FAAS, however, these techniques has not been thoroughly investigated. 

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

Metalloid compounds are usually determined by flowing-stream techniques hyphenated with hydride generation (HG)-atomic absorption or atomic fluorescence spectrometry. The continuous operation mode inherent to flow injection is specially suited for the latter detection technique as the tetrahyd-roborate reagent is a potential source of hydrogen for supporting the flame. Analyte preconcentration is frequently needed to detect the typical levels of metalloid species found in water matrices. In this context, cold trap collection of generated hydrides, sorbent extraction microcolumn methods, sorption... 

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