Vapour-generator system

To obtain the calibration standards, take aliquots ranging from 50 /xL to 300 juL As, from the working standard solution, using an Eppendorf micropipette. Add the appropriate microlitre quantities to the reaction vessel of the vapour generation system, together with 10 mL of hydrochloric acid (AM), delivered from a calibrated dispenser. 

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.

Liquid samples gasouse extraction (Hydride/cold vapour generation system)... 

Atomic Fluorescence System - Millennium Excalibur PSA 10.055 -was used in our work. This system consists of the autosampler, the integrated continuous flow vapour generator and the atomic fluorescence spectrometer with the boosted dischar ge hollow cathode lamp and a control computer. 

Figure S.4 shows a calibration graph of arsenic concentrations obtained by using a Perkin Elmer 2100 atomic-absorption system bnked to a P.S. Analytical hydride/vapour generator (PSA 10.003). An electrically heated tube has been used in this work and the spectral source was an electrodeless discharge lamp. Alternatively, a flame-heated tube can be used.

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. 

Fig. 7.16 Calibration graph of vapour generator linked to gold amalgam preconcentration system.

Typical cold vapour generation AAS system used for mercury determination. The same system can be used with a flame in place of the Pyrex tube to allow the determination of hydride -forming elements. 

Analysis of the pressure versus temperature data for the tests (see Annex- 2) indicated that case (iii) generated permanent gas but that the other cases were vapour pressure systems. For a vapour pressure system, it is the rate of temperature rise at the relief pressure which, determines the relief system size. The relief pressure of 3 bara corresponds to a temperature of approximately TOO °C for cases (i), (ii) and (v)r and to approximately 80°C for case (iv). It can be seen from Table 3.1 that case (ii) gives the highest rate of temperature rise at that.temperature and is therefore the worst of the vapour pressure systems. 

To achieve (b), it is necessary to use relief sizing methods that take account of the dynamics of the pressure relief, event. Pressure relief systems for runaway chemical reactions usually discharge a two-phase mixture (see 4.3). If a steady-state calculation were used to size the relief system, then it would be necessary to size it for the volumetric rate of two-phase mixture equal to the volumetric, rate, of gas/ vapour generation at a particular point (e.g. at the relief pressure for vapour systems). This leads to very large calculated relief system sizes. 

The data required for a tempered hybrid system is similar to that for vapour pressure systems. However, because permanent gas is being generated by the reaction, an open test should be used. This is because ... 

The calculation will usually be performed at the relief pressure. If the superficial velocity, calculated above for the gas and vapour generated by the reaction, does not cause two-phase relief, then the relief system can safely be sized for single-phase gas or vapour relief (provided the mixture is not inherently foamy). The above method should not be used to decide whether two-phase relief would actually... 

Diemer, J. and Heumann, K.G. (1997) Bromide/bromate speciation by NTI-IDMS and ICP-MS coupled with ion exchange chromatography. Fresenius J. Anal. Chem., 357,74-79. Duan, YX., Wu, M., Jin, Q.H. and Hieftje, G.M. (1995) Vapour generation of nonmetals coupled to microwave plasma-torch mass-spectrometry. Spectrochim. Acta B, 50,355-368. Ebdon, L., Hill, S. and Jones, R (1987) Interface system for directly coupled high performance liquid chromatography-flame atomic absorption spectrometry for trace metal determination./. Anal. At. Spectrom., 2, 205-210. 

Nitrogen oxides, as well as nitric acid vapours, generated in the reaction, flow via the ventilating pipe to a system of jars, coolers and absorption towers, where they are absorbed. 

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