Cold Vapour Generation Systems

Liquid samples gasouse extraction (Hydride/cold vapour generation 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. 

AAS determinations based on hydride and cold vapour generation, electrothermal atomization with graphite furnaces have also been used successfully with FI systems to improve the overall performance of these techniques. Special requirements on the atomization-detection systems for hydride and cold vapour generation will be discussed in Chapter 5, and for the graphite furnace, in Chapter 4. 

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. 

Figure 95 A FIA system adapted for hydride generation and cold vapour techniques (Perkin Elmer Corp.)...

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. 

Mercury can be determined in plasma AES by reducing it first to elemental mercury and then transporting the mercury vapour into the plasma. The same reduction methods may be used as for AAS. Commercial hydride generation systems can be adopted to the cold vapour method. The detection limit is about 0.02 mgP ... 

Recent improvements in FAAS include the addition of a flow injection sample delivery system. This allows for automatic dilution and reagent addition, and also provides for automated cold vapour or hydride generation needed for As, Se and Hg (e g., Saraswati et al., 1995). Other new techniques are improving the sensitivity of FAAS. A slotted silica tube placed in the flame improves the sensitivity 8 fold forCd and c. 3 fold forCu, Pb, and Zn. Experimental application of graphite tubes in the flame can improve Pb sensitivity 50 fold (Alvarado Jaffe, 1998). 

Most of the generated vapour is condensed in spray condensers which are equipped with circulation pumps and an EG cooler. The vapour that is still uncondensed is withdrawn from the gas phase with the help of a vapour jet which is located down-stream behind the spray condenser and generates the necessary vacuum in the reaction zone. The most critical part of the spray condenser system is the end of the pipe leading the vapour from the prepolycondensation reactors and the finishers into the spray condenser. The transition from a hot to a cold environment causes deposition of solid material onto the cold walls which has to be removed manually or by means of a mechanical scraper. 

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