Slurry nebulization

Initial work on slurry nebulization shows this to be a very promising method of solid sample introduction for ICP-MS. It has been successfully applied for analysis of geological and soil samples, and industrial catalysts. 

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Slurry nebulization has also proved very popular. In this technique, sample (typically 0.25 g) is placed in a 30 ml plastic bottle and 10 g of expanded zirconia beads are added. A dispersant is added and the bottle is sealed and then placed on a mechanical shaker for several hours. During the shaking, the zirconia beads grind the sample into very fine particles. After dilution to a known volume, the slurry may be aspirated directly into an atomic spectrometric instrument. Other methods of slurry preparation also exist, e.g. using a micronizer, but the bottle and bead method is the most common. 

To date, slurry nebulization has not found widespread or routine application in environmental analysis by flame spectrometry. In the author s experience the time saving is small or even non-existent, because the use of standard additions procedures is time consuming, and samples often have to be processed one at a time when slurry atomization is to be used. 

Solid foods in powder form can be analyzed directly by means of LA- or ETV-ICP-MS to eliminate time-consuming sample dissolution procedures (see Table 8.2). However, this requires the preparation of homogeneous powdered samples and the subsequent analytical determination is not as straightforward as the one based on liquid sample introduction. Another way to perform direct analysis of solid foods is to grind and suspend them into slurries. The viability of slurry nebulization relies on the ability to prepare samples of fine particle size in a reproducible manner and on the adoption of suitable (e.g., high-solids) nebulizers. Otherwise, slurries can be analyzed by ETV-ICP-MS resorting to the ultrasonic slurry sampling technique [72-74]. 

P. J. Mckinstry, H. E. Indyk, N. D. Kim, The determination of major and minor elements in milk and infant formula by slurry nebulization and inductively coupled plasma-optical emission spectrometry (ICP-OES), Food Chem., 65 (1999), 245-252. 

Some authors [46] claim that using slurry nebulization is only advisable when no efficient alternative digestion procedure is available. The dilution factor of the sample in slurry nebulization is at least as high as in normal solution nebulization. The concentrations... 

Work with slurries requires that the slurries are first nebulized and behave just as solutions with respect to the sample introduction into the aerosol. From electron probe micrographs of aerosol particles sampled on Nuclepore filters under isokinetic conditions, it was found that at nebulizer gas flows of 3 L/min, being typical of plasma spectrometry but far below those for flame atomic absorption, particles with a diameter of up to 15 pm can be found in the aerosol (Fig. 44) [117]. This would imply that powders with a grain size of up to about 15 pm could still be nebulized as could a solution. This, however, is not true as the mass distribution in the case of powders may be quite different in the slurry and in the aerosol, as shown for the case of SiC (Fig. 45) [118]. The nebulization limitations for the case of slurry nebulization thus must be investigated from case to case and leads to certain types of restrictions. 

After solvent removal the aerosols produced from slurries deliver solid particles, the diameters of which are those of the powder particles. In slurry nebulization used for flame work or plasma spectrometry, they are injected with a velocity that is less than or equal to the nebulizer gas atom velocities, as viscosity drag forces are responsible for their entrainment into the ICP. The velocity of the gas atoms (vG) can be calculated from the gas temperature at the location considered (TG), the injection velocity (vi) and the temperature at the point of injection (T), as vG = Vi x TG/Ti and the acceleration of particles (d2z/dt2) as a result of the viscosity drag forces is ... 

Van Borm W. A. and Broekaert J. A. C. (1990) Noise characteristics in inductively coupled plasma optical emission spectrometry using slurry nebulization and direct powder introduction techniques, Anal Chem 62 2527-2532. 

Van Borm W., Broekaert J. A. C., Klockenkamper R., Tschopel P. and Adams F. C. (1991) Aerosol sizing and transport studies with slurry nebulization in inductively coupled plasma spectrometry, Spectrochim Acta, Part B 46 1033-1049. 

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