Atomizers and Nebulizers

There are a number of techniques for generating aerosols, and these are discussed in detail in the LBL report (1979) and in volumes edited by Willeke (1980) and Liu et al. (1984). We briefly review here the major methods currently in use these include atomizers and nebulizers, vibrating orifices, spinning disks, the electrical mobility analyzer discussed earlier, dry powder dispersion, tube furnaces, and condensation of vapors from the gas phase. 

Aerosols may be produced by atomizing liquids or suspensions of solids in liquids. Nebulizers are a type of atomizer in which both large and small particles are initially produced but in which the large particles are removed by impaction within the nebulizer. As a result, only particles with diameters 10 /xm exit most nebulizers. 

FIGURE 11.76 Schematic diagram of compressed air nebulizer (from Hinds, 1982). 

The larger droplets are removed by impaction on the curved wall, and the smaller particles exit the device. Detailed descriptions of other types of compressed air nebulizers that differ somewhat in design are found in Raabe (1976) and Hinds (1982). 

These compressed air nebulizers produce polydisperse aerosols. After the aerosol is produced, the size distribution may change due to evaporation of liquid from the droplets. In addition, the particles may be electrically charged due to an ion imbalance in the droplets as they form if such charges become further concentrated due to evaporation, the particle may break up into smaller particles. Thus electrical neutralization of the aerosol, for example, by exposure to a radioactive source, is usually necessary to prevent electrostatic effects from dominating the particle motion, coagulation, and other behavior. 

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Specific advancements ia the chemical synthesis of coUoidal materials are noteworthy. Many types of genera ting devices have been used to produce coUoidal Hquid aerosols (qv) and emulsions (qv) (39—43) among them are atomizers and nebulizers of various designs (30,44—50). A unique feature of produciag Hquid or soHd coUoids via aerosol processes (Table 3) is that material with a relatively narrow size distribution can be routinely prepared. These monosized coUoids are often produced by relying on an electrostatic classifier to select desired particle sizes ia the final stage of aerosol production. 

Niven, Ralph W. Atomization and nebulizers. In Inhalation Aerosols Hickey, A.J., Ed. Marcel Dekker, Inc. New York, 1996 273-312. 

Niven RW. Atomization and nebulizers. In Hickey AJ, ed. Inhalation Aerosols Physical and Biological Basis for Therapy. New York Marcel Dekker, 1996 273-312. 

In the early literature, the terms atomizer and nebulizer tended to be used interchangeably, but modern practice is to reserve the term nebulizer for collision devices that contain a barrier, and an atomizer for those that do not contain a barrier. 

Niven, R. W., Atomization and Nebulizers in Inhalation Aerosols, Anthony J. Hickey, Marcel Dekker, New York, 1996-Vol. 94 of Lung Biology in Health and Disease, Ed. Claude Lenfant, pp. 273-312. 

Precision For absorbances greater than 0.1-0.2, the relative standard deviation for atomic absorption is 0.3-1% for flame atomization, and 1-5% for electrothermal atomization. The principal limitation is the variation in the concentration of free-analyte atoms resulting from a nonuniform rate of aspiration, nebulization, and atomization in flame atomizers, and the consistency with which the sample is heated during electrothermal atomization. 

Flame Sources Atomization and excitation in flame atomic emission is accomplished using the same nebulization and spray chamber assembly used in atomic absorption (see Figure 10.38). The burner head consists of single or multiple slots or a Meker-style burner. Older atomic emission instruments often used a total consumption burner in which the sample is drawn through a capillary tube and injected directly into the flame. 

An inductively coupled plasma formed by passing argon through a quartz torch is widely used for the mass spectroscopic analysis of metal compounds separated by online HPLC.6 Samples are nebulized on introduction into the interface. Plasma impact evaporates solvent, and atomizes and ionizes the analyte. Applications include separation of organoarsenic compounds on ion-pairing F4PLC and vanadium species on cation exchange. 

From the sample solution to be analyzed, small droplets are formed by the nebulization of the solution using an appropriate concentric or cross-flow pneumatic nebulizer/spray chamber system. Quite different solution introduction systems have been created for the appropriate generation of an aerosol from a liquid sample and for separation of large size droplets. Such an arrangement provides an efficiency of the analyte introduction in the plasma of 1-3 % only.6 The rest (97 % to 99%) goes down in the drain.7 Beside the conventional Meinhard nebulizer, together with cooled or non-cooled Scott spray chamber or conical spray chamber, several types of micronebulizers together with cyclonic spray chambers are employed for routine measurements in ICP-MS laboratories. The solvent evaporated from each droplet forms a particle which is vaporized into atoms and molecules... 

Multi-element determination of dissolved metals at ultratrace level may be performed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). U.S. EPA s Methods 200.8 and 1638 present a methodology for measuring trace elements in waters and wastes by the above technique. Sample is acid digested and the solution is introduced by pneumatic nebulization into a radio-frequency plasma. The elements in the compounds are atomized and ionized. The ions are extracted from the plasma through a differentially pumped vacuum interface and separated by a quadrupole mass spectrometer by their mass to charge ratios. The mass spectrometer must have a resolution capability of 1 amu peak width at 5% peak height. 

ICP-MS is a multielement technique that is suitable for trace analysis it offers a long linear range and low background for most elements. ICP-MS is a technique where the ions produced in inductively coupled plasma are separated in a mass analyzer and detected. The sample solution is fed into a nebulizer by a peristaltic pump. The nebulizer converts the liquid sample into a fine aerosol that is transported into the plasma by an Ar gas flow. In the plasma the sample is evaporated, dissociated, atomized, and ionized to varying extents. The positive ions and molecular ions produced are extracted into the mass analyzer. Detailed descriptions of the ICP-MS technique can be found in a number of textbooks.13,14... 

The role of the sample introduction system is to convert a sample into a form that can be effectively vaporized into free atoms and ions in the ICP. A peristaltic pump is typically used to deliver a constant flow or sample solution (independent of variations in solution viscosity) to the nebulizer. Several different kinds of nebulizers are available to generate the sample aerosol, and several different spray chamber designs have been used to modify the aerosol before it enters the ICP Gases can be directly introduced into the plasma, for example, after hydride generation. Solids can be introduced by using electrothermal vaporization or laser ablation. 

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