Spray and Desolvation Chambers

The first form of aerosol modifier is a spray chamber. It is designed to produce turbulent flow in the argon carrier gas and to give time for the larger droplets to coalesce by collision. The result of coalescence, gravity, and turbulence is to deposit the larger droplets onto the walls of the spray chamber, from where the deposited liquid drains away. Since this liquid is all analyte solution, clearly some sample is wasted. Thus when sensitivity of analysis is an issue, it may be necessary to recycle this drained-off liquid back through the nebulizer. 

Having removed the larger droplets, it may remain only to encourage natural evaporation of solvent from the remaining small droplets by use of a desolvation chamber. In this chamber, the droplets are heated to temperatures up to about 150 C, often through use of infrared heaters. The extra heat causes rapid desolvation of the droplets, which frequently dry out completely to leave the analyte as small particles that are swept by the argon flow into the flame. 

Having assisted desolvation in this way, the carrier gas then carries solvent vapor produced in the initial nebulization with more produced in the desolvation chamber. The relatively large amounts of solvent may be too much for the plasma flame, causing instability in its performance and, sometimes, putting out the flame completely. Therefore, the desolvation chamber usually contains a second section placed after the heating section. In this second part of the desolvation chamber, the carrier gas and entrained vapor are strongly cooled to temperatures of about 0 to -10 C. Much of the vapor condenses out onto the walls of the cooled section and is allowed to drain away. Since this drainage consists only of solvent and not analyte solution, it is normally directed to waste. 

Introduction of sample solution via a nebulizer may need both spray and a desolvation chamber, but a well-designed, efficient nebulizer needs neither. 

Nebulizers are used to introduce analyte solutions as an aerosol spray into a mass spectrometer. For use with plasma torches, it is necessary to produce a fine spray and to remove as much solvent as possible before the aerosol reaches the flame of the torch. Various designs of nebulizer are available, but most work on the principle of interacting gas and liquid streams or the use of ultrasonic devices to cause droplet formation. For nebulization applications in thermospray, APCI, and electrospray, see Chapters 8 and 11. 

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Many designs of nebulizer are commonly used in ICP/MS, but their construction and mode of operation can be collated into a small number of groups pneumatic, ultrasonic, thermospray, APCI, and electrospray. These different types are discussed in the following sections, which are followed by further sections on spray and desolvation chambers. 

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

The thermospray device produces a wide dispersion of droplet sizes and transfers much of sample solution in unit time to the plasma flame. Therefore, it is essential to remove as great a proportion of the bigger droplets and solvent as possible to avoid compromising the flame performance. Consequently, the thermospray device usually requires both spray and desolvation chambers, especially for analyte solutions in organic solvents. 

Thermospray nebulizers are somewhat expensive but can be used on-line to a liquid chromatographic column. About 10% of sample solution is transferred to the plasma flame. The overall performance of the thermospray device compares well with pneumatic and ultrasonic sprays. When used with microbore liquid chromatographic columns, which produce only about 100 pl/min of eluant, the need for spray and desolvation chambers is reduced, and detection sensitivities similar to those of the ultrasonic devices can be attained both are some 20 times better than the sensitivities routinely found in pneumatic nebulizers. 

The simplest desolvation chambers consist simply of a tube heated to about 150°C through which the spray of droplets passes. During passage through this heated region, solvent evaporates rapidly from the droplets and forms vapor. The mixed vapor and residual small droplets or particulates of sample matter are swept by argon through a second cooled tube, which allows vapor to... 

Solutions can be examined by ICP/MS by (a) removing the solvent (direct and electrothermal methods) and then vaporizing residual sample solute or (b) nebulizing the sample solution into a spray of droplets that is swept into the plasma flame after passing through a desolvation chamber, where excess solvent is removed. The direct and electrothermal methods are not as convenient as the nebulization inlets for multiple samples, but the former are generally much more efficient in transferring samples into the flame for analysis. 

To assist in the deposition of these larger droplets, nebulizer inlet systems frequently incorporate a spray chamber sited immediately after the nebulizer and before the desolvation chamber. Any liquid deposited in the spray chamber is wasted analyte solution, which can be run off to waste or recycled. A nebulizer inlet may consist of (a) only a nebulizer, (b) a nebulizer and a spray chamber, or (c) a nebulizer, a spray chamber, and a desolvation chamber. Whichever arrangement is used, the object is to transfer analyte to the plasma flame in as fine a particulate consistency as possible, with as high an efficiency as possible. 

Complete descriptions of the particle beam, its operation, its experimental setup, and its utility in protein structural studies have been previously described. (8, 12). Relevant PB dimensions include a 25 pm diameter fused silica capillary for production of the aerosol spray, a 22 cm length desolvation chamber to remove solvent, a single stage momentum separator, and a nozzle-substrate distance of 5 mm. Particle beam deposits ranged in size from 20 pm to 100 pm in diameter, and averaged approximately 50 pm. Deposit were made onto a water insoluble calcium fluoride (CaFj) window (25 mm dia. x 2 mm) from International Crystal Laboratories (Garfield, NJ). 

To assist in the deposition of these larger droplets, nebulizer inlet systems frequently incorporate a spray chamber sited immediately after the nebulizer and before the desolvation chamber. Any liquid deposited in the spray chamber is wasted analyte solution, which can be run off to waste or recycled. 

The Thermospray jet is introduced into a spray chamber which is heated sufficiently to complete the vaporization process. Helium is added through a gas inlet in sufficient quantity to maintain the desired pressure and flow rate. The fraction of the solvent vaporized in the thermospray vaporizer and the temperature of the desolvation chamber is adjusted so that essentially all of the solvent is vaporized within the desolvation region. The Thermospray system allows very precise control of the vaporization so that all of the solvent can be vaporized while most of even slightly less volatile materials will be retained in the unvaporized particles. 

The apparatus in its simplest form is shown schematically in Figure 2. The Thermospray vaporizer is installed in the heated desolvation chamber through a gas-tight fitting. The helium is introduced through a second fitting and flows around the Thermospray vaporizer and entrains the droplets produced in the Thermospray jet. In general the carrier gas flow required is at least equal to the vapor flow produced by complete vaporization of the liquid input. The heated zone within the spray chamber should be sufficiently... 

Figure 11.7. Sample nebulizer, spray chamber, and desolvation apparatus for ICP discharge spectrometry. (Not to scale.) Adapted from R. N. Kniseley, V. A. Fassel, and C. C. Butler, Clin. Chem., 19, 807 (1973), by permission of the publisher.

Spray Chambers and Desolvation Systems. A nebulizer must produce droplets less than 10 /im in diameter in order to achieve a high aerosol transport efficiency (the percentage of the mass of nebulized solution that reaches the plasma), and rapid desolvation, volatilization, and atomization of the aerosol droplets. Pneumatic nebulizers, especially, produce highly poly dispersive aerosols with droplets up to 100 jwm in diameter and these large droplets must be removed by a spray chamber. 

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