Spray chambers typical designs

Three different spray chamber designs (Fig. 3.6) are most often used for ICP-MS the Scott [15] (double-barrel) chamber, a conical chamber with an impact bead, and a cyclonic chamber [14,16,17]. The cyclonic spray chamber typically provides a slightly (up to about a factor of 2 or 3) higher analyte transport efficiency as well as somewhat shorter washout times. In some cases the spray chamber is cooled (such as on the HP 4500 ICP-MS double-pass spray chamber, which is cooled to 4°C) to reduce the amount of water vapor that enters the ICP further so that signals from polyatomic ions containing oxygen are reduced. The cooled spray chamber also helps maintain a stable spray chamber temperature. 

In the cyclonic spray chamber droplets are discriminated according to their size by means of a vortex produced by the tangential flow of the nebulizer. Smaller droplets are transferred with the sample aerosol into the ICP-MS, while the larger droplets collide with the walls and exit by gravity via a drain tube. If compared to the Scott design, the cyclonic spray chamber typically shows higher sensitivity. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

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. 

Contact condensers employ liquid coolants, usually water, which come in direct contact with condensing vapors. These devices are relatively uncomplicated, with typical configurations illustrated in Figure 14. Some contact condensers are simple spray chambers, usually with baffles to ensure adequate contact. Others, incorporate high-velocity jets designed to produce a vacuum. 

Figure 7.33 (a) The components of a liquid sample introduction system for ICP glass concentric nebulizer, demountable three piece quartz torch, torch body, and glass cyclonic spray chamber, (b) The components assembled for use. This torch design is typical of that used in Jobin Yvon ICP instruments. [Courtesy of Glass Expansion Pty, Ltd., Australia (www.GEICP.com).]... 

A wide variety of spray-chamber designs has been employed for fluoride removal from tall cylindrical towers to horizontal vessels with cross- or cocurrent flow of water and gas. In typical spray chambers, water is introduced at several points from nozzles operating with water pressures of 15 to 60 psig. 

These valves are essentially designed to work in the inverted (stem down) position. Depression of the valve stem allows the contents of the metering chamber to be dispensed through the orifice in the valve stem. After actuation, the metering chamber refills from the bulk liquid formulation, once the metering chamber is sealed from the atmosphere and is ready to dispense the next dose. This is essential otherwise, continuous spray would be achieved. Typical volumes that are dispensed range from 25 to 100 pL. The accuracy of the dosing... 

Essentially, three major types of atomizers are used in practice (1) rotary wheel (or disk) atomizers, (2) pressnre nozzle, and (3) two-fluid nozzle. Figure 23.20 shows some typical atomizer designs. Ultrasonic and electrostatic atomizers can also be used for special applications to produce monodisperse sprays, but they are very expensive and low capacity. Most spray dryers operate at slight negative pressnre. New designs with low-pressure chambers enhance drying rates at lower temperatnres to dry heat-sensitive products. 

---