Impact bead

The nebulizer capillary position may be adjustable on a screw thread to permit optimization of sample uptake and drop size. Alternatively or additionally, an impact bead may be placed in the path of the initial aerosol to provide a secondary fragmentation and so improve the efficiency of nebulization. Such a device is illustrated in Fig. 2.9. 

The material of the nebulizer must be highly corrosion resistant. Commonly, the plastic capillary is fixed to a platinum-iridium alloy (90 10) capillary mounted in stainless-steel gas supply inlets. The impact bead is sometime made of a similar alloy or smooth borosilicate glass. 

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. 

Figure 6 Spray chambers (a) Scott, double-pass design, (b) Conical chamber with impact bead, (c) Cyclone spray chamber (top view), (d) Cyclone spray chamber (side view).

Thus, it can be seen that there are several variables associated with the flame atomiser that must be optimised to achieve the best sensitivity and detection limit. The flame must be correctly positioned with respect to the light path. The fuel oxidant ratio should be investigated to establish the optimum chemical environment for atomisation. The nebuliser and impact bead (where fitted) must be optimised to produce, overall, the best signal-to-noise ratio. 

The acid concentration should be as low as possible. The nebuliser fitted as standard may have a stainless steel capillary tube which will be attacked by acid over a period of time. Solutions containing more than 5% mineral acid should be nebulised using a nebuliser having a platinum/iridium capillary. A corrosion-resistant nebuliser having a plastic throat is required for solutions containing hydrofluoric acid. Additionally, in this instance, a teflon impact bead must replace the standard glass one where these are employed. 

Virtually all modern flame AAS (FAAS) instruments make use of a pre-mix nebuliser in combination with a laminar burner design. A typical design is presented in Figure 5. The flows of gaseous fuel and oxidant gases into the nebuliser create a Venturi effect across the exit of a capillary tube. As a result, liquid sample is aspirated through the capillary (at rates of 2 to 6 mL/min) and exits into the nebuhser chamber as an aerosol (with a rather wide range of droplet sizes). An impact bead or a flow spoiler is typically used to further smash up the droplets and to increase turbulence of the flow... 

In the premix burner, the sample, in solution form, is first aspirated into a nebulizer where it forms an aerosol or spray. An impact bead or flow spoiler is used to break the droplets from the nebulizer into even smaller droplets. Larger droplets coalesce on the sides of the spray chamber and drain away. Smaller droplets and vapor are swept into the base of the flame in the form of a cloud. An important feature of this burner is that only a small portion (about 5%) of the aspirated sample reaches the flame. The droplets that reach the flame are, however, very small and easily decomposed. This results in an efficient atomization of the sample in the flame. The high atomization efficiency leads... 

Figure 37 Principle of counter flow (A) and impact bead (B) nebulizers...

Interferences in the condensed phase may also be minimized by reducing particle size or delay time of the sample in the flame. The particle size must be as small as possible so that the evaporation would be fast. Drop size may be controlled by the sample intake rate, use of the impact beads, or counter flow nebulizers, and organic solvents. 

If the analyte concentration is high (more than about 200 times the reciprocal sensitivity), sensitivity should be reduced. In AAS this can be done by removal of the impact bead, by burner rotation, or use of less sensitive absorption lines. However, if these are not practicable, the solution should be diluted to bring it into the best range for measurement. On the other hand, if the concentration is too low, scale expansion or some chemical pretreatment are required. 

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