Page 45 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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Q. Why are different burners needed for different flames?

Q. What are the advantages and disadvantages of air-propane flames?

Q. Why is the air-acetylene flame so popular for AAS?

Q. What causes the red feather observed in the nitrous-oxide acetylene flame?

Q. What advantages does the nitrous oxide-acetylene flame offer for AAS?

2.2.3 Sample Introduction and Sample Atomization

So far we have no analyte atoms in the atom cell! This is usually achieved in the following manner,
although some alternative ways are considered in Chapter 7.

Figure 2.7 shows a typical pneumatic nebulization system for a premixed flame. The sample is sucked
up a plastic capillary tube. In the type of concentric nebulizer illustrated here, the sample liquid is
surrounded by the oxidant gas as it emerges from the capillary. The high velocity of this gas, as it issues
from the tiny annular orifice, creates a pressure drop which sucks up, draws out and 'shatters' the liquid
into very tiny droplets. This phenomenon is known as the venturi effect and is illustrated in Fig. 2.8.
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.

The aerosol then passes along the plastic expansion chamber. Large droplets collect on the walls of the
chamber and, to ensure that only the smallest particles reach the flame, spoilers or baffles may be
placed in the path of the gases. The chamber also allows for mixing of the gases and tends to damp
fluctuations in nebulization efficiency. Some loss of solvent by evaporation will also occur. The
chamber requires a drain tube which must be sealed to provide a back-pressure for the flame. This is
usually