Page 38 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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and selenium), the signal-to-noise ratio, because of the low signal, may so adversely affect detection
limits that electrodeless discharge lamps can offer improvements. The analytical signal is the ratio of I
to I . Therefore, improved intensity of the signal can never improve sensitivity in AAS.
0
2.2.1.3 Source Requirements in AAS

This leads us to a summary of source requirements in AAS. The source must give a narrow resonance
line profile with little background, and should have a stable and reproducible output of sufficient
intensity to ensure high signal-to-noise ratios. The source should be easy to start, have a short warm-
up time and a long shelf-life.

Q. How are the metal atoms produced and excited in a hollow-cathode lamp?

Q. What is the normally preferred filler gas in a hollow-cathode lamp?

Q. Why must quartz windows be used in sources for AAS?

Q. What are the advantages of radiofrequency-excited electrodeless discharge lamps?

Q. Why does greater source intensity not lead to increased absorbance?

2.2.2 Flames

Several types of atom cell have been used for AAS. Of these, the most popular is still the flame,
although a significant amount of analytical work is performed using various electrically heated graphite
atomizers. This second type of atom cell is dealt with at length in Chapter 3, and the material here is
confined to flames.

In AAS, the flame is only required to produce ground-state atoms (cf. AES, where a hot flame is
preferred as atoms must also be excited). Frequently, an air-acetylene flame is sufficient to do this. For
those elements which form more refractory compounds, or where interferences are encountered (see
Section 2.4), a nitrous oxide-acetylene flame is preferred. In either case, a slot burner is used (100 mm
for air-acetylene, 50 mm for nitrous oxide-acetylene) to increase the path length (this arises from Eqn.
2.3, Section 2.1) and to enable a specific portion of the flame to be viewed. Atoms are not uniformly
distributed throughout the flame and, by