Page 51 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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As only moderate resolution is needed (because of the 'lock and key' effect), focal lengths of 0.25-0.50
m and rulings of 600-3000 lines mm are commonly employed. Resolution in the region 0.2-0.02 nm
-1
are claimed.

As atomic emission and atomic absorption take place at the same wavelength, it is useful to be able to
discriminate between the two signals to maximize atomic absorption sensitivity. Figure 2.12 shows how
this may be done. In Walsh's first design (Fig. 2.12a), the light from the lamp passes through the flame
(the source being at the focus of the first lens) in a parallel beam which is focused on the entrance slit of
the monochromator, this being at the focus of the second lens. The flame is placed at the focus of the
second lens so that flame emission is exactly defocused at the monochromator. Thus, the atomic
absorption signal is maximized and the atomic emission signal minimized.

The next development was modulation. A rotating sector (often crudely referred to as a chopper) is
placed in the light beam (Fig. 2.12b). As the

Figure 2.12
Optical and instrumental arrangements for AAS. S= source;
L = first lens; L = second lens; F = flame; f= focal length;
1 2
R = rotating sector; M = monochromator; D = detector.