Page 52 - An Introduction to Analytical Atomic Spectrometry - L. Ebdon

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beam strikes the solid part of the chopper, it is interrupted; the hole in the centre allows it to pass. The
sector is placed between the source and the flame. The atomic absorption signal is now modulated, but
the atomic emission signal is not. An AC amplifier tuned to the atomic absorption signal, via phasing
coils on the rotating sector, gates the amplifier and selectively amplifies the atomic absorption signal as
opposed to the DC atomic emission signal. It is therefore essential that the sector be placed between the
source and the flame. In atomic absorption and atomic emission spectrometers, two rotating sectors may
be found, one for atomic absorption and one for atomic emission (between the flame and the
monochromator). The atomic emission chopper only functions when the source is off and is necessary if
the instrument is fitted with an AC amplifier. When the instrument is switched back to atomic
absorption mode, the sector should lodge in the open mode.

Often flame radiation may be reflected from the back of a rotating sector, which is less than totally
reliable because it is mechanical. Most modern instruments therefore modulate the power applied to
the source and also use this signal to trigger the amplifier (Fig. 12c). Such instruments may still have a
rotating sector (but only for atomic emission) between the flame and the monochromator, or the
amplifier may be capable of being reset for DC signals.

In all cases, a photomultiplier is used as the detector and, after suitable amplification, a variety of read-
out devices may be employed (see Section 2.2.5.3).

2.2.5.1 Double Beam Spectrometers

The systems so far described have all been single-beam spectrometers. As in molecular spectrometry, a
double-beam spectrometer can be designed. This is shown diagrammatically in Fig. 2.13. The light
from the source is split into two beams, usually by means of a rotating half-silvered mirror or by a
beam splitter (a 50%-transmitting mirror). The second reference beam passes behind the flame and, at
a point after the flame, the two beams are recombined. Their ratio is then electronically compared.

Double-beam operation offers far fewer advantages in AAS than it does in molecular absorption
spectrometry, mainly because the reference beam does not pass through the most noise-prone area of
the instrument, the flame. Double-beam systems can compensate for source drift, warm-up and source
noise. This should lead to improved precision and often does. However, as the major source of noise is
likely to be the flame, this advantage is slight and may be more than offset by the significant loss of
intensity in the light signal, and hence lower signal-to-noise ratio.