Page 94 - Tandem Techniques

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wavelength pertinent to a particular element, a chromatogram could be constructed that monitors that
specific element. This could be considered as the atomic emission equivalent to single-ion-monitoring
in chromatography/mass spectrometry tandem systems. Employing data for carbon, hydrogen, oxygen
and nitrogen, the empirical formula for an organic compound can also be approximately determined.

Atomic Absorption Spectroscopy

Atomic absorption spectroscopy is another element specific spectroscopic monitoring system that can
determine the presence of specific elements when they exist at high temperature in a flame or in a
graphite furnace. The device is, in fact, the complement of the atomic emission spectrometer, in that the
absorption of light specific to a particular element is measured, as opposed to the light emitted. The
amount of light absorbed is proportional to the amount of the element present which, in turn, is
proportional to the amount of the element that is continuously fed into the flame or furnace.

The Flame Atomic Absorption Spectrometer

A diagram of a flame atomic absorption spectrometer is shown in Figure 2.19. The light source is a cold
cathode lamp that produces, almost exclusively, the light that would be naturally emitted by the element
to be measured. A whole range of such lamps are available that includes the vast majority of the
elements of general analytical interest. Consequently, the light will contain specifically those
wavelengths that the element in the flame will selectively absorb. The light passes through the flame,
which is usually rectangular in shape so as to provide an adequate path length of flame for the light to
be absorbed, and then into the optical system of the spectrometer. The flame is fed with a combustible
gas, customarily air/acetylene, nitrous oxide/acetylene or air/propane or butane. The sample, dissolved
in a suitable solvent, is nebulized and fed into the gas stream at the base of the burner. The light, having
passed through the flame, can be focused directly onto a photo cell or onto a diffraction grating by
means of a spherical mirror. The diffraction grating is