Gas Chromatography - Tandem Techniques - The Characteristics of Atomic Absorption > The Flame Atomic Absorption Spectrometer > Page 33
Courtesy of the Hewlett-Packard Corporation
Figure 21. The Helium Plasma Atomic Emission Spectrometer
The diode array can have any number of diodes, but the particular instrument manufactured by Hewlett-Packard and shown above has 211 diodes but its position is adjustable. Consequently, by movement of the diode array sensor, different wavelength ranges can be selected for monitoring from the complete spectrum provided by the grating. The total wavelength range covered by the particular instrument depicted in figure 14 is from 170 nm to 800 nm. Each diode element is 61 mm wide and the entrance slit is 50 mm wide. When used in combination with a gas chromatograph, the diode array would be scanned continuously during the development of the chromatogram and the output from each diode stored by the computer. As a consequence a chromatogram can be reconstructed that monitors the presence of a specific element only by selecting the wavelength characteristic of the particular element. From an analytical point of view this procedure can be extremely useful for monitoring certain metal species present in biological samples.
This procedure might be considered as the atomic emission equivalent to single-ion-monitoring in chromatography/mass spectrometry combined systems which will be discussed later. Another interesting application of the chromatography atomic spectrometry combination is its use to obtain an approximate empirical formula of an eluted solute. By monitoring the specific elements carbon, hydrogen, oxygen and nitrogen and comparing peak areas that are corrected for the elements specific response the empirical formula of an organic compound can also often be determined.
Atomic Absorption Spectroscopy
Atomic absorption spectroscopy is another element specific spectroscopic monitoring system. This process can identify the presence of specific elements when they exist at high temperature in a flame or in a graphite furnace. The results can also provide a quantitative estimation of the element content. Atomic adsorption spectrometry is the complement of atomic emission spectrometry, in that the absorption of light specific to a particular element is measured, as opposed to the light emitted. In principle, the amount of light absorbed is proportional to the amount of the element that is 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 22. The light source used in an atomic adsorption spectrometer is usually a cold cathode lamp that is designed to produce (almost exclusively) light of that wavelength that would be naturally emitted by the element of interest. Lamps are available for the majority of the elements of general analytical interest. It follows, that the light generated will contain specifically those wavelengths that the element in the flame will selectively absorb. The light beam passes through the flame which is usually rectangular in shape to provide an adequate path length of flame for the light to be absorbed.
Figure 22. The Flame Atomic Absorption Spectrometer