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associated with this type of transition are found in the UV-visible part of the
electromagnetic spectrum. In this respect atomic spectroscopy is similar to UV-visible
spectroscopy, however, the samples used in atomic spectroscopy are individual atoms
in a gaseous state, whereas those used in UV-visible spectroscopy are molecules
dissolved in liquids. This has important consequences for the nature of the spectra
produced. In atomic spectroscopy the peaks are narrow and well defined, but in UV-
visible spectroscopy they are broad and overlap with one another. The are two major
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reasons for this. Firstly, because absorption or emission is from atoms, rather than
molecules, there are no vibrational or rotational transitions superimposed on the
electronic transitions. Secondly, because the atoms are in a gaseous state they are well
separated from each other and do not interact with neighboring molecules.
The energy change associated with a transition between two energy levels is
related to the wavelength of the absorbed radiation: " E = hc/» , where, h = Planks
constant, c = the speed of light and » = the wavelength. Thus for a given transition
between two energy states radiation of a discrete wavelength is either absorbed or
emitted. Each element has a unique electronic structure and therefore it has a unique
set of energy levels. Consequently, it absorbs or emits radiation at specific
wavelengths. Each spectrum is therefore like a "fingerprint" that can be used to
identify a particular element. In addition, because the absorption and emission of
radiation occurs at different wavelengths for different types of atom, one element can
be distinguished from others by making measurements at a wavelength where it
absorbs or emits radiation, but the other elements do not.
Absorption occurs primarily when electrons in the ground state are promoted
to various excited states. Emission occurs when electrons in an excited state fall back
to a lower energy level. Atoms can exist in a number of different excited states, and
can fall back to one of many different lower energy states (not necessarily the ground
state). Thus there are many more lines in an emission spectra than there are in an
absorption spectra.
Atomic spectroscopy is used to provide information about the type and
concentration of minerals in foods. The type of minerals is determined by measuring
the position of the peaks in the emission or absorption spectra. The concentration of
mineral components is determined by measuring the intensity of a spectral line known
to correspond to the particular element of interest. The reduction in intensity of an
electromagnetic wave that travels through a sample is used to determine the
absorbance: A = -log(I/I ). The Beer-Lambert law can then be used to relate the
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absorbance to the concentration of atoms in the sample: A = a.b.c, where A is
absorbance, a is extinction cofficient, b is sample pathlength and c is concentration of
absorbing species. In practice, there are often deviations from the above equation and
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so it is often necessary to prepare a calibration curve using a series of standards of
known concentration prepared using the same reagents as used to prepare the sample.
It is also important to run a blank to take into account any impurities in the reagents
that might interfere with the analysis.
Atomic Absorption Spectroscopy
Atomic absorption spectroscopy (AAS) is an analytical method that is based
on the absorption of UV-visible radiation by free atoms in the gaseous state. The food
sample to be analyzed is normally ashed and then dissolved in an aqueous solution.
This solution is placed in the instrument where it is heated to vaporize and atomize the
minerals. A beam of radiation is passed through the atomized sample, and the
absorption of radiation is measured at specific wavelengths corresponding to the
mineral of interest. Information about the type and concentration of minerals present is
obtained by measuring the location and intensity of the peaks in the absorption spectra.
Instrumentation
The radiation source. The most commonly used source of radiation in AAS is
the hollow cathode lamp. This is a hollow tube filled with argon or neon, and a cathode
filament made of the metallic form of the element to be analyzed. When a voltage is
applied across the electrodes, the lamp emits radiation characteristic of the metal in the
cathode i.e., if the cathode is made of sodium, a sodium emission spectrum is
produced. When this radiation passes through a sample containing sodium atoms it will
be absorbed because it contains radiation of exactly the right wavelength to promote
transition from one energy level to another. Thus a different lamp is needed for each
type of element analyzed.
Chopper. The radiation arriving at the detector comes from two different
sources: (i) radiation emitted by the filament of the lamp (which is partially absorbed
by the sample); (ii) radiation that is emitted by the atoms in the sample that have been
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