An Overview of Basic Principles of Analytical Methods - Field Sampling: Principles and Practices in Environmental Analysis - page 262

10.7.3.1. Atomic Spectrometric Methods
Atomic absorption spectrometry methods can be of two types—absorption and emission.
If a single wavelength of light (monochromatic) passes through a hot gas that contains
metal * in an atomic state, some will be absorbed. The specific wavelength of needed light
is produced by a hollow cathode lamp (HCL), which has a cathode made of the metal of
interest. A sample is passed through an atomizer and into a flame, which produces the hot
gas through which the light is passed. The intensity of the analytical wavelength of light
is decreased by absorption of the light by the metal in the flame. Any wavelengths not
absorbed by the metal are not affected by it. The decrease in the recorded light intensity
is proportional to the amount of metal present. Thus, the amount of light absorbed can be
related back to the metal present in the sample and from there back to the field.
In atomic emission spectrometry, a wavelength of light characteristic of the metal
being analyzed is isolated and recorded. As described above, the sample is passed
through an atomizer into a flame where the metal is excited and gives off wavelengths of
light specific for that metal. The wavelength of light of interest is isolated using a
monochromator and recorded. The amount of light recorded is proportional to the amount
of metal in the flame and can be related back to its concentration in the sample and the
field.
In both absorption and emission spectroscopy the metal must be excited in a flame. For
many elements the hotter the flame the more sensitive the analysis. There are, however,
some similar techniques using the same basic instrument that do not require a flame,
although the sample is still heated to high temperature. In all cases a wavelength of light,
which may be either in the ultraviolet or visible region of the spectrum, specific to the
metal being analyzed for is used in the analysis.
There is another way in which an atom can be excited so that it gives off light of
specific wavelengths, and this is by being bombarded by X rays. In this method, called X-
ray fluorescence spectrometry, a sample is placed in a beam of high-energy X rays and
irradiated. The metals in the sample are excited, absorb energy, and reradiate this
absorbed energy as light of specific wavelengths. The wavelength of light given off is
specific for specific elements, and the amount of light produced can be related back to the
amount of metal present in the sample. In addition to X rays, neutron and gamma
radiation can also be used, although the instrumentation is different.
Anything done during sampling, transportation, and storage that changes the metal
content of the sample will change the results of this analytical method. This can happen
anytime the sample comes in contact with any metal because almost all metals in
common use are mixtures. Even if the analysis is not for iron, exposure of the sample to
iron could cause it to be contaminated with other metals in the iron. Chromium, nickel,
and manganese are commonly found in materials made from iron and could contaminate
a sample left in contact with it. Any contact between the sample and any metal should be
avoided [6].
* Although this discussion only considers metals, some nonmetals can also be analyzed by this
method.