Environmental Engineering Reference
In-Depth Information
(approximately 360 to 800 nm) used to illuminate the sample (refer to Chapter 8).
This implies that features on the sample smaller than this dimension cannot be
visualized using the light microscopes and a shorter wavelength is needed. In
electron microscopy, this is commonly achieved using a high voltage beam of
electrons. When using scanning electron microscopes (SEM), samples are first
coated with a metal that readily reflects electrons. This coating is also essential to
conducting surface for electrons to avoid charging of the sample. Electrons are
emitted from a wire (usually tungsten or lanthanum hexaboride) that has been
superheated by an electric current. Since the emitted electrons have a Boltzman
energy distribution (see Chapter 9.1, Eq. 9.3), magnetic coils are used to focus and
direct a passing electron beam into the sample. This is analogous to the lens and
prisms used in optical microscopes to select the desired wavelength. When this
incoming condensed electron beam is scanned over the object, an image is formed
by the electrons that bounce off the surface of the object under examination. This
image is collected onto an imaging screen, so the observer sees a picture of the
surface of the sample.
The scanning tunneling microscopes (STM), developed in 1981, are capable of
resolving features on an atomic scale on the surface of a conducting solid surface. In
STM, the surface of a sample is scanned by a very fine metallic tip. This sharp tip is
moved up and down, based on the topographical structure of the surface, by
monitoring the tunneling currents between the tip and sample surface in order to
maintain the tip at a constant distance from the sample surface. A tunneling current
is a current that passes through a medium (vacuum, a nonpolar liquid) that contains
no electrons. The mechanism of current flow can be rationalized by quantum
mechanisms, but is beyond the scope of this text.
The atomic force microscopes (AFM), developed in 1986, allows for the
resolution of individual atoms on both conducting and insulating surfaces. In AFM,
a sharp tip mounted on a soft lever is scanned across the sample surface, while the tip
is in contact with the surface. The atomic force acting on the tip changes according
to the sample topography, resulting in a varying deflection of the lever. The deflec-
tion of the lever is measured by means of laser beam deflection of a microfabricated
cantilever and subsequent detection with a double-segment photodiode.
12.3.3 Screening Methods Using Immunoassay
An immunoassay (IA) is a biochemical test that measures the level of a substance
based on the specific reaction of an antibody to its antigen. It is the only simple
method introduced in this chapter that does not rely on expensive instrumentations,
but is used as an alternative measure of chemical analysis. An antibody is a binding
protein used by the immune system, which is produced in response to the
introduction of an alien substance (an antigen). An antigen not only induces an
immune response but will also react with antibodies. Many of the environmental
contaminants (e.g., pesticides) are not able to induce the production of antibodies but
can react with antibodies already present. They are called haptens. To generate
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