adulterantSee cutting agent.
aerogel A low-density porous trans-
parent material that consists of more
than 90% air. Usually based on metal
oxides or silica, aerogels are used as
drying agents and insulators.
aerosol A colloidal dispersion of a
solid or liquid in a gas. The com-
monly used aerosol sprays contain an
inert propellant liqueÜed under pres-
sure. ChloroÛuorocarbons, such as
dichlorodiÛuoromethane, are com-
monly used in aerosol cans. This use
has been criticized on the grounds
that these compounds persist in the
atmosphere and may lead to deple-
tion of the ozone layer.
AESSee atomic emission spec-
troscopy.
A-factorSee arrhenius equation.
afÜnity chromatography A bio-
chemical technique for purifying nat-
ural polymers, especially proteins. It
functions by attaching a speciÜc lig-
and by covalent bonding to an insolu-
ble inert support. The ligand has to
have a speciÜc afÜnity for the poly-
mer, so that when a solution contain-
ing the ligand is passed down a
column of the material it is speciÜ-
cally retarded and thus separated
from any contaminating molecules.
An example of a suitable ligand is
the substrate of an enzyme, provided
that it does not change irreversibly
during the chromatography.
aÛatoxin A poisonous compound,
C 15 H 12 O 6 , produced by the fungus As-
pergillusÛavus. It is extremely toxic to
farm animals and can cause liver can-
cer in humans. It may occur as a con-
taminant of stored cereal crops,
cotton seed, and, especially, peanuts.
There are four isomeric forms.
AFMSee atomic force microscope.
agarAn extract of certain species of
red seaweeds that is used as a gelling
agent in microbiological culture
media, foodstuffs, medicines, and
cosmetic creams and jellies. Nutrient
agarconsists of a broth made from
beef extract or blood that is gelled
with agar and used for the cultiva-
tion of bacteria, fungi, and some
algae.
agarose A carbohydrate polymer
that is a component of agar. It is used
in chromatography and electrophore-
sis.agateA variety of *chalcedony that
forms in rock cavities and has a pat-
tern of concentrically arranged bands
or layers that lie parallel to the cavity
walls. These layers are frequently al-
ternating tones of brownish-red.
Moss agate does not show the same
banding and is a milky chalcedony
containing mosslike or dendritic pat-
terns formed by inclusions of man-
ganese and iron oxides. Agates are
used in jewellery and for ornamental
purposes.agitatorA bladelike instrument
used in fermenters and *bioreactors
to mix the medium continuously in
order to maintain the rate of oxygen
transfer and to help keep the cells in
suspension.airSee earth’s atmosphere.15 air
a
NONNOHOHNNH 2OHAflatoxin