increase in potential becoming more negatively and
less positively charged. Repolarization is when the
state returns to its resting potential. ACTION POTEN-
TIALs are caused by depolarization in nerve cells. An
action potential is a one-way, self-renewing wave of
membrane depolarization that propagates at rapid
speed (up to 120 m/sec) along the length of a nerve
axon. Julius Bernstein first proposed the concept of
depolarization in 1868. The term is also used in refer-
ence to electrochemical cells to identify a process that
reduces concentration gradients at the electrodes, thus
reducing the output potential. The term is also used in
optics to refer to depolarization of polarized light.
deposition Solidification of vapors by cooling; the
opposite of SUBLIMATION. Process by which water
changes phase directly from vapor into a solid without
first becoming a liquid. More generally, a process that
leads to formation of a solid material on a surface.
derivative A compound that resembles or arises
from the original compound, except that some modifi-
cations in atomic structure are evident, such as the
replacement of one atom with another or with a
group. The term also refers to the product of certain
mathematical operations.
dermal toxicity Adverse health effects resulting from
skin exposure to a toxic substance.
desferal SeeDESFERRIOXAMINE.
desferrioxamine (DFO) CHELATING agent used
worldwide in the treatment of iron overload condi-
tions, such as HEMOCHROMATOSISand THALASSEMIA.
The iron-free form of ferrioxamine.
deshielding SeeSHIELDING.
detachment The reverse of an ATTACHMENT.
See alsoELECTRON ATTACHMENT.
detailed balancing, principle of When equilibrium
is reached in a reaction system (containing an arbitrary
number of components and reaction paths), as many
atoms, in their respective MOLECULAR ENTITIES, will
pass forward as will pass backward along each indi-
vidual path in a given finite time interval. Accordingly,
the reaction path in the reverse direction must in every
detail be the reverse of the reaction path in the for-
ward direction (provided always that the system is at
equilibrium).
The principle of detailed balancing is a conse-
quence for macroscopic systems of the principle of
MICROSCOPIC REVERSIBILITY.detector A device in a radiometer that senses the pres-
ence and intensity of radiation. The incoming radiation
is usually modified by filters or other optical compo-
nents that restrict the radiation to a specific spectral
band. The information can either be transmitted imme-
diately or recorded for transmittal at a later time.
More generally, the term refers to any device used
to sense or measure the amount or kind of radiation or
substance in an analytical system. The term is also
used to refer to any device that can respond to a par-
ticular signal or substance and provide qualitative or
quantitative information about it, e.g., electrochemical
detectors in HPLC, thermal conductivity detectors in
GC, etc.
See also GAS CHROMATOGRAPHY; HIGH-PERFOR-
MANCE LIQUID CHROMATOGRAPHY.detergent An organic compound or compounds com-
posed of molecules containing both hydrophilic (polar)
and hydrophobic (nonpolar) portions. A petroleum-
based surfactant or emulsifier other than soap.detonate To cause an explosion.deuterium A stable isotope of hydrogen whose
atoms are twice as massive as ordinary hydrogen. Dis-
covered in 1932 by Harold C. Urey. An atom of deu-
terium consists of one proton, one neutron, and one
electron. About 0.015 percent of natural hydrogen is
composed of deuterium.74 deposition