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C18

C18 is a designation used in several fields to indicate an 18-carbon unit or octadecyl group. In chemistry and analytical chemistry, C18 typically refers to octadecyl (C18) groups bonded to silica or other supports, producing C18 or octadecylsilane phases. These are used as stationary phases in reversed-phase high-performance liquid chromatography (RP-HPLC) and for solid-phase extraction. The C18 phase retains nonpolar compounds through hydrophobic interactions; retention generally increases with the hydrophobicity and chain length of the analyte. Columns labeled C18 are available in varied pore sizes and particle geometries and are widely used for separating a broad range of organic molecules, peptides, and small biomolecules.

In lipid biochemistry, C18 denotes the class of 18-carbon fatty acids, often written as C18:0 or C18:1

In nuclear science, carbon-18 refers to a rare or highly unstable isotope of carbon with mass number

Beyond these, the designation C18 may appear in other technical contexts where the meaning depends on a

to
indicate
the
degree
of
unsaturation.
They
are
major
components
of
dietary
fats
and
cellular
membrane
lipids.
The
physical
and
biological
properties
of
these
fatty
acids
depend
on
saturation
and
the
configuration
of
double
bonds,
influencing
melting
points,
fluidity
of
membranes,
and
metabolism.
18.
It
is
produced
only
transiently
in
laboratory
experiments
and
has
no
natural
abundance;
due
to
rapid
decay,
it
has
limited
practical
use
outside
experimental
research.
specific
field
or
application.