Chromatography is a commonly used technique in forensic science. Describe the science behind this technique. Use scientific language as appropriate.

Chromatography is a process used for separating the
substances that make up a mixture, when the concentration of such substances is very
low. Thus chromatography is used in forensic science to detect and measure presence of
specific substances in very small quantities in objects collected as
evidence.

Chromatography achieves separation of different
substances in a mixture by passing the mixture through an adsorbent material. The
adsorbent material has the property of attracting various substances in the mixture to
different extents, causing them to separate from one
another.

There are several chromatographic methods.  Three
of the most commonly used are liquid-column, thin-layer, and gas chromatography. In
liquid-column chromatography, a column of adsorbent material in a tube is used.  A
sample of a mixture under examination is placed at one end of the column.  Then a fluid
called an eluant is poured through the column.  The individual substances in the mixture
move through the column at varying rates because of the difference in their tendency to
be absorbed.  This makes it possible to collect each substance separately as it emerges
from the column or is removed from the adsorbent
material.

Thin-layer chromatography uses a thin film of
adsorbent material on a flat plate.  A sample of the mixture under study is applied near
one edge of the plate.  This edge is brought into contact with the eluant, which moves
upward through the adsorbent material.  The substances in the mixture separate on the
plate.

Gas chromatography is used in analysing gases and
substances that can be easily converted into gases when heated. It is similar to gas
chromatography except that a gas such as helium is used as the eluant to move the
gaseous mixture through a column of adsorbent material.