Chromatography is an analytical technique used for
separating a mixture of chemical substances into its
components so that these can be identified or analyzed. Chromatography
comes in many forms, e.g., paper chromatography, liquid chromatography,
gas chromatography, ion-exchange chromatography, but all of these employ
the same basic principles. Chromatography is extensively used in the
semiconductor industry, especially in the identification of contaminants
that cause yield, quality, and reliability problems.
Chromatography can separate a mixture into its components with
In fact, it can be used to distinguish between two very similar
components, such as proteins that may be different only by a single amino
acid. The conditions under which the separation process takes place
are also not severe, allowing the use of chromatography on delicate
products. With the right materials and operating conditions,
chromatography is capable of purifying any soluble or volatile substance.
In all types of chromatography, the
which is a sample of the mixture being analyzed, is applied and allowed
to adhere to a stationary material known as
Another material, known as the
is then made to flow through the adsorbent.
Because the different components of the analyte exhibit varying degrees
of strength of
to the adsorbent, they also travel
through the adsorbent as the eluent flows through it, i.e., components
that adhere more strongly to the adsorbent travel more slowly than those
with weaker adhesion. In effect, this process separates the
various components of the analyte into individual samples that can be
Although the principle above works even in simple techniques such as paper
chromatography, modern chromatographic equipment today employ an
where the actual separation process takes place. The column is
usually made of glass or metal tube that is capable of withstanding the
range of pressure that may be applied to it.
As discussed above, the column contains both a stationary phase and a
mobile phase. A column is said to be a
column if its
stationary phase is in granular form and is packed into the column,
completely filling it as a homogeneous bed. A column is
if it has a
hollow center acting as a 'passageway,' with its stationary phase confined
to the inner wall of the column as a film or layer.
the mobile phase is generally a chemically inert gas, such as nitrogen,
helium, argon, and carbon dioxide. The sample to be analyzed by gas
chromatography is vaporized and then injected into the column. It is
then transported through the column by the flow of the mobile phase.
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the mobile phase is usually a liquid of low viscosity that is flowing
through the stationary phase bed. This bed usually consists of one of the
following: 1) an immiscible liquid that coats a porous support; 2) a thin
film of liquid bonded to the surface of a sorbent; or 3) a sorbent of
controlled pore size.
the stationary phase packings of the column usually consist of
ion-exchange resins bonded to inert polymeric particles of small diameter
(typically 10 microns). For cation separation, the ion exchange resin is
usually sulfonic or carboxylic acid, while for anion separation, the
ion-exchange resin is usually a quaternary ammonium group.
basic chromatography process consists of the following steps: 1)
wherein the analyte is injected into the mobile phase or carrier fluid; 2)
separation of the analyte
in the column into its components as the mobile phase flows through the
stationary phase, by virtue of the varying degrees by which these
components are attracted to the stationary phase; 3)
wherein the different components of the sample will emerge from the column
at different times, with the component that's least bound to the
stationary phase eluting first; and 4)
wherein the eluted components are collected and analyzed, usually by
measuring certain properties of the components, such as the refractive
index, uv absorbence, or solution conductivity.
The output of a chromatographic analysis is referred to as a
It is a plot that consists of several different peaks representing the
different components of the sample mixture.
is a widely used ion-exchange chromatography technique in the
semiconductor industry. This is because it can provide quantitative
analysis of anions in the ppb range, making it capable of detecting
contaminants on the surface of a wafer, die, or package. Since ionic
contamination is a major source of
problems in the industry, ion chromatography is considered to be an
indispensable tool when analyzing water samples suspected to be the cause
corrosion issues on the line.
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