chromatography used in analytical chemistry for separating and analyzing compounds that
can be vaporized without decomposition. Typical uses of GC include testing the purity of a
particular substance, or separating the different components of a mixture (the relative amounts
of such components can also be determined). In some situations, GC may help in identifying
a compound. In preparative chromatography, GC can be used to prepare pure compounds from
a mixture.[1][2] In gas chromatography, the mobile phase (or
"moving phase") is a carrier gas, usually an inert gas such as helium or an unreactive
gas such as nitrogen.
Helium remains the most commonly used carrier gas in about 90% of
instruments although hydrogen is preferred for improved separations.[3] The stationary
phase is a microscopic layer of liquid or polymer on an inert solid support, inside
a piece of glass or metal tubing called a column (an homage to the fractionating column
used in distillation). The instrument used to perform gas chromatography is called a
gas chromatograph (or "aerograph", "gas separator"). The gaseous compounds being analyzed interact
with the walls of the column, which is coated with a stationary phase. This causes each
compound to elute at a different time, known as the retention time of the compound.
The
comparison of retention times is what gives GC its analytical usefulness. Gas chromatography is in principle similar
to column chromatography (as well as other forms of chromatography, such as HPLC, TLC),
but has several notable differences. First, the process of separating the compounds in
a mixture is carried out between a liquid stationary phase and a gas mobile phase, whereas
in column chromatography the stationary phase is a solid and the mobile phase is a liquid.
(Hence the full name of the procedure is "Gasliquid chromatography", referring to the mobile and
stationary phases, respectively.) Second, the column through which the gas phase passes
is located in an oven where the temperature of the gas can be controlled, whereas column
chromatography (typically) has no such temperature control. Finally, the concentration of a compound
in the gas phase is solely a function of the vapor pressure of the gas.[1] Gas chromatography is also similar to fractional
distillation, since both processes separate the components of a mixture primarily based
on boiling point (or vapor pressure) differences.
However, fractional distillation is typically
used to separate components of a mixture on a large scale, whereas GC can be used on a
much smaller scale (i.E. Microscale). Gas chromatography is also sometimes known
as vapor-phase chromatography (VPC), or gasliquid partition chromatography (GLPC). These alternative
names, as well as their respective abbreviations, are frequently used in scientific literature.
Strictly speaking, GLPC is the most correct terminology, and is thus preferred by many
authors.
A gas chromatograph is a chemical analysis
instrument for separating chemicals in a complex sample. A gas chromatograph uses a flow-through
narrow tube known as the column, through which different chemical constituents of a sample
pass in a gas stream (carrier gas, mobile phase) at different rates depending on their
various chemical and physical properties and their interaction with a specific column filling,
called the stationary phase. As the chemicals exit the end of the column, they are detected
and identified electronically. The function of the stationary phase in the column is to
separate different components, causing each one to exit the column at a different time
(retention time).
Other parameters that can be used to alter the order or time of retention
are the carrier gas flow rate, column length and the temperature. In a GC analysis, a known volume of gaseous
or liquid analyte is injected into the "entrance" (head) of the column, usually using a microsyringe
(or, solid phase microextraction fibers, or a gas source switching system). As the carrier
gas sweeps the analyte molecules through the column, this motion is inhibited by the adsorption
of the analyte molecules either onto the column walls or onto packing materials in the column.
The rate at which the molecules progress along the column depends on the strength of adsorption,
which in turn depends on the type of molecule and on the stationary phase materials. Since
each type of molecule has a different rate of progression, the various components of
the analyte mixture are separated as they progress along the column and reach the end
of the column at different times (retention time).
A detector is used to monitor the outlet
stream from the column; thus, the time at which each component reaches the outlet and
the amount of that component can be determined. Generally, substances are identified (qualitatively)
by the order in which they emerge (elute) from the column and by the retention time
of the analyte in the column. Qualitative analysis[edit]
Generally chromatographic data is presented as a graph of detector response (y-axis) against
retention time (x-axis), which is called a chromatogram. This provides a spectrum of
peaks for a sample representing the analytes present in a sample eluting from the column
at different times.
Retention time can be used to identify analytes if the method conditions
are constant. Also, the pattern of peaks will be constant for a sample under constant conditions
and can identify complex mixtures of analytes. However, in most modern applications, the
GC is connected to a mass spectrometer or similar detector that is capable of identifying
the analytes represented by the peaks. Quantitative analysis[edit]
The area under a peak is proportional to the amount of analyte present in the chromatogram.
By calculating the area of the peak using the mathematical function of integration,
the concentration of an analyte in the original sample can be determined.
Concentration can
be calculated using a calibration curve created by finding the response for a series of concentrations
of analyte, or by determining the relative response factor of an analyte. The relative
response factor is the expected ratio of an analyte to an internal standard (or external
standard) and is calculated by finding the response of a known amount of analyte and
a constant amount of internal standard (a chemical added to the sample at a constant
concentration, with a distinct retention time to the analyte). In most modern GC-MS systems, computer software
is used to draw and integrate peaks, and match MS spectra to library spectra.
In general, substances that vaporize below 300 C (and therefore are stable up to that
temperature) can be measured quantitatively. The samples are also required to be salt-free;
they should not contain ions.
Very minute amounts of a substance can be measured, but
it is often required that
the sample must be measured in comparison to a sample containing the pure, suspected
substance known as a reference standard. Various temperature programs can be used to
make the readings more meaningful; for example to differentiate between substances that behave
similarly during the GC process. Professionals working with GC analyze the
content of a chemical product, for example in assuring the quality of products in the
chemical industry; or measuring toxic substances in soil, air or water. GC is very accurate
if used properly and can measure picomoles of a substance in a 1 ml liquid sample, or
parts-per-billion concentrations in gaseous samples.
In practical courses at colleges, students
sometimes get acquainted to the GC by studying the contents of Lavender oil or measuring
the ethylene that is secreted by Nicotiana benthamiana plants after artificially injuring
their leaves. These GC analyse hydrocarbons (C2-C40+). In a typical experiment, a packed
column is used to separate the light gases, which are then detected with a TCD. The hydrocarbons are separated using
a capillary column and detected with a FID.
A complication with light gas
analyses that include H2 is that He, which is the most common and most sensitive inert
carrier (sensitivity is proportional to molecular mass) has an almost identical thermal conductivity
to hydrogen (it is the difference in thermal conductivity between two separate filaments
in a Wheatstone Bridge type arrangement that shows when a component has been eluted). For
this reason, dual TCD instruments used with a separate channel for hydrogen that uses
nitrogen as a carrier are common. Argon is often used when analysing gas phase chemistry
reactions such as F-T synthesis so
that a single carrier gas can be
used rather than two separate ones. The sensitivity
is less, but this is a trade off for simplicity in the gas supply.
Gas Chromatography is used extensively in
forensic science. Disciplines as diverse as solid drug dose (pre-consumption form) identification
and quantification, arson investigation, paint chip analysis, and toxicology cases, employ
GC to identify and quantify various biological specimens and crime-scene evidence..
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