GOS CHROMATIGRAPHY

Gas Chromatography is an instrumental method for separating and analyzing sub microgram amount  of volatile liquid and volatile solids and gases.
The separation takes place as the sample vapor passes through a coil tube and a column, which is housed in a temperature controlled oven.
There are two types of column, capillary column and packed columns.
Packed columns are stainless steel or glass tubes around half a centimeter internal diameter and 1-2m long.

The column is packed with a stationary phase with either a finely divided solid support on which a high boiling inert liquid is being absorbed or just a finely divided solid such as silica or carbon.
Capillary columns are very fine silica tubes between 10-15m long, then about a half a millimeter internal diameter, here the high boiling liquid is chemically bonded on the inside wall of the capillary.
These columns are capable of much better resolution than packed columns.

The sample vapor is transported through the column by a mobile phase, an inert carrier gas such as Nitrogen or Helium.
To start, check that the instrument is turned on. It’s common in many laboratory to leave these instrument permanently switched on.
Check that the carrier gas flow rate is as specified in the instruction sheet, typically 1-2cubic centimeter per minute in the capillary column or 30-60 cubic centimeters per minute in a 1-2meter packed column.

The sample movement vaporized in a pre-heated in the injection pot prior to entry of the column, so set the appropriate injector temperature, then set the column working temperature.
Be careful not to allow the oven temperature not to exceed the upper temperature limit of the column, otherwise the stationary phase may be stripped through the column.

Now leave the instrument to stabilized, this normally takes a few minute, modern instruments displace the word ready at this point.
Since gas chromatography is a very sensitive technique, the sample will usually need to be diluted, make a solution in an inert volatile solvent such as diethyl ether which will elute through the column quicker than the sample.

Typically a sample concentration needed will be about 1%.
The Gas Chromatography is capable of analyzing 1part per million or less.
Wash up the hypodermic syringe with the sample solution and then drop up a measured volume, typically 1-3microliters.
Then inject this through the self sealing septum injection pot taking care not to bend the needle.

Most modern instruments automatically records the time of the injection in this case is the origin where the trace started to draw, otherwise note the injection point.
Gas samples are best introduced into the instrument using a gas syringe, or by means of a fixed volume gas loop, typically at 1cubic centimeter capacity.

The loop is incorporated into pipe or the edge of the column and is filled using a syringe or by drawing the sample gas through it.
Opening the loops isolating valve allow the carrier gas to stripped the gas sample into the column, as the separated component emerge from the column they pass into the detector and the resulting  electrical signal are displayed on the VDU or a moving chart.
Each separated component should give rise to a single peak, a variety of detectors are available Choice depending on the nature of the sample the analyte.

The flame ionization detector is suited to combustible analytes such as Hydrocarbons or alcohol, Nitrogen or Helium is used as the carrier gas.
The electron captured detector is suited to halogenated compounds; the carrier gases used are Nitrogen, Helium or Argon.
The most sophisticated detector is the mass spectrometer.
In a GCMS instrument the separated component pass through the column, at a heated transfer line into a mass spectropheter.

This is the only detector capable of directly determining the identity of the components. Other detectors can only do so by reference to pure samples of the suspected compounds.
The time elapsed between the sample injection and the appearance of peak maximal is called retention times.
Modern instrument will automatically displayed the retention times corresponding to each peak maximum.

The correlation of the component retention time with that of the separately injecting pure compound could be taken as strong evidence of the component’s identity.
But co-injection of the analyte and standard is a more reliable procedure; add a known standard compound solution of the unknown sample and inject the mixture unto the column, if the standard compound is present in the unknown sample then one of the peaks in the case and second will be relatively higher than in the original chromatogram. If not an addition peak will occur.

The area under each peak is proportional to the amount of the compound caught that peak.
But before this could be used for quantitative analysis it is necessary to calibrate the instrument to allow a varying detector response to different compounds and different samples modes.

Commonly, the correction factor are stored in the computerized data handling system of the instrument and used automatically when the sample data is printed out.
An unknown concentration could then be found by injecting the sample solution and determining the peak area from the trace and hence percentage concentration.