Gas Chromatograph

In many branches of natural science, one of the first and foremost methods for analyzing and decomposing of an unknown matter into its components is the use of one of the instruments belonging to large group of analytical separation methods called Gas Chromatography (GC). As implied by its name, these methods are only able to analyze volatile substances in the gas phase. However, this is not necessarily considered a serious limitation, because many of materials can be prepared as liquid solution, which then can be injected to the GC instruments.

The heart of GC consists of a separating column which may be placed in an oven. Inside the separating column, a substance called stationary phase is immobilized in a form of deposited layer over surfaces of particles that are accommodated in the column. However, sometimes the separating column uses capillary tubes whose inside walls coated with a thin layer of stationary phase. The latter type is known as packed column, and the former type is known as capillary column. In both columns, the stationary phase must be chemically inactive and have high thermal stability; its thermal stability must be at least 100 °C higher than the maximum temperature of an oven which enclose the column.

The Gas Chromatography principles are very similar to fractional distillation, in which a complex chemical is separated into its constituents because of the difference in boiling point and vapor pressure. Similarly, in GC a complex chemical is injected into a gas stream called carrier gas. The carrier gas is responsible for transferring the whole complex chemical (with all constituents) through the column from the beginning to the end. Chemical constituents have different vapor pressures; therefore, they have different affinities with the stationary phase immobilized inside the column. As a consequence, although all constituents go into the column at the same time, but they come out of the column gradually in separated intervals. And in this way a complex chemical breaks apart into its constituents.

The other important part of any GC instrument is its detectors which are connected to the end of separating column. Detector task is chemical or physical analysis of the separated constituents (analyte). The two most common types of detectors are Flame ionization detectors (FID) and Thermal conductivity detectors (TCD). The latter uses a flame to wholly decompose an analyte into ions and electrons; therefore, the flame becomes conductive toward electricity. The electrical conductivity is then used as a criterion for determination of molecular weight and concentration. The FID detectors have high sensitivity and are useful for the analysis of organic compounds. The main disadvantage of these detectors is that they cause damage to the analyte. On the other hand, TCD detectors estimate the amount of analyte in carrier gas by measuring the carrier gas thermal conductivity. Being a non-destructive analysis tool is the main advantage of TCD detectors, so it is possible to collect the analyte after separation and detection. The greatest drawback of the TCD is its low sensitivity compared to the other detection methods.