BACKGROUND

Modern gas chromatography (GC) was invented by Martin and James in 1952 , and has become one of the most important and widely applied analytical techniques in modern chemistry. Major milestones in the development of GC, especially in column technology, detection and sample introduction are described in this historical review.(http://www.sciencedirect.com/science/article/pii/S0165993602008063).

Griffin and George (London, UK) probably manufactured the first commercial GC system in 1954, and several companies, including Perkin Elmer, Fisher/Gulf, Barber Coleman, Podbelniak (all U.S.-based) and Pye Unicam (UK), followed shortly in 1955 and 1956.(http://www.chromatographyonline.com/lcgc/Articles/A-History-of-Gas-Chromatography/ArticleStandard/Article/detail/657484)

Many trends in current progress can be seen to originate in the first two decades of the history of GC, but the invention of fused-silica capillary columns greatly increased the application of high-resolution GC across the field of organic analysis; the development of low-cost, bench-top mass spectrometers led to further advances. Progress continues to be rapid in comprehensive 2D GC, fast analysis, detection by atomic emission and time-of-flight mass spectrometry, and in applications to process analysis.First used in the early 1900s chromatography got its name because it was used to separate different mixtures of coloured compounds.By the 1930s the popularity of chromatography had increases chemists realized that this experimental technique could also be used to separate mixtures of colourless compounds.(http://www.sciencedirect.com/science/article/pii/S0165993602008063).

THEORY

Chromatographic analysis is used to separate complex mixtures of compounds. GC is also a frequently used technique in many environmental and forensic laboratories because it allows for the detection of very small quantities. A broad variety of samples can be analyzed as long as the compounds are sufficiently thermally stable and volatile.

All chromatographic systems have two phases,mobile phase and a stationary phase.The mobile phase (=carrier gas) is comprised of an inert gas i.e., helium, argon, or nitrogen. The stationary phase consists of a packed column where the packing or solid support itself acts as stationary phase, or is coated with the liquid stationary phase (=high boiling polymer). Most analytical gas chromatographs use capillary columns, where the stationary phase coats the walls of a small-diameter tube directly (i.e., 0.25 m film in a 0.32 mm tube).

The separation of compounds is based on the different strengths of interaction of the compounds with the stationary phase (like-dissolves-like-rule). The stronger the interaction is, the longer the compound interacts with the stationary phase, and the more time it takes to migrate through the column (=longer retention time). In the example above, compound X interacts stronger with the stationary phase, and therefore lacks behind compound O in its movement through the column. As a result, compound O has a much shorter retention time than compound X.

As the sample in the mobile phase passes throught the stationary phase,the compounds in the mixture will separate because of differences in their affinities for the stationary phase,and differences in their solubilities in the mobile phase.

In chromatographic analysis,the eluted compounds are characterized by retention times,t.Qualitative analysis involves determinatin of t of analytesand comparing them with t of standards.Quantitative analysis is accomplished by comparing the areas of the analyte peaks with those of standards.The separation of compounds is based on the different strengths of interaction of the compounds with the stationary phase (like-dissolves-like-rule). The stronger the interaction is, the longer the compound interacts with the stationary phase, and the more time it takes to migrate through the column (=longer retention time). In the example above, compound X interacts stronger with the stationary phase, and therefore lacks behind compound O in its movement through the column. As a result, compound O has a much shorter retention time than compound X.

Factor that influence the separation of the components.Boiling point

The boiling point of a compound is often related to its polarity (see also polarity chapter). The lower the boiling point is, the shorter retention time usually is because the compound will spent more time in the gas phase.

The polarityof components versus the polarity of stationary phase on column.

If the polarity of the stationary phase and compound are similar, the retention time increases because the compound interacts stronger with the stationary phase. As a result, polar compounds have long retention times on polar stationary phases and shorter retention times on non-polar columns using the same temperature. Column temperatureA excessively high column temperature results in very short retention time but also in a very poor separation because all components mainly stay in the gas phase.

Column lengthA longer column generally improves the separation. The trade-off is that the retention time increases proportionally to the column length and a significant peak broadening will be observed as well because of increased longitudinal diffusion inside the column.(http://www.chromatographyonline.com/lcgc/Articles/A-History-of-Gas-Chromatography/ArticleStandard/Article/ )