Gas Chromatography - Tandem Techniques - Introduction > Page 1
Complex mixtures of volatile compounds have been of great commercial interest to all civilizations for thousands of years. Originally the most important were the essential oils extracted from various plants and subsequently used as perfumes, food flavorings, and for medicinal purposes. Later, with the advent of the industrial revolution, petroleum fractions, solvents and coal products, such as coal tar and benzole mixtures were added to the list. Prior to the introduction of gas chromatography (GC) by James and Martin (1) in the early 1950s, the analysis of complex mixtures of volatile substances was extremely difficult and time consuming to carry out.
At that time, the only effective procedure for separating and analyzing such materials was by distillation using (what was then) high efficiency fractionating columns which, due to their very long equilibrium times, many days (sometimes weeks) were necessary to complete a separation. In addition, the distillation process was only really effective for isolating the major component of the mixture in pure form, the fractions containing the minor components still consisted of complex mixtures. As a consequence, this type of separation was virtually useless as it was often the minor component(s) that had physiological, toxic or organoleptic importance. The introduction of GC had a revolutionary effect on the separation and identification of the components of such complex mixtures and, for the first time, the real nature of many of the essential oils and industrial solvents that were in common use was disclosed.
Even the early examples given by James and Martin (2) in one of their first public lectures on the subject, included dramatic separations of multi-component mixtures that fired the imagination of chemists world-wide. For the first time the complex nature of these important materials could be displayed both quantitatively and qualitatively. An example of the separation of an essential oil using modern GC equipment is shown in figure 1 .
|1. a-Pinene||7. gŠTerpinene||13. Geraniol|
|2. Camphene||8. Terpinolene||14. Neryl Acetate|
|3. b-Pinene||9. Linalool||15. Geranyl Acetate|
|4. Myrcene||10. Terpinene-4-ol||16. Caryophyllene|
|5. p-Cymene||11. a-Terpineol||17. trans-a-Bergamotene|
|6. Limonene||12. Neral||18. b-Bisabolen|
Courtesy of Supelco, Inc.
Figure 7 A Chromatogram of Lime Oil