In gas chromatography (GC), the sample is vaporized and injected onto the head of a chromatographic column. Elution is brought about by the flow of an inert gaseous mobile phase such as helium, argon, nitrogen, carbon dioxide, and hydrogen. In GC, the mobile phase does not interact with molecules of the analyte, and it only transports the analyte through the column. In two general kinds of GC, gas-solid chromatography (GSC) and gas-liquid chromatography (GLC), the mechanisms of analyte retention in the column are thoroughly different. In GLC, the analyte has been participated between a gaseous mobile phase and a liquid stationary phase. While in GSC, the retention of analytes is the consequence of its physical adsorption onto a solid stationary phase. In comparing of GLC and GSC, more widespread use of GLC has been found in all fields of science. This is mainly due to the semipermanent retention of active or polar molecules and the severe tailing of elution peaks, which is a consequence of the nonlinear character of adsorption process, in GSC. In GC, column efficiency requires that sample be of suitable size and be introduced as a plug of vapor. So, the sample preparation is a very important step in GC. The sample should be injected into a flash vaporizer port located at the head of the column, and its temperature is about 50°C above the boiling point of the least volatile component of the sample. So, the components of the sample should be easily vaporized in this temperature, and they should have high heat resistance not to be decomposed. Both of liquid and solid samples can be introduced to the column. But solid samples are ordinarily introduced as solutions or sealed into thin-walled vials that can be inserted at the head of the column and punctured or crushed from the outside. In order to separate and analyze the gaseous, liquid, and volatile solid samples directly, GC is a suitable analytical equipment. When the analyte sample is nonvolatile, the derivatization and pyrolysis GC techniques are crucial. Gas chromatography can be applied to the solution of many problems in various fields such as drugs and pharmaceuticals, environmental studies like air and clinical samples, petroleum industry, pesticides and their residues, and foods. On the other hand, most samples are not ready for direct introduction into instruments. For organics and volatile organics, the sample preparation procedures can be named as extraction, cleanup, derivatization, transfer to vapor phase, and concentration. So, the basic concept of a sample preparation method is to convert a real matrix into a sample in a format that is suitable for analysis by a separation or other analytical techniques. The goals of sample treatment step are as follows: (1) The capability of using smaller amounts of initial sample, especially for trace analysis. (2) Achieving higher specificity and selectivity in analytical determinations. (3) To improve the potential for automation or online methods and minimize the manual operations. (4) The usage of no or small volumes of organic solvents in order to approach the green chemistry techniques with less wastes and more friendly environment. On the other hand, different samples possess a variety of sample treatment methods, for example: (1) In order to treat solid samples and separate a purpose analyte, some enhanced solvent extraction methods include pressurized liquid extraction, microwave- and sonic wave-assisted extraction, supercritical fluid extraction, and superheated water extraction. (2) For analytes in solution, the sample preparation can be attributed to the analyte trapping methods such as -phase extraction, solid-phase microextraction, and stir bar extractions. (3) Also, the extraction of the analytes into a liquid phase can be achieved by other methods like membrane extraction, single-drop microextraction (SDME), and purge and trap. (4) For separation of analytes in the gas phase, trapping analytes from vapor samples and headspace analysis are used. As a result, sample preparation is not only a critical step but also possesses different ways to treat and convert matrix into a suitable sample to inject GC.
Part of the book: Gas Chromatography