Chromatography, in one of its several forms, is the most commonly used procedure in contemporary chemical analysis and the first configuration of chromatography equipment to be produced in a single composite unit and made commercially available was the gas chromatograph. Gas chromatography was invented by A. J. P. Martin who, with R. L. M. Synge, suggested its possibility in a paper on liquid chromatography published in 1941. Martin and Synge recommended that the liquid mobile phase used in liquid chromatography could be replaced by a suitable gas. The basis for this recommendation was that, due to much higher diffusivities of solutes in gases compared with liquids, the equilibrium processes involved in a chromatographic process would be much faster and thus, the columns much more efficient and separation times much shorter. So the concept of gas chromatography was envisioned more than fifty years ago, but unfortunately, little notice was taken of the suggestion and it was left to Martin himself and his coworker A. T. James to bring the concept to practical reality some years later in 1951, when they published their epic paper describing the first gas chromatograph.
The gas density balance, was the first detector with a truly catholic response that was linearly related to the vapor density of the solute and consequently its molecular weight. The gas density balance had a maximum sensitivity (minimum detectable concentration) of about 10-6 g/ml at a signal to noise ratio of two. This detector inspired the invention of a wide range of detectors over the next decade providing both higher sensitivity and selective response.
Modern Gas Chromatograph
The modern gas chromatograph is a fairly complex instrument mostly computer controlled. The samples are mechanically injected, the analytical results are automatically calculated and the results printed out, together with the pertinent operating conditions in a standard format. However, the instrument has evolved over many years although the majority of the added devices and techniques were suggested or describe in the first three international symposia on gas chromatography held in 1956, 1958 and 1960. These symposia, initially organized by the 'British Institute of Petroleum' have been held every two years ever since 1956 and the meetings have remained the major stimulus for developing the technique and extending its capabilities.
Gas chromatography has an entirely different field of applications to that of liquid chromatography. In general, gas chromatography is used for the separation of volatile materials and liquid chromatography for the separation of involatile liquids and solids. There are certain compounds, however, that can be separated with either techniques, and more importantly, many involatile substances such as amino acids, steroids and high molecular eight fatty acids can be derivatized to form volatile substances that can be separated by GC. The derivatization must be highly reproducible and usually proceed to completion in order to maintain adequate accuracy. The capillary columns in GC can have much higher efficiencies than their LC counterpart and thus GC can more easily handle multi-component mixtures such as essential oils. On the other hand, only LC can separate the peptides, polypeptides, proteins and other large biopolymers that are important in biotechnology.
Gas Chromatography Applications
The most common hydrocarbon analysis carried out by GC is probably that of gasoline. The analysis of gasoline is typical of the type of sample for which GC is the ideal technique. It is this type of multi-component mixtures containing very similar compounds that need the high efficiencies available from GC for a successful analysis. The separation of a sample of gasoline carried out on a long open tubular column is shown in figure 35. It is clear that the column had a very high efficiency that was claimed to be in excess of 400,000 theoretical plates. The column was 100 m long and only 250 mm I.D., carrying a film of the stationary phase, Petrocol DH, 0.5 mm thick. Petrocol DH is specially designed stationary phase for the separation of hydrocarbons and consists of bonded dimethylsiloxane, a very dispersive type of stationary phase, retaining the solutes approximately in the order of their increasing boiling points.
Non-polar or dispersive stationary phases are employed for the separation of hydrocarbons (e.g. OV101, which is also a polyalkyl-siloxane, is widely used in packed columns). The flow velocity of 20 cm/sec., appears to have been taken from the ratio of the column length to the dead time. Thus, due to the pressure correction the actual effective linear velocity would be much less than that (see Book 7 Peak Dispersion in Chromatographic Systems). Helium was used as the carrier gas that was necessary to realize the high efficiencies with reasonable analysis times.
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