Aromatic Hydrocarbons

sensitivity could be achieved for each aromatic or group of aromatic hydrocarbons. Diehl et al. used a Hewlett-Packard Model 5890 Series II GC/5965 IRD FTIR, with an open tubular column 60 m long and 0.53 mm I.D., carrying a 5.0 mm film of stationary phase on the internal surface. The column was programmed from 40C to 190 C at 2C per min. and then to 300C at 30C per min. The light pipe and the transfer line were held at 300C. By using appropriate wavelengths, the elution of the aromatic hydrocarbons could be exclusively monitored. The chromatograms obtained are shown in figure 41. The separation was fairly lengthy, extending well over an hour but this investment in time was repaid in selectivity. The aromatic hydrocarbons are exclusively monitored, and the numerous aliphatic and unsaturated hydrocarbons, (also present in the mixture) gave little or no response at the selected wavelength

compounds are the aromatic hydrocarbons. However, just as dipole-dipole interactions occur coincidentally with dispersive interactions, so are dipole-induced dipole interactions accompanied by dispersive interactions. It follows that using an n-alkane stationary phase, aromatic hydrocarbons can be retained and separated by purely dispersive interactions as in GC. This again is demonstrated in the upper chromatogram in figure 1. Alternatively, a polyethylene glycol stationary phase will separate aromatic hydrocarbons largely by dipole-induced dipole interactions combined with some dispersive interactions. This effect is also demonstrated in the lower chromatogram in figure 2. Molecules can exhibit multiple interactive properties. For example, phenyl ethanol possesses both a dipole as a result of the hydroxyl group and is polarizable due to the aromatic ring. Phenyl acetic acid contain groups that will provide dispersive interactions(the methylene group and the aromatic ring),

phase Supelcowax 10. This stationary phase is strongly polar and corresponds to a bonded polyethylene glycol. The strong fields from the hydroxyl groups polarize the aromatic nuclei of the aromatic hydrocarbons and thus retention was effected largely by polar interactions between the permanent and induced dipoles of the stationary phase and solute molecules respectively.   Courtesy of Supelco Inc.   Figure 38 The Separation of 10 ppb Quantities of Aromatic Hydrocarbons from Water   The flow rate was 10 ml/min. in conjunction with the FID detector. The column was held at 50˚C for 8 min. and then programmed to 100˚C at 4˚C per min. More than adequate separation is achieved and even the m and p xylenes are well resolved. This might indicate that a significantly shorter analysis was possible. The aromatic hydrocarbons were present in the original aqueous solution at 10 ppb and so the 5 ml of water contained about 50 pg

Hydrocarbon Analysis Due to the perceived toxicity and carcinogenic character of the aromatic hydrocarbons, the presence of these materials is carefully monitored in all areas where they might enter the human food chain. The analysis of water for aromatic hydrocarbons, particularly surface water in those areas where contamination might take place, is a common assay made by the public analyst. It is essential to be able to measure concentrations in the ppb levels, and thus GC method employing a high sensitive detector is essential. Nevertheless, even if a high sensitivity detector is employed, some sample concentration will be necessary to measure contaminants at such low levels. One method is the purge and trap

areas of application. It was originally used to differentiate aromatic from paraffinic hydrocarbons by measuring the luminosity that the aromatic nucleus imparted to the flame. Contemporary photometric detectors do not usually monitor the total light emitted only light emitted at specific wavelengths. For example, phosphorus and sulfur containing hydrocarbons generate chemi-luminescence at specific wavelengths when burnt in the hydrogen flame. The wavelengths of the light emitted by carbon and hydrocarbons containing sulfur and phosphorus are shown in figure 25.   Courtesy of the HewlettPackard Corporation   Figure 25 Light Emission Wavelengths of Carbon and Hydrocarbons Containing Sulfur and Phosphorus

An example of the use of induced dipoles to separate polarizable substances is afforded by the analysis of some aromatic and nitroaromatic hydrocarbons by LC using silica gel as the stationary phase.   Courtesy of Supelco Inc. Figure 14 The Separation Aromatic and Nitro-Aromatic Hydrocarbons A small-bore column 25 cm long and 1 mm I.D. was employed, packed with silica gel having a particle diameter of 10 m. The mobile phase was n-hexane at a flow-rate 50 ml per min. The solutes of interest are naphthalene and pyrene, the first two peaks. The two solutes are well separated