Separation and Detection Methods

The analytical technique for separating BTEX is GC. Table 14.5 shows information about gas chromatographic columns and conditions, and detection methods used for the analysis of BTEX in water and soil.

A special application of GC, to separate the oxygenates (methyl-tert-butyl-ether and ethyl-butyl-ether) and aromatics like BTEX from hydrocarbons coeluents in groundwater due to gasoline spills, is presented by Gaines et al.,30 where comprehensive two-dimensional GC, in which the eluent of a first column is introduced into a second column with a different separation mechanism, provides additional separation power and precision. A modulator that repeatedly generates sharp concentration pulses from the first column eluent and deposits them onto the second column links the two columns. This modulator consists of a tube with a thick stationary phase to retain and accumulate the first column eluent, and a rotating slotted heater that focuses the analytes and injects them in the second chromatographic column as it passes through the modulator tube.

GC is the most commonly used separation method in the analysis of BTEX from environmental samples. Liquid chromatography (LC) analysis with superheated water31 or water-dimethylsulfoxide (DMSO) mixtures32 has also been reported. In both cases a reduction in the dielectric constant of the mobile phase for the separation of nonpolar analytes was studied. The results showed how the rise in temperature required a decrease in DMSO in order to achieve the same retention time.

If only water is used as mobile phase, higher temperatures have to be used. The dielectric constant of liquid water decreases with decreasing pressure and increasing temperature. Therefore high temperatures are necessary to decrease the dielectric constant or polarity of liquid water in order to be able to use it as reversed-phase liquid chromatography eluent. If the column is at the same temperature along its length, the pressure drop will cause a decrease in the polarity of superheated water, having the same effect as a decrease in the capacity of the stationary phase of the column. Another problem is the selection of a suitable stationary phase that does not decompose or dissolve in hot water, and whose functional groups are not influenced by temperature or aqueous environment. Zirconia-based stationary phases with elemental carbon and polybutadiene functional groups have good thermal stability and are compatible with water as eluent.

Although UV detectors are most commonly used for LC, FID and MS can also be attached to the chromatographic column.

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