Techniques to Measure Organics and Inorganics

Which determinative technique to use is dictated by the physical and chemical nature of the analyte of interest. The organics protocol flowchart introduced in Chapter 2 serves as a useful guide. Let us consider how we would determine which instrumental technique to use for the following example. Ethylene glycol, 1,2-ethanediol (EG), and 1,2-dichloroethane (1,2-DCA) consist of molecules that contain a two-carbon backbone with either a hydroxyl- or chlorine-terminal functional group. The molecular structures for these are as follows  

As first observed in 1903 by M. Tswett, a Russian botanist, when plant pigments were dissolved in a nonpolar solvent such as hexane and this solution was passed through a glass column packed with calcium carbonate, a separation of the two major forms of chlorophyll occurred due to a differential migration through the packed stationary phase. This observation of color writing led to the most used term in the separation sciences today—chromatography  

If a mixture containing EG and 1,2-DCA is introduced into a column, it is possible to conceive of the notion that the molecules that make up each compound would migrate differentially through the packed bed or stationary phase. Let us assume that this hypothetical column tends to retain the more polar EG longer. This separation of EG from 1,2-DCA is shown as follows  

We observe that the dispersion of the molecules as represented by cr is found to be proportional to the distance migrated, z, according to  

We just saw that, experimentally, EG and 1,2-DCA differentially migrate through a stationary phase when introduced into a suitable mobile phase and that chromatography arises when this mobile phase is allowed to pass through a chemically selective stationary phase. It is not sufficient to merely state that EG is retained longer than 1,2-DCA. It is more accurate to state that EG partitions to a greater extent into the stationary phase than does 

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