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led to the term "reversed phase" chromatography (even though today RPLC is so widespread that in fact it should be considered normal!). Retention in RPLC is due to hydrophobic interactions of the solute with the stationary phase. Since nearly all organic molecules have hydrophobic regions in their structure, this retention mechanism is nearly universal. Both neutral and ionic solutes can be separated by this technique.

Stationary phases in RPLC are usually solid particles with surfaces chemically modified by attachment of organic moieties. Silica is typically used as the support material, although recently zirconia is gaining ground owing to its better chemical stability, especially at pH extremes. Other materials used in RPLC include alumina, carbon and various polymers. The nature of the organic moiety determines the polarity of the stationary phases. Most separations are carried out using nonpolar stationary phases, including C-8 (octyl) and C-18 (octadecyl). Table 3.3 presents examples of LC stationary phases.

In RPLC, the solute continuously partitions between the stationary phase and the mobile phase. The nature of the partitioning between the two phases is very similar to partitioning between two immiscible liquids. For example, the process is noncompetitive and the sorption isotherms are linear. As a result, peaks are usually symmetrical, and the separations are very reproducible. RPLC is by far the most popular liquid chromatographic technique currently in use. Other separation modes are usually considered only after RPLC fails to deliver desirable results.

Ions and easily ionizable substances can be conveniently separated using ion-exchange chromatography. In this method, retention is based on electrostatic attraction between mobile phase ions and charged sites bound to the stationary phase. The sample ions are separated according to their relative affinity to the stationary phase compared to the mobile phase counter ions. In general, ion-exchangers tend to bind ions with multiple charges, small hydrated radius or large polarizability more strongly. Ion exchange finds application in nearly all areas of chemistry. In environmental analysis, it is most often used for the separation of inorganic and organic ions (both cations and anions). In this implementation, the technique is known simply as ion chromatography. Since its introduction, ion chromatography has revolutionized the analysis of ions and replaced many tedious wet chemical procedures.

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