Introduction

The separation of polar, ionizable substances can cause problems for simple partition- or adsorption-based chromatographic systems and this is as true in thin-layer, or planar, chromatography (TLC) as for any of the other liquid chromatographic techniques. Often the problem can be dealt with by the simple expedient of controlling the pH of the solvent used for chromatography, in order to suppress the ioniz-ation of the solute and thereby reduce its polarity. However, with very polar compounds this is not always a successful strategy and in such circumstances it may be more practical to employ an ion pair (IP) reagent (often referred to as soap chromatography in the early literature). A further benefit of using such an approach is that a hydrophobic IP reagent may, in addition to masking the ionizable group, usefully modify the overall polarity of the complex and thus cause a dramatic change in chromatographic properties. Whilst IP-TLC has not been as widely employed as IP-high performance liquid chromatography (IP-HPLC: described elsewhere in this encyclopedia), there have been sufficient examples of the technique to demonstrate its utility for both acids and bases.

In addition to simply using IP reagents in order to obtain suitable chromatographic separations for substances that are too polar to chromatograph in any other way, this type of TLC has other uses. For example, faced with a mixture of neutral and ioniz-able compounds, an IP reagent may enable the ana

Frey H-P and Zieloff K (1993) Qualitative and Quantitative Thin Layer Chromatography (in German). Weinheim: VCH Publishers.

Geiss F (1987) Fundamentals of Thin Layer Chromatography. Heidelberg: Hü thig.

GrinbergN (ed.) (1990) Modern Thin Layer Chromatography. New York: Marcel Dekker.

Jork H, Funk W, Fischer W and Wimmer H (1990) Thin Layer Chromatography. Reagents and Detection Methods, vols 1a and 1b. Weinheim: VCH Publishers.

Sherma J and Fried B (eds) (1998) Handbook of Thin Layer Chromatography. New York: Marcel Dekker.

lyst to introduce additional selectivity into the system. This can enable the RF of more acidic or basic compounds to be altered whilst leaving those of neutral solutes unchanged and thereby effecting a separation. The addition of an IP reagent to a system can also provide a rapid indication of which compounds in a mixture are ionizable. There have also been several examples of the use of chiral IP reagents for the separation of enantiomers.

The scope and practice of IP-TLC are described below.

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