Precoated layers chiral phases

HPTLC separation material is available in the form of precoated layers supported by glass, plastic sheets or aluminum foil. Although precoated layers of aluminum oxide, cellulose, polyamide, ion exchange materials, reversed phase silica (alkyl bonded) have been commercially available for quite some time, the vast majority of TLC separations is carried out on normal phase silica gel, particularly in quantitative TLC analysis. Recently introduced plates modified with amino, cyano and diol functional groups bonded to the silica can be used as multimodal media. Depending on the developing solvent used, they extend normal phase or reversed phase chromatographic properties. These layers may affect the predominant role of normal phase silica gel in TLC to a certain extent. They are described in Chapter 4 of this book. Also chiral layers for the separation of optical isomers are available and cause increased interest (11). 

Chapter 3 through 6 deal with the commercial and noncommercial stationary phases used for the direct and indirect enantioseparations by means of TLC and with the chiral modifiers of mobile phases, which are used exclusively in direct separations. Chapter 3 describes the commercial chiral and nonchiral sorbent materials and commercial precoated layers used in chiral separations. Thus, it deals with silica gel native and esterified cellulose chiral plates (reversed phase plates impregnated with a chiral selector) and C-18, C-18W, diol, diphenyl, and C-2 chemically bonded silica gel. At the end of this chapter, the author discusses the quantification of enantiomers by using densitometry, depending on the type of the stationary phase employed. 

Literature coverage in this chapter will not be comprehensive, but selected, important examples of enantiomer separations and analyses utilizing all of the types of systems involving commercial, precoated plates that are mentioned above will be given with corresponding references. Mechanisms of separations with chiral layers and mobile phases are described in Chapter 7 and will not be covered in detail in this chapter. Applications of homemade plates from commercial batch sorbents will not be covered in this chapter. 

An improvement in the use of phases impregnated with Pirkle selectors in planar chromatography was proposed by Witherow et al. [48], which prepared a two-phase modified amino-bonded stationary phase by immersing the lower portion of the precoated silica gel 60 HPTLC NH2 F254S plate (Merck) in a solution of CS2. The layer portion ionically modified with the chiral selector was employed for the enantioseparation of the two aforementioned test materials. 

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