Planar chromatography interactions

Nowadays, almost all commercially available HPLC stationary phases are also applicable to planar chromatography. In addition to the polar hydroxyl groups present on the surface of native silica, other polar functional groups attached to the silica skeleton can also enter into adsorptive interactions with suitable sample molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol functions, attached to the silica skeleton by alkyl chains, all of which have been well proven in HPLC, have also been developed for TLC (34). 

Self-associative lateral interactions can only occur with the AB-type analytes, chromatographed in sufficiently mild chromatographic conditions. In planar chromatography, this type of lateral interaction was first demonstrated on monocarboxylic fatty acids and a,co-dicarboxylic acids, chromatographed on microcrystalhne cellulose with aid of decalin and 1,4-dioxane as monocomponent eluents, respectively [8,20,23]. 

Pieniak, A., Densitometric investigation of the effect of lateral interactions exerted on retention and separation of analytes in the non-linear variant of adsorption planar chromatography (in Polish). Ph.D. thesis, The University of Silesia, Katowice (Poland), 2006. 

It is important to know the influence of the physicochemical parameters of the mobile phase (dipole moment, dielectric constant, and refractive index) on solvent strength and selectivity. The main interactions in planar chromatography between the molecules of the mobile phases and those of solutes are caused by dispersion forces related to the refractive index, dipole-dipole forces related to the dipole moment, induction forces related to a permanent dipole and an induced one, hydrogen bonding, and dielectric interactions related to the dielectric constant. Solvent strength depends mainly on the dipole moment of the mobile phase, whereas the solvent selectivity depends on the dielectric constant of the mobile phase. 

One of the most crucial influencing factors in planar chromatography is the vapor space and the interactions involved. The fact that the gas phase is present, in addition to stationary and mobile phases, makes planar chromatography different from other chromatographic techniques. Owing to the characteristic of an open system the stationary, mobile, and vapor phases interact with each other until they all are in equihbrium. This equilibrium is much faster obtained if chamber saturation is employed. This is the reason for differences in separation quality when saturated and unsaturated chambers are used. However, the humidity of the ambient air can also influence the activity of the layer and, thus, separation. Especially during sample application, the equihbrium between layer activity and relative humidity of the... 

In normal-phase chromatography, polar stationary phases are employed and solutes become less retained as the polarity of the mobile-phase system increases. Retention in normal-phase chromatography is predominately based upon an adsorption mechanism. Planar surface interactions determine successful use of NPC in separation of isomers. The nonaqueous mobile-phase system used in NPC has found numerous applications for extremely hydrophobic molecules, analytes prone to hydrolysis, carbohydrates, and sat-urated/unsaturated compounds. In the future, with the advent of new stationary phases being developed, one should expect to see increasingly more interesting applications in the pharmaceutical industry. 

Two-dimensional planar chromatography (2D-TLC) is frequently used in combination with autoradiography or digital autoradiography (DAR) in studies on metabolism. Examples of 2D-TLC-DAR will be given in the analysis of pharmaceutical products. Other applications generally use either different types of development, or utilise different interactions for separation, or different stationary phases, such as elution-displacement absorption-partition normal phase-reversed phase ion exchange-normal phase. 

Various binders have been used to give mechanical stability to the layer spread on the suppon plate. Basic requirements are that the binder should not interfere with solute-sorbent interactions, with elution, and detection procedures. At the same time, these binders have to provide compact and adherent layers together with the sorbent. There are various binders that have been applied to prepare the stationary phase for planar chromatography (Table 10.7). 

Physicochemical description of retention processes in liquid chromatography (planar chromatography included) is far from complete and, therefore, new endeavors are regularly undertaken to improve existing retention models and/or to introduce the new ones. The excessive simplicity of already established retention models in planar chromatography is—among other reasons—because some types of intermolecular interaction in the chromatographic sys-... 

In planar chromatography the separation process occurs in a three-phase system of stationary, mobile, and vapor phases, all of which interact with one another and with the operating parameters. Selection of the chamber type and vapor space is a variable offered only by planar chromatography, as the third dimension of the chromatographic parameters. The role of the vapor phase in TLC is well known, although only little attention is made in practice (39). 

In optimizing planar chromatography, peak capacity in 2-D TLC far exceeds HPLC (13). PRISMA has also been very helpful by developing computerized and statistical choices for solvents. Demixing remains a major problem in predicting Rys and the ultimate experimental outcome vs. predicted. Again, 20 chromatograms define experimental variables for optimum Rf. Solvent selectivity (14) has been discussed based on proton donation, acceptance, or dipole interactions (IS). 

Successively, Wainer et al. [45] employed the tt-acceptor N-(3,5-dinitrobenzoyl)-/ -( - )-a -phenylglycine (CS1), bound to an aminopropyl silanized silica gel through ionic interactions, for the resolution of racemic 2,2,2-trifluoro-l-(9-anthryl)ethanol (5 fj-g) by planar chromatography eluting with n-hexane/isopropanol (9.5 1, v/v). CSl was bound to Zorbax BP-NH2 plates (DuPont, USA) by continuous development with a tetrahydrofuran solution (20 ml) containing 1 g of the chiral selector. 

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