Extraction and Planar Chromatographic Separation Techniques in the

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LODI et al. 1991, GOCAN et al. 1996, ESSIG and KOVAR 1997, SUMMANEN et al. 1998,
SZABADY et al. 1999, POTHIER et al. 2001, GALAND et al. 2002).
6.2.2. Method development in TLC
Method development for TLC generally starts with the selection of the basic separation
conditions. Besides the choice of stationary phase, the chosen developing technique, the size
and type of developing chamber and the vapor space have a pronounced effect on the
separation (GEISS 1987, NYIREDY 2002). In practice, however, only little attention is
usually devoted to this initial, yet important step.
Unlike the case in HPLC, the most important stationary phase in TLC is by far silica and it is
usually selected as the sorbent to start method development with. However, a general
approach to stationary phase selection has also been proposed based on the properties of the
sample components (POOLE and DIAS 2000). The various method development procedures
for TLC presented in the literature usually involve only the optimization of the mobile phase
using the previously chosen separation conditions. In contrast, the PRISMA optimization
system also includes the selection of these basic parameters and appropriate development
mode and operating conditions (NYIREDY 2002).
Most of the published literature on TLC method development focus on optimization of the
composition of the mobile phase as it is the second most important factor after sorbent
affecting the quality of the separation (GEISS 1987). Generally, the methods rely on empirical
data and therefore some experimental runs are required.
The only recently proposed method that does not involve any preliminary experiments is the
LSChrom software, which automatically calculates the recommended solvent strength of the
mobile phase based on the functional groups of the analytes, and suggests several solvent
combinations of this particular solvent strength (PALAMAREVA et al. 2003). It should,
however, be noted that the proposed solvent combinations do not take into account the
optimum separation of the analyte mixture, the software only suggesting mobile phases which
ensure that the retention of all the analytes is in the range of 0 < RF < 1.
The retardation factor RF describes numerically the position of the analyte on the TLC plate.
It is the ratio of the distance travelled by the analyte in relation to the distance travelled by the
solvent front on the plate, and is described by the following equation:
RF = zx/zf - zo
where zx is the distance travelled by the analyte from the starting position, zf is the distance
travelled by the solvent front starting from the bottom of the plate, and zo is the distance of the
start position of the analyte from the bottom of the plate.