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Computer-Aided Mobile Phase Optimisation in 2D-TLC Separation of Metal Ions

Biological and non-biological materials containing metal constituents are very often the subject of analysis. Conventional laboratory methods that provide detailed qualitative and quantitative information about samples are generally time consuming and there is a need for fast screening methods. TLC as screening technique enables faster and chipper approach also in metal analysis. Optimisation of a simple and sensitive screening system for simultaneous detection of metal constituents present in samples is presented in this paper. Separation of 10 metal ions as model was investigated: Al3+, Be2+, Cr3+, Cu2+, Fe2+, Fe3+, Mn2+, Ni2+, Pb2+ and Sn2+. Separations were carried out on microcrystalline cellulose precoated layers using acetonitrile:HCl:H2O solvent system for separation in x direction and methanol:HCl:H2O for y direction. Chromatographic plates were developed to a distance of 7 cm with previous chamber saturation. After development the ions were visualised by successive spraying with visualisation reagents, exposed to ammonia vapours and finally the chromatograms were captured under white light (reflectance) by means of a highly sensitive CCD colour video camera. Image acquisition, processing and archiving were controlled via Camag Video Store 2 documentation software. RF values were determined from stored chromatograms via Vista Scan TLC/HPTLC program. Preliminary experiments were used to find suitable region for mobile phase composition for which reasonable RF values could be obtained. After that experimental points for statistical optimisation were selected from ternary diagrams. Experimental results were used to optimise mobile phase compositions in 2D-TLC by means of four factor statistical scanning technique in two developing directions. Optimisation can be successful only if a proper objective function is used and one must select an optimisation criterion that faithfully reflects the quality of separation. In presented paper a new quality criterion for separation optimisation in 2D-TLC was introduced. By using this criterion we were able to optimize separation not only in terms of distances between spots but also with regard to position of individual spots on chromatogram using additional penalty function. The present study has shown that it is possible to obtain optimal developing conditions for 2D-TLC separation in preselected TLC systems in short period of time with reduced number of experiments.

2D-TLC; HPTLC microcrystaline celulose plates; mobile phase optimisation; metal ions

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