TABLE OF CONTENTS The concentration zone concept offers a number of benefits for certain types of analysis. The benefits were first described by Abbott and Thomson in 1965 [77] and elaborated on further in 1969 by Musgrave. [78] The TLC/HPTLC concentration zone plate consists of two different layer sections with a sharply defined borderline. There is no gap in between. The lower zone is used for sample application and covers the full width of the plate up to 25 mm. This lower zone is composed either of kieselguhr or synthetic porous silicon dioxide of medium pore volume, but very high pore diameter (50 000 ) and extremely small internal surface area (less than 1 m 2 g ?1). Kieselguhr being a natural material, mainly composed of silica, and approximately 10% of other metal oxides' can cause variations in chromatographic behaviour. The upper section of the plate is coated with normal silica gel 60 sorbent. Samples are applied as spots or bands to the concentration zone. Usually these are more dilute (typically 5 10 times) than what would be applied to a normal silica gel 60 TLC layer. On elution the analytes of interest migrate at or near the solvent front. On reaching the interface they form a concentrated band. This continues to migrate into the silica gel 60 layer and development continues normally. Although there are obvious advantages to the use of such concentration zone plates such as eliminating the need for precise positioning of...
