Figure 4 :This was the mobile phase transfer system prior to putting the three parts together. Unfortunately the producer of the plastic screw cap has changed the production. From 2010 on the cap has a too thin upper plastic part not stable enough to hold the 5.0 mm phase transfer tube, which contains the phase transfer wick. Technical solution of this problem: The 5.0 mm PTFE tube is now part of an insert, which is shown in this ‘figure’ (click on PTFE-insert-and-tube-part) The new type of screw cup, still available through www.lansche-versand.de , gets a 5.1 mm hole in its center -also visible in the just mentioned ‘figure’. The PTFE insert is packed with the phase transfer wick and pushed through the cap hole. These two parts are now firmly screwed onto the micro bottle until tightly connected. This way the phase transfer system is completely inert. The PTFE-insert-and tube-part needs for production a turning lathe, which may be available at mechanic shops.
The PTFE outer diameter of 5.0 mm must loosely - but not too loosely - fit into the 5 mm cover glass plate hole. Thus the PTFE tube part must have checked quality. As the 5 mm hole into the quite thick glass plate is made by water cooled diamond drills, the final i.d. is in the range of 5.02..5.05 mm. Because of this plus 0.02 to 0.05 mm hole tolerance the wick tube part is free movable up and down when sitting in the cover glass upon the HPTLC layer. This assures a close enough contact but a free flow of the mobile phase and no surface damage to the stationary phase. A later possible roboterized µ-PLC equipment would need quite some control effort for the optimized cooperation of mobile and stationary phase. Therefore the author is quite satisfied that this critical part of µ-PLC hardware is based on such a most simple pairing of materials and dimensions. At the open end of the PTFE tube the wick reaches about half mm out of the tube. At the inner side of the tube two crossed 0.2 mm steel wires fix the cotton wick in its position. This way the phase transfer remains optimal. The little glass bottle can well be cleaned by a gas flow flashed into. It is filled with one ml mobile phase using a clean glass precision syringe. If the mobile phase must consist of a quantitative mixture of more than two liquids, the mix can be made using smaller precision syringes or an analytical balance. Accuracy in the mobile phase composition is something a little bit under developed in to days HPTLC practice.
The brown glass bottle of 1.5 ml is of special quality for medicinal use. This special bottle is available from order number 4300 B, minimum size of the order: 50 pieces.