Removal of Spots from the Plate and Elution Technique

Removal of Spots from the Plate and Elution Technique 

After the spots have been marked, they can be scraped off with a razor blade, spatula or similar tool [69, 122, 197, 213, 215, 636, 648]. The best way is first to remove the adsorbent from a zone around the spot and then to scrape off the spot itself on to a sheet of smooth cellophane. It is then extracted by shaking with the chosen solvent (cf. p. 153) and the extract freed from adsorbent by filtering or centrifuging. 

Filter papers are imsuitable for filtration. The membrane filters so far commercially available, could not be used on account of their poor resistance to organic solvents. The successful use of methanol-insoluble membrane filters in the extraction of TLC spots, has however been recently reported [799]. Glass or porcelain filters are generally used. The filtrate from a G4 sintered glass filter can be sucked directly into a small standard flask in an evacuated vessel [213, 215]. An extractor functioning under increased pressure has also been employed [695]. 

To prevent losses, air currents must be strictly avoided while scraping off the spot and transferring the powder to a suitable vessel. Various authors have accordingly used vacuum extractors, with which the localised spots could be sucked on to the filter surface or into the extraction flask and then eluted [65, 70,123, 318, 349, 437, 451,463, 578, 605, 651]. 

An extractor of this type is shown in Kg. 80 [437]. Continuous extraction according to the Soxhlet principle [329,535] for example, is not recommended since even sparingly soluble components of the binder or impurities can be dissolved these may interfere in the quantitative determination [338] (cf. p. 150). An automatic zone collector has been described for quantitative evaluation of radioactive spots [647]. 

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