Zonal Scanning of Thin-Layer Chromatograms
Thin-layer chromatography (TLC) plates have been assayed for radioactivity by autoradiography [1–3], continuous scanning [4–9], area scraping [10–13], and zonal scanning . Direct counting of the thin-layer adsorbent [10,11] in a liquid scintillation system provides the simplest, most quantitative and most efficient means of counting weak-energy beta emitters of low-activity samples, since elution steps are not required and the maximal counting efficiency is possible. The scintillation solution of choice for counting carbon-14 and tritium under these conditions is shown in Table I. The water in this solution serves to deactivate the silica, since adsorption of radioactivity in less polar solvent systems can result in self-absorption losses (10% for C14 and 25% for H3) on 10- to 25-μ silica particles. If radioactivity persists in partitioning between particles and solvent, Cab-O-Sil must be used to suspend the particles, and correction factors must be used . Silica, iodine, dichlorofluoroscein and rhodamine-6G have no quenching properties in this system, whereas elemental carbon and high levels of AgNO3-treated silica can cause severe quenching . Neatan, a polyvinyl propionate used for preserving thin-layer chromatograms, causes no color quenching effect, but it can cause a significant decrease in counting efficiency (because of self-absorption) when it coats partidies containing adsorbed radioactivity. This report elaborates on the zonal scanning technique and demonstrates some typical data.
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