Stability of mixed-ligand zinc(II) carboxylate complexes in thin-layer chromatography

The TLC stability of mixed-ligand zinc(II) carboxylate complexes (ZC) of the type Zn(RCOO)2L n , where L is caffeine, nicotinic acid, thiourea or phenazone (n=1 or 2), has been studied on silica gel and octadecyl-bonded silica layers. All the ZC examined were found to undergo partial decomposition during the chromatographic process when eluted both with non-aqueous and buffered mobile phases. According to off-line analyses performed by atomic absorption and mass spectrometry, the chromatographic stability of ZC was highly dependent on their molecular structure. The nature of organic base ligand has a dominant impact on stability, which increases in the order thiourea < phenazone < nicotinic acid < caffeine derivatives. The influence of the carboxylic ligand is of secondary importance and seems to be attributed to its steric properties.