RESUMEN
Batchwise equilibrium study was carried out on the retention of nonpolar and polar organic compounds to octadecylsilyl (ODS) silicas of different properties at atmospheric pressure. While nonpolar compounds were retained only by distribution on the ODS phase, polar compounds were retained by both distribution and Langmuir-type adsorption on residual silanol. Retention on ODS silica with more silanol proceeded at a higher rate than retention on ODS silica with less silanol and was reversible on this solid phase extraction time-scale. An increase in surface density of ODS decreased the distribution constant, due to a decrease in fraction of ODS functioning as an extracting medium and also decreased the saturated adsorption amount, due to reductions of the residual silanol and the functioning ODS. The ODS silica with the lowest ODS density showed a distribution constant 31 times higher and a saturated adsorption amount 27 times higher than the ODS silica with the highest ODS density. On the other hand, because the interaction between the organic part and the ODS group introduced at higher density is strengthened, the adsorption constant is increased by about 5 times compared to the low density ODS silica. The electronic effects of substituents to nitrogen- and oxygen-containing compounds on retention were discussed. In conclusion, ODS silica with an appreciable amount of residual silanol is superior for solid phase extraction.
RESUMEN
Batchwise studies have demonstrated that the retention of acetylacetone (Haa) from an aqueous phase in non-endcapped octadecylsilyl (ODS) silica with silanol intentionally left (NEC) is stronger than the retention in endcapped ODS silica under ambient pressure, due to the enhanced wettability and the contribution of adsorption to silanol (-SiOH) as well as of the distribution into the ODS phase. Equilibrium analysis on solid phase extraction (SPE) of copper ion with Haa to NEC has indicated the adsorption of 1:1 species by the proton replacement reaction with silanol to give [-SiOCu(aa)] at relatively low Haa concentrations and the adsorption of [Cu(aa)2] to silanol, as well as the distribution of [Cu(aa)2] into the ODS phase. These adsorbed and distributed species of the same composition could be differentiated by ESR spectroscopy. The advantages of residual silanol in SPE of metal ions are discussed.