Fundamental studies in the solid-phase extraction of organic cations and neutral compounds: The role of hydrophobic and ionic interactions
AuthorGonzalez, Ricardo Rene
AdvisorBurke, Michael F.
MetadataShow full item record
PublisherThe University of Arizona.
RightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
AbstractThe work presented is a systematic investigation of silica-based sorbents using solid-phase extraction (SPE). Properties of various cation exchangers and mixed beds comprised of strong cation exchange particles and alkyl-modified silica particles were explored. The role of ionic and nonpolar interactions was examined in terms of retention and elution of analytes. A series of displacement experiments was used to characterize the interactions and energy-minimized, three-dimensional models were used to illustrate these interactions. Selectivity of cation exchangers was probed by introducing cations differing in size, hydrophobic nature and charge before and after introduction of the analytes. The ionic strength, the solvent composition and the pH of the liquid phase were parameters that were varied in order to define the type of interaction that was responsible for retention, lack of retention, or elution during the SPE procedure. Application of mixed sorbents toward the extraction of basic drugs of abuse, as well as the use of reversed-phase silica and a resin for the extraction of a neutral organonitrogen species were also investigated. With propylene-linked cation exchangers (under appropriate pH conditions), it was shown that the primary mode of interaction is through ionic forces. As a consequence, there is no selectivity of these sorbents for cations based on hydrophobic nature. With ethylbenzene and octylene-linked exchangers, there is an increase in selectivity for cations with increasing hydrophobic nature. The linker chain is adequately long enough to allow for nonpolar interactions with the aliphatic side groups, just as a resin-based exchanger allows. With short and long chain exchangers, there is selectivity proportional to the charge of the ion. The proposed reason for this is a combination of multiplesite attachment of polyprotic species to the surface and increased probability of single-site interaction proportional to the number of charged nitrogen atoms on the molecule. It was shown that mixed sorbent beds offer advantages in operator control over the retention of organic bases in a high ionic strength sample matrix. The retention mechanism was elucidated, demonstrating a clear contribution of nonpolar forces to retention with the mixed sorbents as was observed with the long-chain cation exchangers.
Degree ProgramGraduate College