Arlo McGinn and John D Lamb, Chemistry and Biochemistry

Ion chromatography is an instrumental technique known for its ability to determine and quantify mixtures of ions in solution. In ion chromatography, a solution containing different ions is passed through a column packed with a stationary phase containing active groups (chemical compounds that exhibit binding properties). These active groups show affinity for the various ions in solution. Chemical separations are achieved in ion chromatography by means of electrostatic interactions and coordinate covalent bond formation between the active groups in the stationary phase of the chromatography column and the ions present in the mobile phase or eluent. Strong interactions effectively slow down the rate at which the ions make their way through the labyrinthine packing material. Ions which are not attracted to the stationary phase elute faster and ions which interact with the active groups in the packing material elute slower. The elution of the ions can be identified by a conductivity detector which also helps us to quantify ion concentrations in solution.

Pioneering work has been done here at BYU with the synthesis of a novel class of molecules, namely substituted resorcinarenes. These resorcinarene bowls have been substituted with four macrocyclic ligands, enabling them to selectively bind cations in solution. The resulting positively-charged tetranuclear metal complex has been observed to electrostatically bind anions. The ligating groups bound to the resorcinarene bowl also play an important role in the selectivity of the ligand for ions. To first determine the selectivity of the ligands we adsorbed some of the active ligating groups, which we synthesized, onto an ion chromatography column with neutral divinylbenzene MPIC resin as the packing material. This allowed us to ascertain which anions the charged groups would show the most affinity for.

We found the different ligands to show distinct differences in selectivity for several anions. The changing of the eluent also affected the selectivity and the separation the peaks due to different anions. We found some ligands that showed selectivity for nitrates, sulfates, and many other polynuclear anions.

It has been hypothesized that the distances between the four coordinated metal centers may provide a cavity in which large anions such as selenate and pertechnetate may be bound by cooperative electrostatic interactions. Selenate and pertechnetate are both toxic and harmful anions that have been introduced to the environment. Living organisms require selenate in trace amounts but large amounts are highly toxic. This anion is introduced to the environment through mining waste and the irrigation of arid lands. Pertechnetate is a radioactive component of nuclear waste and constitutes a potential danger to the environment. The ability to detect and quantify these anions effectively would, therefore, be of great benefit to the scientific community and the environment. Though we were unable to find a ligand that bound these anions with high selectivity, the work is shedding insight into the binding character of different ligands. Armed with this information, we will be able to design ligands that we believe will show selectivity for these anions of interest.