Factors affecting the selectivity and efficiency of solid phase extraction of ionizable compounds

Abstract

Solid phase extraction (SPE) is an extension of conventional liquid chromatographic methods. In the case of SPE the desired separation is digital in nature. Under one solvent condition total retention occurs while under a second solvent condition total elution occurs. The role of SPE as a sample preparation tool is becoming more dominant in the field of analytical chemistry. The need for high throughput, decreased solvent usage, and automation makes SPE the method of choice over other extraction methods. Ionic interactions at modified silica sorbents are studied with respect to the impact on the isolation and purification of analytes. The interactions are characterized by quantitating absolute recoveries for a variety of analytes under different conditions, and qualitative analysis of analytical interferences. Extractions of ionizable compounds were found to be dependent on a myriad of factors. These factors include sample pH, buffer composition, solution concentration, and elution solvent composition. Selectivities for ionizable analytes can be increased by utilizing ionic interactions, particularly in a mixed-mode operation. The extraction of basic compound compounds is generally robust, while methods for the acidic analytes are more highly dependent on the type of counter-ion. Ion selectivities observed were different than those recorded for polymer-based ion-exchange sorbents. The organization of water in the near surface region of the silica-based sorbents is the reason for the disparity between the two types of materials. The utility of a polymeric polystyrene divinylbenzene sorbent for the analysis of analytes from biological matrices is demonstrated. It was shown that these sorbents can withstand harsh chemical environments that would preclude using a silica-based material. Analysis times were shortened and extraction efficiencies were comparable to the silica materials. The extractions were not as selective as those performed on mixed-mode sorbents however.