Factors affecting selectivity of covalent chromatography.

Abstract

Of the interactions utilized in the separation of chemical species, the reversible covalent bond is the strongest and most selective. In order to exploit the selectivity of this interaction, an understanding of the effects of several factors on the formation of the covalent bond have been studied. Covalent chromatography is most useful in solid phase extraction. The strength of the covalent bond will allow for high distribution coefficients needed to quantitatively retain chemical species. The selectivity of the covalent interaction allows for the retention of a specific compound or class of compounds. Under conditions where the covalent bond is no longer formed or will break, a low distribution coefficient is possible, and the compounds may be eluted from the sorbent in a small volume allowing for preconcentration. The sorbent consists of three parts, the active functionality capable of forming reversible covalent bonds, the solid support and the spacer arm that tethers the active functionality to the support. Silica supports demonstrate several advantages over organic supports. However, silica supports have been limited by the activity of the residual silanols and the use of hydrophobic spacer arms. This research describes the preparation and characterization of a modification method for silica with a hydrophilic spacer arm that restricts the sample from the residual silanols. Immobilized phenylboronic acid (PBA) interacts with compounds containing polar functionalities in the correct configuration, but this interaction is dependent on the local environment. Controlling the local environment allows for the control of the interaction provided the effects are understood. Diagnostic chromatography was used to determine the effects of the solvent strength, ionic strength, pH and composition of the mobile phase and the effects of the spacer arm on the interactions of PBA with several compounds. Three phases selective for thiols were also characterized. Thiopyridone attached to mercaptopropyl bonded silica through a disulfide linkage, is used for isolation and detection. Immobilized phenylmercury was utilized for the extraction of thiol containing species while immobilized phenylarsonous acid selectively extracts dithiol compounds. A unique and powerful means of separation of monothiol from dithiol compounds has been demonstrated.