There are numerous variants of producing solid-phase extraction material, but the most widely used techniques are based on the physical adsorption of the extractant into the pores of the support material. The preferred method for incorporating the chosen extractant into the support is often dependent upon the type of extractant-metal complex and is therefore determined by the intended separation process. Preparation routes reported in the literature during the last forty years primarily involve adsorbing the extractant into the lattice of a polymer substrate — known as impregnation methods — or adding the extractant to a mixture of monomers during the bead polymerization process, i. e. the well-known Levextrel resins which incorporate the extractant in polystyrene-divinlybenzene during the copo­lymerization step (Kauczor et al., 1978, Poinescu et al., 1985, Ionue et al., 1987, Yoshizuka et al., 1990). Other techniques such as adsorbing the extract­ant in silica gel, silica and organic polymer mixtures or adding the extractant to a dissolved polymer and then precipitating the mixture have also been reported. Excellent reviews of the classical preparation techniques have been presented by Warshawsy (1981) and Cortina et al., (1997) and the reader is directed to these works for detailed descriptions of the synthetic methods. Variants of the classical impregnation methods have received the most recent attention in terms of developing solid-phase materials with extractant systems relevant to An and Ln separations. A brief review of the impregnation methods is therefore germane to this work in order to famil­iarize the reader with this production scheme.

Recalling the assumption that a solid-phase extraction resin represents a complexing agent dispersed homogeneously within a solid polymeric medium, the impregnated extractant should behave as in liquid-liquid extraction, but maintain a strong affinity for the solid polymer matrix. In order to approximate these criteria, Warshawsky (1981) presents the fol­lowing requirements for the extractant, polymeric support, and the impreg­nation process:

• The extractant should be a liquid or retained in the liquid state by the addition of an appropriate diluent.

• The extractant should have a very minimal solubility in the aqueous solution containing the solute to be extracted.

• The polymeric support should be fully expanded and remain so during the impregnation process — macroporous polymers exhibit minimum volume variations during impregnation and are therefore preferred.

• The impregnation process should not have a deleterious effect on the properties of the extractant or polymer.

Previous reviews by Warshawsy (1981) and Cortina et al. (1997) have classified the methods for producing extractant impregnated resins as follows: [14]

contacted with the polymer in batch mode for a period of time necessary to obtain maximum saturation of the extractant within the polymer pores. The diluent is then removed via vacuum evaporation resulting in a two-component, polymer-extractant material.

• Wet impregnation — an extractant or extractant and diluent is adsorbed into the polymer as described above, but the water-immiscible organic diluent is not removed by evaporation.

• Modifier addition method — a modifier is added to the extractant-diluent and the mixture is adsorbed into the polymer as described in the above methods. The diluent is then removed by evaporation, leaving a poly­mer-extractant-modifier resin. The chosen modifier is typically more polar than the extractant and is added to enhance water penetration into the porous network of the polymer.

Of course, slight modifications to these methods may be used depending on the desired application of the solid-phase extraction material. The dry method is preferred for impregnating the more hydrophilic extractants such as amines, ketones, and esters and is the most used technique for making solid-phase extractants applicable to An or Ln separations. The preparation of a small batch of resin using the dry impregnation method is shown in Fig. 13.2.

It should also be noted that the class of inert, macroporous or macrore­ticular polymers have become the most widely used substrates in the con­temporary synthesis of solid-phase extraction media for separating the An

and Ln elements (Horwitz et al., 2006). These polymeric macroporous resins have a rigid three-dimensional structure that provides minimum solvent swelling during the impregnation process. The inert macroporous resins are also capable of adsorbing large amounts of the extractant due to a high specific surface area (500-900 m2/g) and typical porosity fraction ranging from 0.4 to 0.6 (Van Hecke et al., 2006). The macroreticular polymers are typified by a continuous gel phase and a continuous pore phase and are manufactured with varying degrees of hydrophobicity. Thus the specific polymer can be matched with the properties of a given extractant to opti­mize the hydrophobic/hydrophilic balance. This is important in the solid — phase extraction media since the goal is to minimize losses of the extractant to the aqueous phase while maintaining enough hydrophilicity to maximize mass transfer of the solute into the resin pores and between phases. Examples of the macroreticular polymers frequently used are the non-ionic Amberlite XAD and Amberchrom CG series resins.

Most studies support the assumption that immobilization of the extract­ant on the internal surface of the macroporous, inert resins is a combina­tion of adsorption via van der Waals forces (Hommel et al., 1983, Bobozka et al., 1985, Cote et al., 1987, Handley et al., 1991, Cortina et al., 1994a, Cortina et al., 1993, Villaescusa et al., 1992) and potentially physical trap­ping of the ligands within the pores of the resin beads (Handley et al., 1991). Impregnation of the extractant within the stationary phase is con­sidered to be a combination of pore filling and surface adsorption. The extractant first fills the pore space beginning with the smallest pores and moving up to pore sizes of approximately 10 nm. Surface adsorption then becomes the dominant retention mode in the larger pores (Guan et al., 1990). Although it is not yet possible to perform a precise a priori predic­tion of the adsorptive and retention properties of a given ligand-polymer system, the hydrophobic or non-polar sections of a ligand molecule are generally attracted to hydrophobic polymers and the hydrophilic or polar molecules to hydrophilic surfaces. For example, the non-ionic, hydrophilic XAD-2 and XAD-4 resins (Amberlite) are often used with many of the ligands important to An and Ln separations because the non-polar vinyl and styrene groups of the polymeric matrix serve to anchor the extractants through their alkyl chains and/or aromatic rings, while the functional groups in the ligand remain capable of forming the desired metal complex (Fig. 13.3). This has been shown by various studies (Bobozka et al., 1985, Cote et al., 1987, Cortina et al., 1994a, Cortina et al., 1993) wherein the authors concluded that this weak interaction between the extractant mol­ecules and the polymer support is the primary mode of ligand retention, but does not adversely affect the complexing characteristics of the extractant.

13.2

Depiction of ligand arrangement on polymer substrate.