Multimodal sorbent

Boonjob W et al (2010) Online hyphenation of multimodal microsolid phase extraction involving renewable molecularly imprinted and reversed-phase sorbents to liquid chromatography for automatic multiresidue assays. Anal Chem 82(7) 3052-3060... 

In the case of gel permeation or size-exclusion HPLC (HP-SEC), selectivity arises from differential migration of the biomolecules as they permeate by diffusion from the bulk mobile phase to within the pore chambers of the stationary phase. Ideally, the stationary phase in HP-SEC has been so prepared that the surface itself has no chemical interaction with the biosolutes, with the extent of retardation simply mediated by the physical nature of the pores, their connectivity, and their tortuosity. In this regard, HP-SEC contrasts with the other modes of HPLC, where the surfaces of the stationary phase have been deliberately modified by chemical procedures by (usually) low molecular weight compounds to enable selective retardation of the biosolutes by adsorptive processes. Ideally, the surface of an interactive HPLC sorbent enables separation to occur by only one retention process, i.e., the stationary phase functions as a monomodal sorbent. In practice with porous materials, this is rarely achieved with the consequence that most adsorption HPLC sorbents exhibit multimodal characteristics. The retention behavior and selectivity of the chromatographic system will thus depend on the nature and magnitude of the complex interplay of intermolecular forces... 

The term multimodal has been used in two ways in TLC, to designate layers such as bonded cyano sorbents that can operate with two or more mechanisms (see Section IV.C) or, in the context of this section, to specify multidimensional separations that are performed by coupling TLC, HPTLC, or OPLC (223) with another technique, such as gas chromatography, supercritical fluid chromatography (224), countercurrent chromatography (225), and, most commonly, HPLC (145, 226-228), in order to improve the separation capacity available from either of the individual methods. For example, the combination of adsorption AMD-HPTLC and partition HPLC for water analysis produced as many as 700 individual densitometric peaks (49). Multimodal TLC separations have been reviewed (229-231). 

---