Monolithic stationary phases preparation

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Sinner and Buchmeiser prepared a new class of functionalized monolithic stationary phases (norbornene monoliths) by a ROMP process that tolerates a huge variety of functional monomers [69]. They used this approach in order to chemically attach (3-cyclodextrin onto the monomer prior... 

Lammerhofer et al. [127] demonstrated the use of a strong anion-exchange stationary phase, prepared in a monolithic format, for the separation of a mixture of four NSAIDs—ibuprofen, naproxen, ketoprofen, and suprofen. The separation, presented in Figure 29, was achieved in 13 min with high column efficiencies of up to 231,000 plates/m. 

One of the problems inherent to high-flow operation is increased back pressure, particularly when methanol is used in the mobile phase. This difficulty has been circumvented by the introduction of monolithic stationary phases, which employ a unique contiguous biporous structure prepared from sol-gel chemistry [90,91]. Reduced flow resistance is accomplished by throughpores (2 /zm), while smaller mesopores (13 nm) provide the surface-area capacity needed for adequate chromatographic separation. High flow rates can be employed with monolithic columns due to reduced back pressure and the ability for facile mass transfer. 

Preparation of Polyacrylamide Monoliths Hjerten et al. [149] introduced the monolithic stationary phases based on acrylamides in the late 1980s. The cross-linked polyacrylamide can be directly synthesized within the mold by a one-step free-radical chain polymerization. Acrylamide, methacrylamide, or piperazine diacrylamide are often employed as monomers, while V,V -methylene-fcM-acrylamide is used as a cross-linker. 2-Acrylamido-2-methylpropane sulfonic acid, vinylsulfonic acid, butyl methacrylate, or stearoyl methacrylate are usually added to the polymerization mixture to provide charge and functional groups [140]. 

Monolithic stationary phases have the same advantages as packed chromatographic beds in a capillary or a microchannel for electrochroma-tographic analysis, including high surface area and easily controlled surface chemistry. In particular, the preparation of monolithic materials is relatively easy as a prepolymerization solution with low viscosity is simply introduced into the channels. The resulting monoliths in microfluidic chips are well anchored onto the inner walls of... 

The CIM disks, with low-volume monolith stationary phase, used in this application allowed the desired separation of the analyte under low-pressure conditions. The use of a mixing chamber and the precise control of timing allowed the inline preparation of the necessary high number of standard solution mixtures for the multivariate calibration. The mixing chamber also provided a homogeneous mixture before the transport to the detector. The large sampled volume (1 mL) allowed the recording of spectra without dispersion effects (D = 1). Samples were diluted and their pH was adjusted to 8.5 before injection. Nevertheless, validation for the tartaric acid concentration was not possible with the enzymatic kits used in this work. 

---