Polymer monoliths columns

This section provides an overview of properties of polymer monolith columns related to 2D-HPLC. Monolithic organic polymer columns, having longer history than silica monoliths, have been reviewed in detail recently by S vec and by Eeltink including their preparation methods and performance (Eeltink et al., 2004 Svec, 2004a). Polymer monolith columns commercially available include polyfstyrene-co-di vinyl benzene) (PSDVB) columns and poly(alkyl methacrylate) columns. 

Table 7.1 shows the pore properties of several polymer monolithic columns prepared from styrene/DVB, methacrylates, and acrylamides along with the feed porosity and column efficiency, summarized from several recent publications. Some important points seem to be clearly shown in Table 7.1, especially by the comparison of the properties between methacrylate-based polymer monoliths and silica monoliths. 

Polymer monolithic columns with small diameter have been successfully employed for proteome analysis. Karger and coworkers reported MALDI-TOF of separated fractions spotted on a plate from a polymeric reversed-phase column that showed high peak capacity (Chen et al., 2005). Huber and coworkers reported separation and detection of about 200 peaks within 5 min by using a PSDVB column (Premstaller et al., 2001). 

FIGURE 5.11 Selection of (a) monomers with functional groups, (b) cross-linkers, (c) charged monomers for EOF generation, and (d) chemical initiators used for the preparation of polymer monolithic columns. 

The reactive porous polymer monoUths can be used as bioreactors to immobiUze specific enzymes, proteins, and DNA [29,126-135]. Svec, Frechet and colleagues reported a patterned polymer monolith column containing reactive vinylazlactone, which reacts with amines, through post-polymerization modification [130]. The... 

It is of much interest to compare polymer monoliths with monolithic silica columns for practical purposes of column selection. Methacrylate-based polymer monoliths have been evaluated extensively in comparison with silica monoliths (Moravcova et al., 2004). The methacrylate-based capillary columns were prepared from butyl methacrylate, ethylene dimethacrylate, in a porogenic mixture of water, 1-propanol, and 1,4-butanediol, and compared with commercial silica particulate and monolithic columns (Chromolith Performance). 

To improve chromatographic separation, another analytical column could be used in addition to the monolith (Xu et al. 2006). The monolith column served as an extraction column only. Hsieh et al. (2000, 2002) utilized a polymer-coated mixed function (PCMF) Capcell C8 column (4.6 x 50 mm, Phenomenex) to provide dual functions—online plasma extraction and analyte separation. The silica was coated with a polymer containing both hydrophilic polyoxythylene and hydrophobic groups. The diluted plasma samples (1 1 to 1 3) were injected directly. No column deterioration was observed after 200 injections. 

Several research groups used another interesting column technology as an alternative to the modification of the capillary surface. This method is inherited from the field of electrophoresis of nucleic acids and involves capillaries filled with solutions of linear polymers. In contrast to the monolithic columns that will be discussed later in this review, the preparation of these pseudostationary phases need not be performed within the confines of the capillary. These materials, typically specifically designed copolymers [85-88] and modified den-drimers [89], exist as physically entangled polymer chains that effectively resemble highly swollen, chemically crosslinked gels. 

Chirica and Remcho first created the outlet frit, packed the column with ODS beads, and then fabricated the inlet frit. The column was filled with aqueous solution of a silicate (Kasil) and the entrapment achieved by heating the column to 160 °C [105,106]. The monolithic column afforded considerably reduced retention times compared to the packed-only counterpart most likely due to a partial blocking of the pores with the silicate solution. This approach was recently extended to the immobilization of silica beads in a porous organic polymer matrix [107]. 

While only a few reports concern the in situ preparation of monolithic CEC columns from silica, much more has been done with porous polymer monoliths and a wide variety of approaches differing in both the chemistry of the monomers and the preparation technique is currently available. Obviously, free radical polymerization is easier to handle than the sol-gel transition accompanied by a large decrease in volume. 

Commercially available monolithic columns are based either on silica or organic polymer and are generally characterized as a polymeric skeleton with macropores, with a diameter of approximately 2 pm, and mesopores, with a diameter of approximately 13 nm. The role of the macropores (through-pores) is to provide channels with high compounds permeability, which permits the use of higher flow rates with respect to columns based on conventional particle size, and an extended surface area, which is comparable to conventional columns packed with 3 pm particles. 

Monolithic columns with the chiral anion exchange-type selectors incorporated into the polymer matrix obtained through in situ copolymerization process of a chiral monomer (in situ approach) [80-83,85] or attached to the surface of a reactive monolith in a subsequent derivatization step (postmodification strategy) [84], both turned out to be viable routes to enantioselective macroporous monolithic columns devoid of the limitations of packed columns mentioned earlier. 

Current research in CEC involves the use of monolith capillaries, which are fritless, packed capillaries having stationary phase bound to the capillary wall. Using porous polymer monoliths, the retention of a packed column can be found in an open tubular capillary. In general, CEC remains unsettled. Frit technology is unreliable and research into monolithic capillaries is still a work in progress. Recent progress in CEC can be found in the reviews by Colon and co-workers. 

In general the preparation of a polymer monolithic rod is performed as a multi-step procedure (Figure 11). Generally, the stages involved are pre-treatment and preparation of the monolithic matrix by polymerization and derivatization or functionalization. Pre-treatment of the bare capillary is sometimes needed in order to obtain good physical stability. Most columns are therefore polymerized in silanized columns. The capillary column is first washed with a strong alkaline solution such as 1.0 M sodium hydroxide so that the siloxane groups at... 

In a related study this group also demonstrated the use of non-volatile solvents in CEC-MS without compromising the quality of spectra that has also been demonstrated using polymer-based monolithic column prepared by in situ copolymerization of butyl methacrylate with sulfonic acid functionalities. 

---