Polymer, monolith

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. 

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). 

The monomers commonly used for the preparation of polymer monoliths are either hydrophobic, for example, styrene/divinylbenzene and alkyl methacrylates, or hydrophilic, for example, acrylamides. The polymerization is usually accomplished by radical chain mechanisms with thermal or photochemical initiation, as detailed in the reviews (Eeltink et al., 2004 Svec, 2004a and b). Internal structures of polymer monoliths are described to be corpuscular rather than spongy this means through-pores were found to be interstices of agglomerated globular skeletons as shown in Fig. 7.1 (Ivanov et al., 2003). Porosity is presumably predetermined by the preparation... 

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). 

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. 

A rather limited range of mesopores in terms of size and volume were observed in the skeletons of polymer monoliths. The porosity of the polymer monolith seems to be lower than that of silica monolith. The total porosity of these monoliths is in the range of 0.61-0.73, whereas interstitial (through-pore) porosity and mesopore porosity are 0.28-0.70 and 0.03-0.24, respectively. In the case of poly(butyl methacrylate-co-ethylene dimethacrylate), the observed porosity is around 0.61-0.71, resulting in permeability 0.15-8.43 x 10 14 m2, whereas the observed porosity of silica monoliths prepared in a capillary is 0.86-0.96 and the permeability is 7-120 x 10 14 m2. Higher permeability will be advantageous for 2D applications, as mentioned later. 

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). 

As mentioned earlier, high-speed separation is necessary to carry out fast, comprehensive 2D HPLC. The polymer monoliths have not been employed in such 2D HPLC, probably because permeability of polymer monoliths is not high enough to allow fast elution of the second dimension (2nd-D) in simple 2D operation, and the gradient cycle at the 2nd-D cannot be so fast to allow online 2D operation without reducing peak capacity at first dimension (lst-D). 

The application of polymer monoliths in 2D separations, however, is very attractive in that polymer-based packing materials can provide a high performance, chemically stable stationary phase, and better recovery of biological molecules, namely proteins and peptides, even in comparison with C18 phases on silica particles with wide mesopores (Tanaka et al., 1990). Microchip fabrication for 2D HPLC has been disclosed in a recent patent, based on polymer monoliths (Corso et al., 2003). This separation system consists of stacked separation blocks, namely, the first block for ion exchange (strong cation exchange) and the second block for reversed-phase separation. This layered separation chip device also contains an electrospray interface microfabricated on chip (a polymer monolith/... 

Svec, E (2004a). Preparation and HPLC applications of rigid macroporous organic polymer monoliths. J. Sep. Sci. 27, 747-766. 

The use of silica and polymer monoliths (Svec 2004 Peterson et al. 2003 Yang et al. 2005) can also significantly increase the surface area-to-volume ratio. In addition, the porous monoliths can be equipped to comprise reactive functionalities or even reactants to perform specific chemical or biochemical reactions (Svec 2004). Two main problems, however, may be encountered during the operation of monoliths (a) the... 

Peterson DS, Rohr T, Svec FK, Frechet JMJ (2003) Dual-function microanalyt-ical device by in situ photolithographic grafting of porous polymer monolith integrating solid-phase extraction and enzymatic digestion for peptide mass mapping. Anal Chem 75 5328... 

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. 

The number of monomers that may be used in the preparation of polymer monoliths is much larger than those used for classical suspension polymeriza-... 

D. J. Throckmorton, T.J. Sheppodd and A.K. Singh, Electrochromatography in microchips Reversed-phase separation of peptides and amino acids using photopattemed rigid polymer monoliths. Anal. Chem. 74 (2002) 784—789. 

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. 

---