Columns, capillary monolith silica

Three main types of columns are used in CEC packed capillary columns, monoliths, and open tubular columns, all of them being made from fused silica capillaries. 

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). 

Chromatographic use of monolithic silica columns has been attracting considerable attention because they can potentially provide higher overall performance than particle-packed columns based on the variable external porosity and through-pore size/skeleton size ratios. These subjects have been recently reviewed with particular interests in fundamental properties, applications, or chemical modifications (Tanaka et al., 2001 Siouffi, 2003 Cabrera, 2004 Eeltink et al., 2004 Rieux et al., 2005). Commercially available monolithic silica columns at this time include conventional size columns (4.6 mm i.d., 1-10 cm), capillary columns (50-200 pm i.d., 15-30 cm), and preparative scale columns (25 mm i.d., 10 cm). 

FIGURE 7.3 Scanning electron micrographs of monolithic silica prepared from sol-gel methods, (a) monolithic silica prepared from TMOS in a test tube, and monolithic silica columns prepared from a mixture of TMOS and MTMS, (b) in a 50-pm fused silica capillary, (c) in a lOO-pm fused silica capillary, and (d) in a 200-pm fused silica capillary tube (reproduced from the reference, Motokawa et al. (2002), with permission from Elsevier). 

Correlation was found between domain size and attainable column efficiency. Column efficiency increases with the decrease in domain size, just like the efficiency of a particle-packed column is determined by particle size. Chromolith columns having ca. 2 pm through-pores and ca. 1pm skeletons show H= 10 (N= 10,000 for 10 cm column) at around optimum linear velocity of 1 mm/s, whereas a 15-cm column packed with 5 pm particles commonly shows 10,GOO-15,000 theoretical plates (7 = 10—15) (Ikegami et al., 2004). The pressure drop of a Chromolith column is typically half of the column packed with 5 pm particles. The performance of a Chromolith column was described to be similar to 7-15 pm particles in terms of pressure drop and to 3.5 1 pm particles in terms of column efficiency (Leinweber and Tallarek, 2003 Miyabe et al., 2003). Figure 7.4 shows the pressure drop and column efficiency of monolithic silica columns. A short column produces 500 (1cm column) to 2500 plates (5 cm) at high linear velocity of 10 mm/s. Small columns, especially capillary type, are sensitive to extra-column band... 

FIGURE 7.13 Two-dimensional separation of tryptic digest of BSA in simple 2D-HPLC. Capillary monolithic silica-C18 column (0.1 mmi.d., 10 cm) was used as 2nd-D column. Mobile phase for 2nd-D gradient started with 0% B at 0.5 min, increased to 50% B at 3.3 min, to 100% B at 3.5 min, then returned to the initial condition and held for the last 0.5 min. Flow rate 3.0 pL/min in capillary, and 2 mL/min at the pump. Other conditions are similar to those for Figure 7.11 (reproduced from the reference, Kimura et al. (2004) with permission from Wiley). 

Chankvetadze, B., Yamamoto, C., Tanaka, N., Nakanishi, K., Okamoto, Y. (2004). High-performance liquid chromatographic enantioseparations on capillary columns containing monolithic silica modified with cellulose tris(3,5-dimethylphenylcarbamate). J. Sep. Sci. 27, 905-911. 

Kobayashi, H., Kajiwara, W., Inui, Y., Hara, T., Hosoya, K., Ikegami, T., Tanaka, N. (2004). Chromatographic properties of monolithic silica capillary columns for polar and nonpolar compounds in reversed-phase HPLC. Chromatographia 60, S19-S25. 

Horvath et al. sintered the contents of a capillary column packed with 6 pm oc-tadecylsilica by heating to 360 °C in the presence of a sodium bicarbonate solution [101]. These conditions also strip the alkyl ligands from the silica support, thus significantly deteriorating the chromatographic properties. However, the performance was partly recovered after resilanization of the monolithic material with dimethyloctadecylchlorosilane allowing the separation of aromatic hydrocarbons and protected aminoacids with an efficiency of up to 160,000 plates/m. 

Although Fields already mentioned the possible preparation of monolithic silica-based CEC columns, the lack of experimental data leads to the assumption that this option has not been tested [111]. In fact, it was Tanaka et al. who demonstrated the preparation of monolithic capillary columns using a sol-gel transition within an open capillary tube [99,112]. The trick was in the starting mixture that in addition to tetramethoxysilane and acetic acid also includes poly(ethylene oxide). The gel formed at room temperature was carefully washed with a variety of solvents and heated to 330 °C. The surface was then modified with octadecyl-trichlorosilane or octadecyldimethyl-A N-dimethylaminosilane to attach the hy-... 

Fig. 18. Scanning electron micrograph of monolithic silica-based capillary column. (Reprinted with permission from [205]. Copyright 2000 American Chemical Society)... 

Fig. 19. Separation of alkylbenzenes C6H5CnH2n+1 (n=0-6) on an in situ prepared monolithic silica column. (Reprinted with permission from [99]. Copyright 2000 VCH-Wiley). Conditions voltage 900 V/cm, capillary column 100 pm i. d., total length 33.5 cm, active length 25 cm, iso-cratic separation using 90 10 acetonitrile-50 mmol/1 TRIS buffer pH = 8, column efficiency 58,000 plates/m...

Fig. 10. Scanning electron micrographs of monolithic poly(divinylbenzene) capillary column. Note that the porous monolith is surrounded by an impervious tubular outer polymer layer resulting from copolymerization of the monomer with the acryloyl moieties bound to the capillary wall. This layer minimizes any direct contact of the analytes with the surface of the fused-silica capillary...

FIGURE 8.10 SEM image of the allylorgano-silica monolith in a 50- xm ID fused-silica capillary column. The monolith was fabricated using allyl-TMS TMOS (1 4 mol ratio). (Reprinted from H. Colon et al., Chem. Comm., 2826 (2005). With permission. Copyright Royal Society of Chemistry 2005.)... 

Jia et al. (2005) developed a two-dimensional (2-D) separation system of coupling chromatography to electrophoresis for profiling Escherichia coli metabolites. Capillary EC with a monolithic silica-octadecyl silica column (500 x 0.2 mm ID) was used as the first dimension, from which the effluent fractions were further analyzed by CE acting as the second dimension. Multi-dimensional separations have found wide applications in biomedical and pharmaceutical analysis. 

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