Commercially prepared capillary columns

Schematic structures of both a dimethyl polysiloxane and a polyethylene glycol liquid phase were given in Chapter 4. There is, however, one difference between packed column and capillary column liquid phases capillary column phases are extensively cross-linked. By heating the freshly prepared capillary column at high temperatures (without column flow) the methyl groups form free radicals which readily cross-link to form a more stable, higher molecular weight gum phase. There is even some chemical bonding with the silanol groups on the fused silica surface. These cross-linked and chemically bonded phases are more temperature stable, last longer and can be cleaned by rinsing with solvents when cold. Most commercial capillary columns are cross-linked.

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). 

Materials and instrumentation. Experiments were performed using a special home-made cell coupled to a dual current supply with a maximum output of 10 V/40 mA. A detailed technical description of this system is published elsewhere (10). H NMR spectra were recorded on a Varian Mercury vx300 instrument at 25 °C. GC analysis was performed on an Interscience GC-8000 gas chromatograph with a 100% dimethylpolysiloxane capillary column (DB-1, 30 m x 0.325 mm). GC conditions isotherm at 105 "C (2 min) ramp at 30 °C min to 280 °C isotherm at 280 °C (5 min). Pentadecane was used as internal standard. The ionic liquid [omim] [BF4] was prepared following a published procedure and dried prior to use (8). All other chemicals were purchased from commercial sources (> 98% pure). 

These examples clearly demonstrate the benefits of the facile tuning of surface chemistry afforded by the monolithic media. The wealth of commercially available monomers possessing a wide variety of functionalities, together with the extreme simplicity of the preparation of the monolithic columns, makes this approach an appealing option for the design of capillary columns with high selectivities. 

The particle-packed capillary column was the most favorable column style for CEC due to the mature arts and crafts in adopting commercially available HPLC stationary phases with the great versatility of surface functionality and particle structure. The preparation of a packed capillary involves the preparation of end frits and column packing. Similarly, the chaimel in a microfluidic chip may be packed with these particle materials, providing low mobile-phase mass transfer resistance for p-CEC. Stationary phase is usually held in the position by weir or other barrier stmctures within the microchannels. 

If large samples need to be separated, the diameter, and also often the length, of the column are increased. (Obviously, open capillaries cannot be used for this purpose.) Preparative GC is an attractive approach (though the fraction collector needs to be cooled) but few commercial instruments are available. Preparative LC is the most important technique in organic synthesis, biochemical research, downstream processing in biotechnology, and for the commercial preparation of certain chemicals or drugs. 

They have not yet become commercially available. Bruner and co-workers [25, 37] showed that preparation of effective capillary columns with GTCB impregnated with a liquid phase is easier. 

M -CHiOH), 111 (24%, M -C3H7), 93 (17%, Nf.H2O-C3H7), 69 (90%, CsH9, and 41 (100%, 3115 ), indicative of a monoterpene alcohol. The con )ound was tentatively identified as the monoterpene alcohol lavandulol (1) by conq)arison with database spectra. The identification was verified with retention time matches on 3 GC capillary columns (DB-5, DB-17, and DB-WAX), and the mass spectrum matched t t of an authentic standard prepared by hydrolysis of commercial lavandulyl acetate (TCI America). 

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