Packed capillary columns monolithic

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. 

Several thousands of different columns are commercially available, and when selecting a column for a particular separation the chromatographer should be able to decide whether a packed, capillary, or monolithic column is needed and what the desired characteristics of the base material, bonded phase, and bonding density of selected column is needed. Commercial columns of the same general type (e.g., CIS) could differ widely in their separation power among different suppliers. Basic information regarding the specific column provided by the manufacturer, such as surface area, % carbon, and type of bonded phase, usually does not allow prediction of the separation or for the proper selection of columns with similar separation patterns. 

Monolithic stationary phases have emerged in the last few years as an attractive alternative to particle-packed capillary columns as a method to completely eliminate the need for bed-retaining frits and their associated problems [302,306,307]. The dimensional stability of the monolithic structures results from their rigidity and/or chemical attachment to the inner wall of the capillary. Several synthetic strategies have been described, but the most useful are based on molded porous polymers [306,326-331], molded porous sol-gel continuous beds [302], hydrothermal immobilization of packed beds [332] and particle-fixed continuous beds [302,333,334]. 

CEC columns are generally made of fused-silica tubing, usually packed with the appropriate stationary phase. Today, the most commonly used CEC columns have i.d. of 100 p,m or less, with 50 and 75 p,m i.d. being the most popular. The stationary phase is retained in the column by two frits. Column designs can be categorized into two major types OT columns and packed structures, which include packed columns, monolithic columns, and microfabricated stractures (open or continuous beds). Packed capillary columns are most commonly used, as has been demonstrated in numerous papers [9-11]. They can be subdivided into three different categories columns packed with particles, columns with continuous beds fabricated in situ creating a rod-like monolithic structure, and columns with immobilized or entrapped particulate materials. 

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). 

Gusev, I., Huang, X., Horvath, C. (1999). Capillary columns with in situ formed porous monolithic packing for micro high-performance liquid chromatography and capillary electrochromatography. J. Chromatogr. A 855, 273-290. 

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. 

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. 

Monolithic columns are comparatively easy to prepare. This is particularly true for capillary columns, which are known to be tedious to pack with particles. Furthermore, the reproducibility of microcolumn packing is low. 

Chankvetadze et al. prepared a 20-cm-long silica capillary column modified by in situ coating with amylase tris(3,5-dimethylphenylcarbamate) [199] for the fast separation of enantiomers. They showed the separation of 10 pairs of enantiomers. The monolith columns exhibit a slightly lower resolution but significant faster separation compared with a conventional 25 cm packed HPLC column. 

Samples are introduced into the capillary by either electrokinetic or hydrodynamic or hydrostatic means. Electrokinetic injection is preferentially employed with packed or monolithic capillaries whereas hydrostatic injection systems are limited to open capillary columns and are primarily used in homemade instruments. Optical detection directly through the capillary at the opposite end of sample injection is the most employed detection mode, using either a photodiode array or fluorescence or a laser-induced fluorescence (LIF) detector. Less common detection modes include conductivity [1], amperometric [2], chemiluminescence [3], and mass spectrometric [4] detection. 

Electroosmosis refers to the movement of the liquid adjacent to a charged snrface, in contact with a polar liquid, under the influence of an electric field applied parallel to the solid-liquid interface. The bulk fluid of liquid originated by this electrokinetic process is termed electroosmotic flow. It may be prodnced either in open or in packed or in monolithic capillary columns, as well as in planar electrophoretic systems employing a variety of snpports, such as paper or hydrophilic polymers. The origin of electroosmosis is the electrical donble layer generated at the plane of share between the snrface of either the planar support or the inner wall of the capillary tube and the surronnding solntion, as a consequence of the nneven distribntion of ions within the solid/liquid interface. 

In the first edition of this book, I forecast that the ultimate HPLC column would be a wall-bonded capillary column that would avoid the voiding and back-pressure problems seen with packed columns. A new type of column, the monolith silica column, recently emerging from research laboratories very closely fits this description. A monolith column has a honeycomb foam of silica, which is bonded with an organic separating phase, completely filling the inside of the column. 

An alternative to sintering frits, which deserves mention here, is to form frits via UV photopolymerization of a glycidyl methacrylate and trimethylolpropane trimethacrylate solution (UV radiation, 365 nm for 1 hour) [135]. The photopolymerization process is similar to that used in the fabrication of monolithic columns (Chapters 5 and 6). Frits fabricated with this method have shown to be reproducible since there is no sintering of packing material, weakening of the capillary column by removal of the polyimide coating and/or alteration of the stationary phase at the frit are avoided. 

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