Column, capillary optimized preparative columns

Under such optimizations, MIP micromonoliths have shown much promise in CEC (see Chapter 20, Sec. II). The ease of fabrication combined with the minimal consumption of materials (approximately 10-100 nmol of template for a typical 1.5- liL capillary tube) makes this method especially convenient [38]. Since polymerization is in situ, long columns may be prepared without the problems associated with packing. 

Usually, two 100 cm long monolithic columns were prepared from the same reaction mixture, and two-four 33 cm long columns were obtained from the two 100 cm long silica capillaries containing silica monolith. The capillary columns (100 pm I.D.) showed 10,000-12,000 theoretical plates for the effective length of 25 cm under optimized conditions in a pressure-driven mode, and up to 40,000 plates in the CEC mode. The use of smaller-sized capillaries, e.g., 50 pm I.D., and the modification of the preparation method of mesopores, resulted in a monolithic silica column of higher efficiency and higher mechanical stability [25-27], Under optimized conditions, 80,000 plates were obtained with a 25 cm column in CEC. 

Molecularly imprinted polymers with a variety of shapes have also been prepared by polymerizing monoliths in molds. This in situ preparation of MIPs was utilized for filling of capillaries [20], columns [21], and membranes [22, 23]. Each specific particle geometry however needs optimization of the respective polymerization conditions while maintaining the correct conditions for successful imprinting. It would be advantageous to separate these two processes, e.g., to prepare a molecularly imprinted material in one step, which then can be processed in a mold process in a separate step to result the desired shape. 

Improvements in resolution on a capillary column after it has been prepared can be made only by the adjustment of retention time, which alters the partition ratio of the compounds, and by the optimization of the carrier gas used. Increases in retention time which result in a partition ratio of greater than 5 afford very little improvement in resolution and are done at the expense of analysis time. Similarly, the use of nitrogen instead of helium as a carrier gas results in an increase in resolution of 1.14 at the expense of doubling the analysis time. 

One-step method for the preparation of highly enantioselective monolithic columns for CEC has been developed by Frechet et al. The chiral polymer bed of defined pore distribution and chiral ligand concentration has been synthesized within the confines of untreated fused silica capillaries using a mixture of O-[2-(methacryloyloxy)ethylcarbamoyl]-10,ll-dihydroquinidine 76, ethylene dimethacrylate (EDMA), and glycidyl methacrylate or 2-hydroxyethyl methacrylate (HEMA) in the mixture of cyclohexanol and 1-dodecanol as porogenic solvents. Under optimized synthetic and chromatographic conditions, these materials with the desired characteristics were demonstrated to efficiently separate a model racemic DNZ-Leu, Figure 13.24 [146],... 

The UV-Visible detector is the universal detector used in analytical and preparative CCC. It does not destroy solutes. It is used to detect organic molecules with a chromophore moiety or mineral species after formation of a complex (for instance, the rare earth elements with Arsenazo III ). Several problems can occur in direct UV detection, as has already been described by Oka and Ito 1) carryover of the stationary phase due to improper choice of operating conditions, with appearance of stationary phase droplets in the effluent of the column 2) overloading of the sample, vibrations, or fluctuations of the revolution speed 3) turbidity of the mobile phase due to difference in temperature between the column and the detection cell or 4) gas bubbling after reduction of effluent pressure. Some of these problems can be solved by optimization of the operating conditions, better control of the temperature of the mobile phase, and addition of some length of capillary tubing or a narrow-bore tube at the outlet of the column before the detector to stabilize the effluent flow and to prevent bubble formation. The problem of stationary phase carryover (especially encountered with hydrodynamic mode CCC devices) can be solved by the addition between the column outlet and UV detector of a solvent that is miscible with both stationary and mobile phases and that allows one to obtain a monophasic liquid in the cell of the detector (a common example is isopropanol). 

Coupled systems include multidimensional and multimodal systems. Multidimensional chromatography involves two columns in series preferably two capillary columns, with different selectivity or sample capacity, to optimize the selectivity of some compounds of interest in complex profiles or to provide an enrichment of relevant fractions. In multimodal systems, two chromatographic methods or eventually a sample preparation unit and a chromatographic method are coupled in series. Coupled systems that received much interest in recent years are multidimensional CGC (MDCGC), the combination of high-performance liquid chromatography with CGC (HPLC-CGC) and the on- or off-line combination of supercritical fluid extraction with CGC (SFE-CGC). Multidimensional and multimodal techniques in chromatography arc described in detail in [65],... 

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