Column combined devices

Cf-FAB in all its forms is a low flow-rate technique, i.e., 1-15 pl/min. Therefore, one should use either a microbore or packed microcapillary column, or a conventional colunm in combination with a post-column splitting device [47-48]. 

If no rational starting point for the separation of a racemate can be found, a random screening procedure has to be applied. In most cases 250 X 4 or 4.6 mm columns are used for this purpose. Modern HPLC systems often include a column switching device with up to 12 columns. Most systems have programmable software options for the set of different mobile phase compositions. Combination of UV and polarimetric detection allows even very small selectivities to be found and used as a starting point for further optimization. Commercial systems with deconvolution and optimization options are available, together with a given set of 12 different chiral stationary phases (www.pdr-chiral.com). After an initial detection of selectivity on a CSP, the mobile phase composition and additives as well as temperature have to be varied either manually or by means of an automated system. 

The column internals are these important imte of the apparatus idiidi are cxmstructed to ensure i oper conditions fo oi ration of die packiii Ihere are support plates, hoU-4own phrtes, liquid distributors and redistributors, gas (vapour) distributors, gas-liquid phase separators, and liquid collators. As an additional type of column internals, combined devices rinch ensuring redistribution of the phases act also as mass transfer vices. 

The combined devices, as already mentioned, ensure redistribution of the phases and simultaneously additional mass transfer in the column. Because of their double effect th will be considered separately here. 

Reactive distillation is a technique for combining a number of process operations in a single device. One company has developed a reactive distillation process for the manufacture of methyl acetate that reduces the number of distillation columns from eight to three, also eliminating an extraction column and a separate reactor (Agreda et al., 1990 Doherty and Buzad, 1992 Siirola, 1995). Inventory is reduced... 

An LC-LC coupling experiment system can be performed by employing a commercially available HPLC apparatus and involving various combinations of HPLC columns, eluents, additives, switching devices and detectors. 

SFC-SFC is more suitable than LC-LC for quantitation purposes, in view of the lack of a suitable mass-sensitive, universal detector in LC. Group quantitation can be achieved by FID. The ideal SFC-SFC system would consist of a short (10-30 cm) packed-capillary primary column, interfaced to a long (5-10m) open-tubular column, but such a combination is difficult to realise, due to the different flow-rates required for each column type. Coupled SFC-SFC is often configured with a solute concentration device prior to valve switching on to the SFC. The main approaches to this concentration stage are the use of absorbent material or cryofocusing. Davies el at. [924] first introduced two-dimensional cSFC (cSFC-cSFC), and its use has been reported [925,926]. 

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