Packed column preparation conditioning columns

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

Preparative chromatographic processes are of increasing importance particularly in the production of fine chemicals. A mixture of compounds is introduced into the liquid mobile phase, and this then flows through a packed column containing the stationary solid phase. The contacting scheme is thus differential, but since the adsorption characteristics of the compounds in the mixture are similar, many equivalent theoretical stages are required for their separation. Chromatographic processes are mostly ran under transient conditions, such that... 

With the exception of a few organic polymers, silica gel represents the basic material used to pack HPLC columns. The silica gel used for chromatography is quite different from crystalline silica (Si02), which is used for its preparation. It is prepared under conditions of controlled hydrolysis by polymerisation of tetraethoxysilane in the form of an emulsion giving rise to microspheres of uniform diameter in the order of 2 to 5 pm (Fig. 3.7). A sol-gel is formed in the process and these very small particles must grow in a regular manner in order to obtain the diameter of a few micrometres. The material has to be free of metallic ions. The silica gel particles obtained must be of uniform diameter to avoid the presence of preferential pathways in the packed bed in the column. 

The molecular imprinting strategy can be applied for the recognition of different kinds of templates from small organic molecules to biomacromolecules as proteins. Some examples of separations investigated with MIP monoliths in CEC and LC are shown in Table 2. The influence of the imprinted monolithic phase preparation procedure and of the separation conditions on the selectivity and chromatographic efficiency have been widely studied [154, 157, 161, 166, 167, 192]. The performance of imprinted monoliths as chromatographic stationary phase has also been compared to that of the traditional bulk polymer packed column [149, 160]. It was shown that the monolithic phases yielded faster analyses and improved chiral separations. 

Preparative HPLC can be carried out in pre-packed HPLC columns or self-pack systems (e.g. DAC columns). The conditions used to slurry pack various self-pack hardware is discussed in Chapter 4. 

It is most often prepared by acid hydrolysis of sodium silicate followed by emulsification in an alcohol water mixture and subsequent condensation to give solid silica gel. This is then washed and dried for use as HPLC column packing. The exact conditions under which these procedures are carried out (e.g. pH, catalysts, temperature) will affect the properties of the resulting material. The most important qualities with regard to the chromatographic performance of the gel are the average particle size, the particle shape, the specific surface area and the pore size. Other factors which are also important are the pH of the gel surface, the number of active silanol groups and the presence of metal ions. 

A preparative high-performance column packed with 5, 7 or 10 pm material and available from various manufactures is used after the analytical separation optimization stage (columns with coarser particles of around 40 pm are more suitable for easy separations requiring less than 100 theoretical plates). A 10 mm i.d. column is designed for 10-100 mg samples, rising to 21.5 mm (o.d. 1 in) for samples between 100 mg and 1 g. The transfer from analytical to preparative conditions works best if both columns are packed with the same stationary phase. 

Figure21.2 Preparative separation ofthe reaction mixture. Conditions sample, 1 ml of solution containing 250 mg guard column, 5 cm x 4.5 mm i.d., dry-packed with silica (40gm) column, 25cm x 21.5mm i.d. stationary phase, silica, 7 xm ...

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