Packed column separations, optimization

Most contemporary instruments for supercritical fluid chromatography are optimized for either open tubular or packed column separations and may provide a poor compromise for column types other than those they were designed for [146]. The small volumes and low flow rates associated with open tubular columns make greater demands on instrument design than typical packed columns, which have dimensions similar to those commonly used in liquid chromatography. 

Examination of the parameters in the van Deemter expression (Equation 2.44), term by term, provides a basis for optimizing a packed column separation. The plate height, h, of a packed column may be represented as the sum of the eddy diffusion, molecular diffusion and mass transfer effects. Thus, to attain maximum column efficiency, each term in the plate height equation should be minimized ... 

While open tubular columns are comparable with packed columns in terms of efficiency under optimal flowrate conditions, longer open tubular columns provide more theoretical plates and thus have greater resolving power. A typical 30-min-long packed-column separation with a 2-m column will generate a typical peak capacity of 30-100 peaks. A 50-m-long open tubular column can generate a peak capacity of several hundred peaks in the same timeframe. 

In the previous two chapters, equations were developed to provide the optimum column dimensions and operating conditions to achieve a particular separation in the minimum time for both packed columns and open tubular columns. In practice, the vast majority of LC separations are carried out on packed columns, whereas in GC, the greater part of all analyses are performed with open tubular columns. As a consequence, in this chapter the equations for packed LC columns will first be examined and the factors that have the major impact of each optimized parameter discussed. Subsequently open tubular GC columns will be considered in a similar manner. 

The curves in figure (5) show the relationship between maximum sample volume and the separation ratio of the critical pair for a fully optimized column and were obtained using equation (20) The curves give the first, indication of the limitations of open tubular columns in LC and the reason why they have not achieved the popularity and success of the packed column... 

It is seen from Table (3) that an open tubular column with an I 0 of about 86 micron and 20 meters long, operating at an inlet pressure of only 1 ps.i. can complete the very difficult separation in about a week. A very long analysis time, perhaps, but not much longer than would be required by an optimized packed column and in this case the separation is earned out with a simple tube to coat and with no high pressure pump required. 

The optimized operating conditions for each analytical method including the detector system of choice are reported in Table II. The reported columns and operating conditions yield satisfactory peak shapes and resolution of all the potential interferents evaluated for HCCP and HCBD. Two potential interferents—tetrachloro-l,2-difluoroethane and 1,2-dichloroethane— could not be separated from 1,2-DCP with conventional packed columns. Tetrachloro-l,2-difluoroethane, a compound with physical properties similar to 1,2-DCP, is not likely to be found with... 

Future work. As mentioned earlier, use of the simplex algorithm for the systematic optimization of SFC separations is still in its early stages. The success already achieved, however, merits continued research along these lines. Research opportunities include (i) extension of the simplex method to less ideal variables and/or greater than 4 variables (ii) investigation of the benefits of the simplex method to packed columns and modified mobile phases and (iii) development of the capability to predict, for a given type of sample, the best combination of variables to optimize. 

When, on the other hand, a nondifferentially pumped system is used, direct coupling generally will be impossible. In that case either an open split device or a carrier gas separator must be installed. When using a jet or membrane separator, one should keep in mind that these devices have been designed for packed columns. Thus, make-up gas must be added to the column effluent to restore optimal working pressure and purge unswept volumes. 

The smaller the particle size the more efficient a separation will be. However, the smaller the particle the higher the back pressure in the packed column, which limits the practical size that can be used due to pressure limitations of the hardware. Typically, most equipment will be capable of withstanding the back pressures generated by 5 xm particles at optimized flow rates. 

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