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liquid chromatography column - Scott R.P.W.

Scott R.P.W. liquid chromatography column - John Wiley & Sons, 2001. - 144 p.
Download (direct link): liquidchromatographycolumntheory2001.djvu
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Performance Criteria
1/ A defined resolution must be obtained.
2/ The analysis must be completed in the minimum time.
3/ The analysis must be completed with the minimum solvent consumption. 4/ The maximum mass sensitivity must be realized.
Instrument Constraints
Certain operating limits are inherent in any liquid chromatograph and these limits will vary with the purpose for which the instrument was designed. For example, the preparative chromatograph will have very different operating characteristics from those of the analytical chromatograph. The first, and obvious operating limit, will be the maximum column inlet pressure that the solvent pump will provide. It will be seen that the maximum inlet pressure that is available (or can be tolerated) will determine the optimum column length, the optimum particle diameter of the packing materia! and, as a consequence, the the minimum analysis time. It should also be noted that it is not the available pressure that the pump can provide, that usually limits the available column inlet pressure, but more often, the maximum pressure that the sample valve can tolerate under continuous operation Most pumps can provide pressures of at least 6,000p.s.i but the sample valve operating pressure will often limit the column inlet pressure to as little as 3,000p.s.i. Although, it Is claimed by many manufactures, that their sample valves will operate at 6,000p.s.i. or even 10,000 p.s.i. the life time of the valves, when operated at these
pressures, is often relatively short. As a consequence, for successful ano continuous operation over an extended period of time, the operating pressure of the chromatographic system may well be limited by the long term pressure tolerance of the sample valve and not by the available pressure from the solvent pump.
The maximum and minimum flow rate available from the solvent pump may also, under certain circumstances, determine the minimum or maximum column diameter that can be employed and, as a consequence, place limits on the mass sensitivity of the chromatographic system as well as the solvent consumption. However, in practice, almost all commercially available LC solvent pumps have a range of flow rates that will embrace the optimum flow rates that may be required for most LC analyses
Another extremely important instrument specification is the total dispersion that takes place in the sample valves, connecting tubes and detector cell of the chromatograph. The subject of extra column dispersion has already been discussed in the previous chapter. It has been shown that the extra column dispersion determines the minimum column radius and thus, both the solvent consumption per analysis, and the mass sensitivity of the overall chromatographic system. The overall extra column variance, therefore, must be known and quantitatively specified.
Finally the speed of response of the detector sensor and the associated electronics once played an important part in optimum column design. The speed of response, or the overall time constant of the detector and associated electronics, would be particularly important in the analysis of simple mixtures where the analysis time can be extremely short and the elution of each peak extremely rapid. Fortunately, modern LC detector sensors have a very fast response and the associated electronic circuits very small time constants and thus, the overall time constant of the detector system does not significantly influence column design. The Instrument constraints can therefore be summarized as follows,
Instrument Constraints
1/ The maximum operating pressure
2/ The extra column dispersion 3/ The minimum flow Rate
4/ The maximum Flow Rate
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5/ The response time of the detecting system.
Elective Variables
The choice of variables remaining with the operator, as stated before, is somewhat restricted and is usually confined to the selection of the phase system. Preliminary experiments must be carried out to identify the best phase system to be used for the particular analysis under consideration The best phase system will be that, which provides the greatest separation ratio for the critical pair of solutes, and at the same time ensures a minimum value for the capacity factor of the last eluted solute. Unfortunately, at this time, theories that predict the optimum solvent system that will effect a particular separation are largely empirical and those that are available can be very approximate to say the least. Nevertheless, there are commercially available experimental routines for selection of the best phase system, the results from which can be analyzed by supporting computer software. The program may then suggest further routines based on the initial results and, by an iterative procedure, eventually provides an optimum phase system as defined by the computer software.
Whether the optimum phase system is arrived at by a computer system, or by trial and error experiments (which are often carried out, even after computer optimization), the basic chromatographic data needed in column design will be identified. The phase system will define the separation ratio of the critical pair, the capacity ratio of the first eluted peak of the critical pair and the capacity ratio of the last eluted peak. It will also define the viscosity of the mobile phase and the diffusivity of the solute in the mobile phase
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