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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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it would appear from the data available at this time, that the Van Deemter equation would be the most appropriate to use in column design.
References
(1) E.Katz, KL.Ogan and R.P W.Scott, J.Chromatogr 270( 1983)51.
(2XJ.J. Van Deemter ,F.Zuiderweg and A.Klinkenberg, Chem.Eng.Sci.,^ 1956)271. (3XJ.C.6iddings, J Chromatogr.^ 1961 )46.
(4XJ.F.K.Huber and J.A.R.J. Hulsman, Anal Chim. Acta.,J8( 1967)305.
(5) G.J.Kennedy and J.H.Knox, J Chromatogr. Sc/., 10( 1972)549.
(6) Cs. Horvath and H.J.Lin, J. Chromatogr. 149( 1976)401
(7) J. Calvin Giddings," The Dynamics of Chromatography" ,Dekker, New York, (1965)55
(8) R. P. W. Scott and P. Kucera, J Chromatogr., 149(1978)93.
(9) J. H. Purnell, Nature,(Ltmb/^Supp1. 26(1959)184, .
(10) E. D. Katz and R. P W Scott, J. Chromatogr. 270( 1983)29
Chapter 10 Extra Column Dispersion
The resolution of two solutes on a chromatographic column depends on the extent to which the dispersion of the individual solute bands are constrained and this can be described by the Van Deemter equation. However, as it has already been mentioned, that there are other dispersion processes that can occur outside the column that can contribute to the overall band variance as measured by the detector. The sum of the individual variances that occur outside the column is called the extra column dispersion and it has been shown in a previous chapter that this should not exceed \0% of the band variance contributed by the column. Dispersion in LC equipment has been discussed by Scott and Kucera, (1) Martin et a/ (2) and Knox and Gi Ibert
(3) and the measurement of extra column dispersion in some commercially available chromatographs has been carried out by Scott and Reese (4). Excluding the response time of the detector sensor and detector electronics (which today, as a result of modern electronic technology has been rendered virtually insignificant) there are six main sources of extra column dispersion; viz, the sample volume; the sample valve; the connecting tube between injection valve and column, the column frits; the connecting tube between column and detector cell (this will include connecting tubes exterior and interior to the detector ); and finally the the detector cell itself. These sources of dispersion will now be discussed in detail.
The Effect of Sample Volume
The dispersion effect of the sample volume was discussed in Chapter 4 and little more needs to be said about it. It will be seen later that the sample volume controls both the concentration and the mass sensitivity of the chromatographic system and thus, should be made as large as possible. This means that all other sources of band dispersion must be kept to an absolute minimum to permit the maximum possible sample volume to be used. A better understanding of the causes of band dispersion has resulted in
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improved design of the chromatographic apparatus and, as a consequence, high mass sensitivities can be realized from modern LC systems.
The Sample Valve.
The elution profile of a solute band leaving a sample valve is controlled largely by the geometry of the sample volume itself (i.e whether it takes the form of an exterior tube or a groove in the spigot of the valve). However, it does also depend to some extent on the geometry of the exit ports to the valve-column connecting tube. Today, low dead volume unions have virtually eliminated dispersion in unions and thus, these will not be considered as a significant source of extra column dispersion. If the sample volume consists of an external tube, then any dispersion that takes place in it will be the same as that, which would occur in a connecting tube of equivalent length, and will be considered later. Dispersion resulting from an internal sample volume consisting of grooves cut in the valve spigot together with dispersion arising in the sample valve exit ports are extremely difficult to treat theoretically and need to be determined experimentally. Such information should be provided by the manufacturer but, unfortunately, even if known, it is rarely, if ever, made available to the customer. This leaves the onus on the user to determine the extra column dispersion and a method for doing so will be given later. Theoretically, it should be possible to design exit ports to provide secondary flow and thus, increase the effective diffusivity of the solute, and consequently, reduce band spreading Unfortunately, there appears no evidence to show that this approach has been adopted by the manufacturers in the design of modern sample valves
Connecting Tubes
The dispersion that takes place in the sample-valve/column connecting tube and the column/detector cell connecting tube will both result from the parabolic velocity profile that occurs in open tubes and thus, will be considered together.
The dispersion that takes place in an open tube is described by the Golay equation (5),(ref. chapter 8) under conditions where there is no stationary phase and thus, -0. As a result,the variance due to an open tube (otu2)will be
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