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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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(1) A.J.P. Martin and RL.Synge, Biochem. J. 35( 1941 )1358
(2)" Gas Chromatography " Second Edition, AI.M.Keulemans (Ed.C.G.Verver) Reinhold Publishing Corporation, New York, (1959)106
(3)" Theory and Mathematics of Chromatography", AS.Said, Dr. Alfred Huthig Veriag GmbdH Heidelberg (1981)126
Chapter 3
The LC Column Dead Volume
The dead volume of a liquid chromatography column has been the subject of a considerable number of investigations, dissertations and public discussion-some of which have bordered on acrimony. As a consequence, the subject has been studied extensively from both a theoretical and experimental point of view. (1-7). The dead volume of a liquid chromatography column is important in both kinetic and thermodynamic measurements and is particularly so, when attempts are made to correlate solute retention with the composition of the mobile phase. In the past, the pertinent column volumes that have been considered to be important have been the column interstitial volume, (the volume of mobile phase existing between the particles), the column pore volume, (the volume of mobile phase within the particle) and the volume of stationary phase in the column. However, certain assumptions, tacitly made in early work, may not be chromatographically acceptable. For example, it has often been assumed that all the interstitial volume of the column contains moving phase and none of the interstitial volume is static. In addition, it has been assumed that the pore contents of a column have the same composition as the mobile phase, whereas there is considerable evidence that this is not so (8-10). Another assumption that has been made, is that all the stationary phase in the column is chromatographically available and, probably more important, the exclusion characteristics of the packing material can be completely ignored. Obviously these assumption are fallacious, and in order to avoid assumed errors of this kind, a simple rational approach must be made to identify the nature of all the different volumes that exist in a liquid chromatography column.
The internal volume of a column is occupied by three substances, the mobile phase, the stationary phase and the support. The term mobile phase is a misnomer as it impl'ies that all the mobile phase is moving, which is not so. The mobile phase within the pores is also stationary and thus constitutes part of the stationary phase. Nevertheless, it is such a well established term, it will still be used to denote the total mobile phase in the column, muviuy and sialic. The term moving phase will be uSeu for liiat fraction of the mobile phase that actually moves, whereas the term static phase will be used for that fraction of the mobile phase that is trapped in the pores, or in the interstices of the support particles, and does not move.
The mobile phase may be a single solvent or a solvent mixture. The support is usually silica gel, although in some circumstances, it may be alumina or, in ion chromatography, perhaps an ion exchange resin. The most common form of chromatography employs a bonded phase, in which case the stationary phase consists of silica gel with an organic moiety bonded to the surface. The organic moiety can be aliphatic, aromatic or heterocyclic in nature and contain a single organic species or a mixture of species.
It follows that -
where (Vc) is the total column volume,
(Vm) is the total volume of mobile phase in the column,
(V$) is the total volume of stationary phase in the column, (Vsi) is the total volume of support (in this case silica) in the
It should be noted that (Vm) is not the same as (Vm) from the plate theory as (Vm) is moving and some of (Vm) will be static. Similarly, (Vs) is not the same as (Vs) from the plate theory as (Vs) refers only to the stationary phase, whereas (Vs)in these circumstances will also include static mobile phase.
For simplicity, the two phases will be considered liquid/liquid in nature. Whether, in fact, a reverse phase does constitute a liquid-like phase or not is a moot point and even now a subject of some controversy. In any event, the arguments about to be put forward are independent of the exact nature of the interactions of any solute with the stationary phase, so either form may be assumed. The volume of stationary phase, (Vs) can be replaced by (Adf), (A is the surface area of the stationary phase in the column and (df) is the effective "film thickness" of the stationary phase) if so desired, and the arguments and the conclusions will remain the same.
Now the volume of the column is given by:-
Vc = Vm + Vs + Vsi
Vc = nr2l
where, (r) is the column radius, and (I) is the column length.
Vm+ Vs + Vsi = nr?i
As some of the mobile phase exists in the interstices of the packing and some inside the pores, (Vm ) can be initially divided into two parts,
Vm = V| + Vp ...................................................................(4)
where, (V|) is the interstitial volume between the packing, and (Vp) is the pore volume of the packing.
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