Plate Theory and Extensions - The Column Dead Volume > Page 28

The Column Dead Volume

An accurate estimation of the dead volume is very important when measuring retention data, particularly if the corrected retention volume is small and commensurate with the dead volume. The dead volume comprises a number of different components of the column volume, and the distribution of the total column volume into those parts of chromatographic significance is a little complicated The composition and measurement of the dead volume has been discussed by a number of workers (4-9).

Originally, the important column volumes were considered to be the interstitial volume (the volume between the particles), the column pore volume (the volume within the particle) and the volume of stationary phase. However, in using these basic column volumes, certain invalid assumptions were made. Firstly. it was assumed that the mobile phase in the interstitial volume was moving phase and none was static, which, although possibly true in GC (particularly in capillary columns), is certainly not true for packed column in LC. Secondly, the pore contents had the same composition as the bulk mobile phase which, for solvent mixtures has been proved not to be so (10). Thirdly, it was assumed that all the stationary phase was available to the solute and, thus, exclusion effects were ignored.

The different column volumes that are chromatographically pertinent can be logically broken down into the individual volumes involved. The column contains three materials, the mobile phase, the stationary phase and the support. It has been assumed that all the mobile phase is mobile, which is not true, for that portion of the mobile phase contained in the pores, is static. However the term mobile phase is well established and so its use to designate all that phase that is not stationary phase will be maintained. However, that part of the mobile phase that actually moves will be designated, the moving phase.