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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

Chromatographic Dead Volumes The two so called "dead volumes" are important in both theoretical studies and practical chromatographic measurements. They are the kinetic dead volume and the thermodynamic dead volume. The kinetic dead volume is used to calculate linear mobile phase velocities and capacity ratios in peak variance studies. The thermodynamic dead volume is relevant in retention measurements for identification purposes, thermodynamic studies and, in particular, for constructing vant Hoff curves. In equation (39) (for an incompressible mobile phase) the kinetic dead volume (which is the volume of moving phase only) is (Vi(m)). Thus, at a flow rate of (Q) ml/s, the dead time (to)

nbsp; The expression for the thermodynamic dead volume is more complex than that for the kinetic dead volume and will depend on the size of the solute molecule. In common with the kinetic dead volume, it contains the volume of moving phase VI(m). However,it also includes that portion of the interstitial volume that is size dependent (Y), together with the pore volume available to the solute (also size dependent (W). Equation (39) shows the major retention factor, (xK3VS(A)), is also  molecular size dependent ((x) is not unity), thus, unless the values for (Y), (W) and (x) are available or can be determined, it is not possible to determine the retention volume difference between two solutes accurately. This is particularly true for LC, when porous stationary phases (supported on silica) are

by differential interactive forces would provide a value for the thermodynamic dead volume. The maximum retention volume was obtained for methanol and water (viz. about 2.8 ml) which can be taken as the thermodynamic dead volume for small molecules (i.e., for concentrations of methanol above 10%v/v where the Langmuir adsorption isotherm become constant, see The Mechanism of Chromatographic Retention). It should be noted that there is no significant difference between the retention volume of water and that of methanol over the complete range of solvent compositions examined, which confirms the validity of this method for measuring the thermodynamic dead volume. Again, however, the lower concentrations of methanol, where the surface area of the stationary phase was not completely covered with methanol and the Langmuir adsorption isotherm would apply, can not be used. It must also be stressed, that this method of measuring thermodynamic dead volume will only be valid for

, have designed low dead volume unions which are now generally available. Actual data reporting the extent of the dispersion that takes place in such unions does not, however, appear to be readily available.  Scott and Simpson also measured the relative dispersion that occurred in normal, low dead volume unions and drilled-out unions. Drilled-out unions allow the ends of the connecting tubes to butt against one another, or against the frit of a microbore column and thus reduce the union volume dead volume to virtually zero. The design of low dead volume and drilled out unions are depicted in figure 15. Figure 15. The Design of Low Dead Volume and Drilled Out Unions

20 % 22.61 ml 18.75 ml 15.39 ml 13.00 ml 11.45 ml 30 % 13.35 ml 11.28 ml   9.63 ml   8.20 ml   7.14 ml 40 %   9.51 ml   7.85 ml   6.46 ml   5.52 ml    4.90 ml 50 %   6.36 ml   5.51 ml   4.06 ml   4.05 ml   3.44 ml Dead Volume = 2.785 ml   The retention volume and the dead volume were measured in duplicate at 0.1, 0.2, 0.3, 0.4 and 0.5 volume fractions of ethanol in hexane and at temperatures of 5˚C, 15˚C, 25˚C, 35˚C and 45˚C respectively. The corrected retention volume was taken as the difference between the retention volume of the solute and the retention volume of the peak obtained for pure ethanol, for each solvent mixture. The retention data for each enantiomer, (S)