Mobile phase compressibility

Flow Programming with a Non-Compressible Mobile Phase... 

Flow programming with a compressible mobile phase is a far more complicated process to examine theoretically. We shall assume that, under flow programming conditions, the mass flow rate will be increased linearly with time, i.e. (Qo(t)) = (Q o + otQt) where (Q o) is the initial exit flow rate, (Qo(t)) is the exit flow rate after time (t) and (aq) is the flow program rate. 

D. P. Poe and D. E. Marthe, Plate height theory for compressible mobile phase fluids and its application to gas, liquid and supercritical fluid cliromatography , 7. Chromatogr. 517 3-29(1990). 

Fig. 3 Resolution of the six major iso-a-acids in beer into five bands by reverse-phase HPLC. Mobile phase acetonitrile water methanol 0.2 M magnesium acetate (aq) formic acid trifluoroacetic acid (840 490 480 24 3.6 1.2 v/v/v/v/v/v). Column NovaPak C18 5 /rm cartridge (100 X 8 mm i.d.) under radial compression. Mobile phase flow rate = 2 ml min-1. (Chromatogram redrawn from original).

The retention time can be taken as the product of the distance on the chart between the dead point and the peak maximum and the chart speed, using appropriate units. As in the case of the retention time, it can be more accurately measured with a stopwatch. Again, the most accurate method of measuring F for a non-compressible mobile phase, although considered antiquated, is to attach an accurate burette to the detector exit and measure the retention volume in volume units. This is an absolute method of measurement and... 

For an analysis of the chromatographic separation of a binary mixture we will again assume the same, constant linear flow rate v = LI tm = H/t throughout all plates, as one can expect with a non-compressible mobile phase. 

Column ixBondapak CIS radial compression Mobile phase MeCN 0.085% phosphoric add 20 80 Flowrate 1.5 Ipjection volume 25 Detector UV 237... 

Column 5 mm i.d. pBondapak CIS radial compression Mobile phase MeOH water-.glacial acetic acid 75 24 1 Flowrate 0.8-1.2 Ipjection volume 20 Detector UV 276... 

For an understanding of band broadening in chromatographic systems, the linear velocity of the mobile phase is more important than the column volumetric flow rate. Complications in identifying a suitable velocity arise for compressible mobile phases... 

For the illustration in a (m2, m3)-diagram the nondimensional parameter of the net flow rate ratio is used (Eq. (9.46)), (26). If a compressible mobile phase is used, the ntj in the zones are no longer constant since they depend on density. In order to keep a simple illustration of the separation regime in a (m2, m3)-diagram, a new ratio rt is defined, which uses the mass flow rate inside each zone (Eq. (9.55)), (58). The reference density po is set to 1000kg/m. Eor the special case of constant densities the parameter can be converted into each other according to Eq. (9.56). 

Column NovaPak C18 radial compression Mobile phase MeOH MeCN 10 mM KHjP04 9 2.5 90 Flow rate 2 Ii jection volume 25... 

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