Column hold-up time

Fig. I.l. Evaluation of the releniion data from a chromatogram. EmfEo) column hold-up volume, i.e.. the volume of the mobile phase in the column measured as the elution volume of a non-relained solute imdo) column hold-up time VRiffRi) and VriUrj) retention (elution) volumes (times) of retained sample...

Here t0 is the column hold-up time, F is the phase ratio and dq/dc is the derivative (slope) of the isotherm function that is evaluated at the plateau concentration Ct. The retention time of a small injection of a compound in a column equilibrated with pure mobile phase without the compound (such as for analytical chromatography) gives the retention time under linear conditions, through the equation ... 

A constant gradient, like 5%/min or 10%/min tends to push all the less retained solutes together and spends excessive time at the end effecting small changes in retention. A doubling gradient offers the best trade-off between speed and resolution in SFC. The modifier concentration starts at some low level like 1% and doubles every column hold-up time. 

Column plate count Column hold-up time (s) Column length (m) Reduced plate height Reduced velocity Separation impedance... 

Tf = final program temperature. To = initial program temperature, t = time, tp = program time for a linear temperature program, tM = column hold-up time, k = initial value of the retention factor at the start of the temperature program, wp = peak width at base and N = column plate count. 

Even on the small diameter columns used (i.e., 50 pm) diffusion coefficients dictated an optimum linear velocity (popt) in the order of 0.1 cm s Since columns tended to be 10 m long, the optimum column hold-up time was in the order of 16.7 min. Since this is unrealistically long for routine analysis, most workers operated up to 10 times Popo making the column hold-up time less than 2 min with typical run times of 30 min to 2h. Efficiency dropped from 200000 plates near optimum to roughly 20000 plates at the higher velocity. Very small column diameters made it difficult to make reproducible injections. 

The retention factor k was defined as a retention parameter that is in contrast to the retention time fg independent of column dimensions and the mobile phase flow rate F. It is characteristic for the given analyte under given stationary and mobile phase conditions as well as temperature. The retention factor can be derived from the peak retention time fg and the column hold-up time (elution time of a nonretained compound, the so-called inert marker) and equals the ratio of the so-called net retention time fg (difference of retention time and hold-up time) and the hold-up time ... 

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