Plate columns pressure drop

The particle size of a packing affects two major chromatographic properties the column pressure drop and the column performance in terms of plate number. For simplicity ... 

The fastest possible separation with a given column pressure drop (AP) and Nreq is obtained at the minimum reduced plate height (hmin) corresponding to the optimum reduced mobile phase velocity (Vopt)- This will be achieved at a specified optimum particle size (dpopt) according to... 

A number of theoretical studies concluded that a pressure drop greater than 20 bar would cause a severe efficiency losses in packed colunrn supercritical fluid chromatography [1,2,5,6,82]. With such small a pressure drop, the maximum column efficiency would be limited to approximately 20,000 plates. In practice packed columns providing over 100,000 plates per meter with a total plate number more than 200,000 and operated with a column pressure drop of about 160 bar have been demonstrated [86,109-112]. Coupled columns providing the highest total plate number were obtained with mixed mobile phases of low compressibility and demonstrate that the theoretical performance limit proposed for packed column supercritical fluid chromatography is too pessimistic. 

Series coupling of columns containing the same stationary phase is used to enhance efficiency and with different stationary phases to fine tune selectivity [8]. Series coupling of packed columns became popular after it was demonstrated that the column pressure drop did not limit the total column efficiency to the extent that had been predicted (section 7.4.2). Serial coupling of 2 or 3 standard columns is practical for routine applications and provides a total plate count in excess of 50,000. There is no theory for selectivity optimization for coupled packed columns but suitable conditions often can be estimated from separations on the individual columns. Effective selectivity changes require that the coupled columns have different retention properties. A number of practical examples for the separation of polymer additives, polycyclic aromatic hydrocarbons, phytic acid impurities and enatiomers have been described [137,195,196]. Series coupling of open tubular columns with different stationary phases is less common, but changes in selectivity are predictable, at least when the pressure drop is low [197]. 

The largest variation in properties between conventional packed columns and capillary columns is associated with the column permeability. For this reason, capillary columns offer much less flow resistance and can be used in much longer lengths. Ultimately, the comparison of different column types is between the efficient use of column head pressure. Thus, a packed column containing 10 pm particles can generate 50 000 theoretical plates per meter but requires a head pressure of 20 MPa m whereas a 70 m capillary column of 50 pm internal diameter can provide over one million theoretical plates with a column pressure drop of 2.2 MPa. 

Elution volume, exclusion chromatography Flow rate, column Gas/liquid volume ratio Inner column volume Interstitial (outer) volume Kovats retention indices Matrix volume Net retention volume Obstruction factor Packing uniformity factor Particle diameter Partition coefficient Partition ratio Peak asymmetry factor Peak resolution Plate height Plate number Porosity, column Pressure, column inlet Presure, column outlet Pressure drop... 

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