Diameter, average particle chromatography

Five column types have been used in gas chromatography. Classical packed columns have internal diameters greater than 2 mm and are packed with particles in the range 100 to 250 p,m. Micropacked columns have diameters less than 1 mm and a similar packing density to classical packed columns (dp / dc less than 0.3, where dp is the average particle diameter and dc the column diameter). Packed capillary columns have a column diameter less than 0.6 mm and are packed with particles of 5-20 p,m diameter. SCOT... 

Usually, in chromatography the volume-average particle diameter is employed. For comparison, the particle-size distribution based on the number and the volume average is shown for the same silica measured by the same technique (Table 3.14). 

Decrease in particle size results in increased column efficiency. In preparative chromatography, in practice it is not possible to increase the column efficiency indefinitely, for both practical and economical reasons. Pressure drop is inversely proportional to the square of the particle diameter. Ease of packing of columns decreases with decreasing particle size. Current average particle sizes for reversed-phase PHPLC are in the range of 7-20 p,m. In preparative liquid carbondioxide-based SFC, smaller particles in the 5-10 p.m range are used due to the low viscosity of the mobile phase. 

Physical properties of TLC alumina are somewhat similar to TLC silica in terms of particle size, surface area, and average pore diameter. Aluminas for chromatography have surface areas between 50 and 350 m /g, average pore diam-eters..between 20 and 150 A, and specific pore volumes between 0.1 and 0.4 ml/ g (Rossler, 1969 Hahn-Deinstrop, 1992 Robards et al., 1994). 

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