STUDY OF THE PHYSICOCHEMICAL PROPERTIES OF SOME PACKING MATERIALS .2.GENERAL-PROPERTIES OF THE PARTICLES

Scanning electron microscopy reveals the shape and illustrates the par ticle size distribution of four different brands of spherical silica p articles used in preparative chromatography (C-18-bonded Kromasil, Vyd ac, YMC, and Zorbax), Pycnometry provides a direct estimate of the una ccessible volume fraction in a column. The retention volumes of benzen e in dichloromethane and of uracil in methanol provide two independent estimates of the total column porosity which are in excellent agreeme nt. Within 2 to 3%, the sum of these two fractional volumes is equal t o unity, as expected, in spite of a possible disagreement between the two sets of measurements due to the fact that the C-18 bonded chains a re collapsed during the pycnometric measurements and are dissolved in the mobile phase during the chromatographic measurements. Finally, est imates of the volume fractions of the column occupied by the bonded la yer and the silica skeleton for two of the packing materials suggest t hat the closed pore porosity is negligible (1 to 2% at most).