ELSEVIER Journal of Chromatography A, 736 (1996) 21-30 JOURNAL OF CHROMATOGRAPHY A Study of the physico-chemical properties of some packing materials II. General properties of the particles a b c Hong Guan " , Georges Guiochon a'b'*, Dorothy Coffey , Evelyn Davis d, Kim Gulakowski d, David W. Smith a "Department of Chemistry, The University of Tennessee, Knoxville, TN 37996-1600, USA "Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6120, USA CMetals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064, USA ~Micromeritics, Norcross, GA 30093-1877, USA Received 9 October 1995; revised 12 December 1995; accepted 13 December 1995 Abstract Scanning electron microscopy reveals the shape and illustrates the particle size distribution of four different brands of spherical silica particles used in preparative chromatography (C ~8-bonded Kromasil, Vydac, YMC, and Zorbax). Pycnometry provides a direct estimate of the unaccessible volume fraction in a column. The retention volumes of benzene in dichloromethane and of uracil in methanol provide two independent estimates of the total column porosity which are in excellent agreement. Within 2 to 3%, the sum of these two fractional volumes is equal to unity, as expected, in spite of a possible disagreement between the two sets of measurements due to the fact that the C,s bonded chains are collapsed during the pycnometric measurements and are dissolved in the mobile phase during the chromatographic measurements. Finally, estimates of the volume fractions of the column occupied by the bonded layer and the silica skeleton for two of the packing materials suggest that the closed pore porosity is negligible (1 to 2% at most). Keywords: Stationary phases, LC; Scanning electron microscopy; Preparative chromatography I. Introduction The reproducibility of packed columns for chro- matography has always been a problem of serious concern for practitioners [1]. So far, the reproducibil- ity of the thermodynamic properties has attracted most of the attention. Progress made over the years in the manufacturing processes used to prepare silica particles and to derivatize them when needed has *Corresponding author. Address for correspondence: Department of Chemistry, University of Tennessee, Knoxville, TN 37996- 1600, USA. resulted in the present situation. Batch-to-batch repeatability of chromatographic separations per- formed with a given brand of packing is quite reasonable for neat silica and alkyl-bonded silica materials. There are still some significant fluctuations of the retention factors, much smaller variations for the separation factors. The results are generally less satisfactory for some nonconventional bonded phases, in which case there may be considerable batch-to-batch variations in the yield of the bonding reactions. Hence, the repeatability of some otherwise difficult separations is not assured [2]. Until recently, the repeatability of the other col- 0021-9673/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0021-9673(95)01355-5