Handbook of Size Exclusion Chromatography and Related ...

thespecificporevolume(mL/gadsorbent)andVSS isthevolumeofpuresolidper
gram. Equation (11) presents the relationship between particle porosity and
internal porosity:
eP ¼eSP(1 ei) (11)
Therange for theinterparticleporosity ei listed inTable3islargelybased ondata
fromRef.33.ItwasfoundthatGFCcolumnspackedwithsphericalparticleshave
interparticle porosities ranging from 0.35 to 0.39, but columns packed with
irregularparticlesshowedVi valuesashighas0.47.Thesevaluesareinreasonable
agreement with earlier findings from Giddings (53), who reported ei values in the
range 0.37–0.43. Experiments by the authors with spherical 5-mm, 100 A ˚ pore
sizesilicashaverepeatedlyfound avalueof0.40for theinterparticleporosityand
0.75–0.80 for the mobile-phase porosity. Values as low as 0.34 for ei were
measuredwhenthesesilicasweremorefragileandhadmobile-phaseporositieseT
of 0.80–0.84. Examples of these two types of silicas are shown later. Engelhardt
reported0.42fortheinterstitialporosityofsolidglassbeadsand0.80–0.88forthe
mobile-phase porosity of totally porous supports (56).
For particles with very large pores, porevolume is sometimes sacrificed for
mechanical stability.For example, when particles varying in pore size from 10 to
385 nm,butwithnearlyidenticalporosities,weresubjectedtopressuretests,those
with the largest pore sizes collapsed at lower pressure drops (see Ref. 23, p. 174).
Thus, the mechanical stabilityof larger pore size particles can only be maintained
by reducing the pore volume. Alternatively,larger pore size particles must be
slurry packed at lower pressures, thereby decreasing the stability and lifetime of
the packed bed.
Chemical modification of the silica surface results in aloss of porevolume.
Thus, the bonded phase layer must be optimized to reduce effectively interactions
with silanol groups while minimizing the thickness of the bonded layer to avoid
reducing the pore volume and preventing slow transport kinetics in the stationary
phase. For example, the thickness of the stationary phase layer was estimated as
0.56 nmforaC3-alkylfunctionalgroupand2.45 nmforC18-alkyl,assumingthat
the ligands stand upright on the surface (57). This assumption is thought to be
correct under conditions that fully solvate the stationary phase layer, which is the
case in GFC as well as GPC, in which the stationary and mobile phases have
similar polar or nonpolar characteristics, respectively. Under such conditions,
however, the bonded phase layer can be partially penetrated by the solutes and,
thus, the loss of porevolume is smaller than expected based on thevolume of the
bonded-phase layer. Henry recently showed the shift in the pore diameter
distribution for apolyethyleneimine phasewith alayer thickness of 0.85 nm (26).
The averagepore size of modern analytical HPLC packings is 100 A ˚ ,range
60–120 A ˚ .Figure 5shows the internal surface area vs. pore diameter for four
© 2004 by Marcel Dekker, Inc.