Intra-particle sorption rate and liquid chromatographic bandbroadening in porous polymer packings .3. Diffusion in the polymer matrix as the cause of slow sorption

In order to discover the physical cause of the slow intra-particle sorption rate of naphthalene in 10-mu m spheres of the macroporous poly(styrene-divinylbenzene) (PS-DVB) polymeric HPLC sorbent Hamilton PRP-1, which has been shown to cause excessive bandbroadening of eluted peaks, the sorption-rate curve for naphthalene from methanol-water (85:15) was measured on PRP-1 using the shallow-bed technique. Sorption on PRP-1 follows a two-term theoretical rate equation for sorption on a biporous particle. From the (fast) first term it is found that 91% of the naphthalene is sorbed on the walls of the large pores and that the diffusion coefficient in these large pores is 3X10(-6) cm(2)/s. This is close to the free-solution diffusion coefficient, which demonstrates that large-pore diffusion is nearly unhindered. From the (slow) second term in the rate equation it is found that 9% of the naphthalene is sorbed into the polymer matrix of PRP-1, in which the effective diffusion coefficient is no larger than 10(-12) cm(2)/s. It is clear from these results that the cause of the slow intra-particle rate, and therefore of excessive chromatographic bandbroadening, is slow diffusion into the polymer matrix of PRP-1. To provide additional information on the PS-DVB polymer matrix, the sorption rate of naphthalene was also measured on Hamilton PRP-infinity which is a 19-mu m diameter, spherical, nominally nonporous PS-DVB chromatographic packing. The sorption is slow and follows the theoretical rate equation for hindered diffusion into a homogeneous sphere. The effective diffusion coefficient is (4+/-1)x10(-9) cm(2)/s. Diffusion through the polymer matrices in PRP-1 and PRP-infinity could be either hindered diffusion through micropores in a rigid matrix or diffusion through a flexible polymer 'gel'.