Effects of Slow Mass Transfer When Using Microporous Adsorbents in Gas-- Solid Chromatography. Page: 4 of 46
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structure (1), many separations are possible due to the ex-
clusion of larger molecules from the large internal surface
area. Thus, straight-chain and branched-chain hydrocarbons
are separated (2), as also are aromatic and cyclic hydro-
carbons from aliphatic species (3). In addition to such
molecular effects, the activity of the zeolite surface al-
lows separations based on adsorption. Examples are the
separations of permanent gases (4) and low molecular weight
hydrocarbons on the large internal surface, and also the
separation of heavier, bulkier molecules on the smaller
external surface (5).
Since the majority of these separations involve adsorb-
ate molecules which can enter the pores of the zeolites to
some degree, the possible effects of slow or restricted
intraparticle diffusion on peak shape and retention behavior
are of interest. In static gas adsorption measurements, the
presence of slow diffusion into and out of small pore open-
ings has been noted. Anomalously low results for surface
area were found for coals and other active porous carbons
(6). In many cases, the adsorption of gases on various natural
and synthetic zeolites showed a very slow approach to equi-
librium (7). The effects of porosity under the dynamic
conditions of gas chromatography have been the subject of a
limited number of investigations. Kiselev (8) has shown
that the height equivalent to a theoretical plate (HETP) in-
creased as pore diameter decreased. His studies with bulk-
and surface-porous glasses revealed increases in HETP as
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Moreland, A. K. & Rogers, L. B. Effects of Slow Mass Transfer When Using Microporous Adsorbents in Gas-- Solid Chromatography., report, October 31, 1970; Lafayette, Indiana. (digital.library.unt.edu/ark:/67531/metadc870695/m1/4/: accessed January 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.