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Tchapla et al.  examined methylene selectivity for ª], C6, C8, C14, and C,„ monomeric-bonded phases as a function of solute carbon number nc. They observed a gradual decrease in selectivity up to a certain solute carbon number ncriv which corresponded to the chain length of the stationary phase nhp, at which a large step decrease in selectivity occurred. Their ex-planation of this phenomenon was that the stationary-phase-bonded chains solvate the alkyl chain of the solute molecule and, as long as the length of the solute alkyl chain was less than that of the bonded phase, increasing the number of methylene groups in the solute causes a constant stationary-phase contribution to selectivity. Once the length of the solute alkyl chain exceeds that of the bonded phase, the remaining solute methylene groups no longer penetrate
Reversed-Phase Stationary Phases
the chains; they undergo weaker dispersive interactions than those that penetrate, causing a sharp decrease in selectivity. This behavior was not observed with the Ñ,-bonded phase, for solutes will not penetrate it. Tchapla and Heron  expanded on this investigation of methylene selectivity and observed a second break in the curve, referred to as the saturation value, nsat. For solutes with nc < ncrit, the value of the methylene selectivity was dependent on the functionality of the different homologous series tested. For solutes with ncrit < nc < nsat and nc > wsat, the value of the methylene selectivity was the same, regardless of the series, but was lower when nc > nsat. Additionally, the value of nsat decreased with increasing stationary phase-bonding density.
Shape selectivity refers to the discriminate retention of solutes based on their three-dimensional structure. Sander and Wise , in their review of a series of investigations, examined the retention behavior of rigid, nonpolar PAHs, relative to their overall structure, on a variety of monomeric and polymeric alkyl-bonded stationary phases. They determined that polymeric C,8 phases are typically more shape-selective than monomeric C18 phases, and that polymeric phases are better able to separate complex mixture of PAHs, especially isomeric mixtures. Wise and Sander  investigated the ability of a monomeric C18 column (2.52 fxmol m-2) and six polymeric C18 columns, with coverages between 2.7 and 5.9 /xmol m 2, to separate 11 PAH isomers of molecular weight 278. The monomeric column gave poor resolution of the mixture. The resolution was much better on the polymeric phases, but only the high-load columns (above about 5.1 /õòî 1 m~2) were able to resolve all of the isomers. In general, the elution order of the solutes followed the trend of increased retention with increasing “length/breadth ratio” (L/B) , which is a descriptor of the two-dimensional structure of the solutes. Nonplanar isomers eluted earlier than predicted based on L/B values.
Wise and Sander  also investigated the ability of four C18 columns to differentiate between linear versus nonlinear solutes and planar versus nonplanar solutes of comparable molecular weights, overall shapes, and molecular dimensions. They compared a monomeric phase with a bonding density of 3.01 äòî¿ m 2 and three polymeric phases with bonding densities of 3.55, 4.67, and 4.91 /xmol nr2. They observed that nonplanar solutes were eluted before planar solutes and that the selectivity between these solutes increased with increasing planarity/nonplanarity differences between the solutes. Similarly, nonlinear solutes were eluted before linear solutes, and selectivity increased with greater linearity/nonlinearity solute differences. They also observed that the selectivity of these compounds increased with in-creasing stationary-phase-bonding density.
Several studies [77,93] have found that certain planar and nonplanar PAHs exhibit dif-ferent elution behavior on monomeric versus polymeric C18 columns. These observations have led to the development of a mixture of three PAH compounds by the National Institute of Standards and Technology (NIST) as a Standard Reference Material (SRM) for the empirical characterization of the shape selectivity of C18 reversed-phase columns [94-96]. The elution order of this mixture (SRM 869) is indicative of the type of phase. SRM 869 contains benzo[a]pyrene (BaP), phenanthro[3,4-c]phenanthrene (PhPh), and 1,2:3,4:5,6:7,8-
tetrabenzonaphthalene (TBN). BaP is planar, whereas PhPh has a helical structure and is the most nonplanar of the three test solutes, and TBN is a nonplanar saddle-shaped solute (Fig. 3). The elution order for monomeric C18 phases is usually BaP < PhPh < TBN, whereas for polymeric phases, it is usually PhPh < TBN < BaP. The selectivity of TBN to BaP (qTBN/Bap) is a convenient means of classifying phase selectivity. Polymeric C18 phases yield àòâû/âàÐ — 1 and f°r monomeric Cj8 phases aTBN/BaP > 1.7, whereas phases that give 1 < «ten/âàð < 1 -7 (elution order of PhPh < BaP < TBN) are termed intermediate and are often either lightly loaded polymeric phases, such as oligomeric phases [771, or densely loaded monomeric phases.