Shape selectivity mobile-phase composition

A very minor effect on shape selectivity has been observed for changes in mobile phase composition [109]. Shape selectivity increased slightly with an increase in percent organic modifier for water-organic mobile phase systems and increased in order of methanol acetonitrile < ethanol. Changes in the shape selectivity factor... 

FIGURE 9.19 Effect of mobile phase composition on shape selectivity with a monomeric octadecylpolysiloxane stationary phase, column using (a) SRM 869a (b) triphenylene/o—terphenyl (c) chrysene/benzo[a]anthracene with column outlet pressure 20.0 MPa and flow rate 1 mL/min at pump head. (Reprinted from J. W. Coym, J. G. Dorsey, J. Chromatogr. A, 971 61 (2002). With permission.)... 

The first results of optimization in chromatography were published in 1975 Since then a growing number of optimization experiments in HPLC using the Simplex procedure has been reported (table 9). The examples are mainly reversed-phase separations, in which the composition of the ternary or binary mobile phase composition is optimized. The factors optimized are usually a selection from flow rate, column temperature and length, the eluents constitution (e.g. organic modifier content, buffer concentration and pH), the gradient shape. Seven years after the first applications of Simplex optimization had appeared, the first fully automated optimization of HPLC separations was published by Berridge in 1982. This development coincid-... 

Mobile phases are usually binary or ternary mixtures of solvents. Selectivity is affected mostly by mobile phase composition rather than strength, and peak shape and retention are both influenced by the addition of organic modifiers.101 Some compounds naturally have 77-donor or 77-acceptor groups and can be resolved directly. In many cases, however, introduction of 77-donating groups by derivatization steps is necessary. Figure 2.20 shows the proposed three-point interaction of 3-aminobenzo[a]pyrene, a polycyclic aromatic hydrocarbon (PAH), with a Pirkle-type stationary phase.111 Two possible interactions are illustrated, showing the best orientations for maximum interaction. 

The initial and final mobile phase compositions and the gradient shape are the properties required to define a suitable gradient for a separation. Solvent selection rules are the same as for isocratic separations (section 4.6.2). The initial mobile phase composition... 

Resolution of the three HBCD diastereomers is easily achieved by reversed-phase LC on C18 columns, often in less than 15 min. The usual elution order for the diastereomers is a-, P-, y-HBCD however, an alternate elution order (i.e., a-, y-, P-HBCD) was reported by Dodder et al. [Ill] for shape-selective columns operated with methanolic mobile phases (see Figure 13.8). The reversed elution order for P-HBCD and y-HBCD provides the basis for the development of orthogonal methods that may help eliminate matrix interferences. Yu et al. [112] also studied the influence of mobile phase composition on HBCD diastereomer selectivity and show that MS sensitivity was also affected by this parameter. 

The majority of LC separations of fat-soluble vitamins are based on RP chromatography on C18 columns, but the use of a triacontyl stationary phase (C30) is becoming more common, due to its superior shape selectivity [90,91]. Shape discrimination with C30 columns improves at subambient temperature, permitting resolution of geometric and positional isomers partly or fully, depending on the mobile phase composition. Nevertheless, C30 columns are less efficient and the chromatographic peaks broader than those obtained with C18 columns. 

It will be clear from figure 6.7 that the nature of the mobile phase (compare figures 6.7a and 6.7b) and the stationary phase (compare figure 6.7c with figures 6.7a and 6.7b) have a great effect on the character of the retention vs. composition plots and hence on the shape of the required (optimum) gradient. It will also be clear that, unlike the situation in GC, the selectivity may be greatly influenced by variations in the mobile phase. 

---