No abstract

No abstract

The retention mechanism in reversed-phase liquid chromatography (RPLC) has been investigated by examining the temperature dependence of retention, with emphasis on the role of the stationary phase in the retention process. Both chromatographic temperature studies and differential scanning calorimetry were used to examine the role of alkyl chain bonding density on the retention mechanism in RPLC. Phase transitions of reversed-phase stationary phases were observed at bonding densities greater than 2.84 mumol/m2. Thermodynamic constants for the transfer of a solute from the mobile phase to the stationary phase (delta H degrees and delta S degrees) were calculated for low bonding density columns, and comparison of these values to previously reported values for the partitioning of a nonpolar solute from the bulk organic liquid to water indicated that the chromatographic retention process is not well-modeled by bulk-phase oil-water partitioning processes. In addition, this data showed that the entropic contribution to retention becomes more significant with respect to the enthalpic contribution as the stationary-phase bonding density is increased, providing additional support that partitioning, rather than adsorption, is the relevant model of retention.

A qualitative informational similarity technique has been used to describe the informational orthogonality of projected two-dimensional (2-D) chromatographic separations of complex mixtures from their one-dimensional 1-D separations. The reversed-phase liquid chromatography (RPLC), supercritical fluid chromatography (SFC), gas-liquid chromatography (GLC), and micellar electrokinetic capillary chromatography (MECC) retention behavior of up to 46 solutes of varying molecular properties was studied by 2-D range-scaled retention time plots and information entropy calculations. One hundred five combinations of technique/stationary phase pairs were used to simulate the 2-D chromatographic analyses. The informational entropy of one and two dimensions, the mutual information, the synentropy or "cross information", and the informational similarity were calculated to describe the informational orthogonality. In addition, pattern descriptors were used to qualitatively describe the 2-D peak distribution. With the solutes tested, informational orthogonality, zero informational similarity, was observed with MECC-SDS/SFC-C1, MECC-SDS/SFC-Carbowax, MECC-TTAB/SFC-Carbowax, HPLC-C18/GLC-DB-5, HPLC-PBD/SFC-phenyl, SFC-Carbowax/GLC-DB5, and HPLC-phenyl/SFC-phenyl 2-D chromatographic systems. Conversely, with the solutes tested, informational nonorthogonal behavior described by range-scaled retention time plots to moderate to severe band overlap and data clustering was observed with 2-D chromatographic systems with high informational similarity and moderate to high degrees of synentropy. These results should prove useful for predicting complementary 2-D techniques as well as for choosing a second separation technique for confirmation of separation or peak purity.

The use of mlcellar mobile phases can provlde unlque selectivities In llquld chromatography. A major drawback In all published reports, however, Is a loss of efflclency when compared to tradltlonal hydroorganlc moblle phases. This inefflclency is shown to arise from slow mass transfer, which comes principally from poor wettlng of the stationary phase. Low concentrations of organic modlflers are useful for modlfylng the surface of the stationary phase and provlding the wetting needed for good mass transfer. Elevated temperatures are also shown to be useful In overcomlng the hlgher vlscoslty of mlcellar mobile phases and thereby serving to Improve peak shape. Mobile phases contalnlng about 3% propanol and temperatures of about 40 OC provide efflclencies approachlng those of hydroorganic moblle phases.The popularity of modern liquid chromatography continues to increase. One of the reasons for this is the unique selectivities that can be generated in the mobile phase by the addition of chemical modifiers. Retention in reversed-phase LC is dominated by solvent-solute interactions, with stationary phase-solute interactions making secondary contributions. The key to separation is then to be able to change the solvent-solute interactions in such a way as to shift the retention of any overlapping compounds. In some cases the selectivity of the chromatographic system can be modified by simply changing the eluent strength or ratio of the hydroorganic mobile phase. Most often, however, changes in selectivity reached in this fashion are accompanied by concomitant changes in the retention of all sample components, and an appreciable change in relative retention is achieved only with capacity factors that fall outside the practical range of 0.3 < k' < 5. It is desirable to find ways to change the selectivity of the chromatographic system for closely eluting sample components without drastic changes in the eluent strength as a whole.One way that this has been done, particularly for ionogenic solutes, is by the addition of low concentrations of ionic surface active agents having an opposite charge to the solute. The surfactant will then coulombically interact with the solute and drastically alter its retention characteristics. Because of the nature of the surfactant, this technique was dubbed "soap chromatography" ( I ) and is now more commonly known as ion-pair chromatography.Until recently, only low concentrations of surfactant were used, and investigators intentionally stayed below critical micelle concentrations. Armstrong and Henry first effectively demonstrated the usefulness of reversed-phase mobile phases containing surfactants above the critical micelle concentration (2). They showed that the micelle can provide a hydrophobic site for interaction with the solute in the mobile phase and can be used in place of an organic modifier. Since then other reports have appeared (3-8) and certain advantages have been shown, including the selectivity of the micellar interaction and economy of operating expense when compared to expen...

Five nonaqueous solvents (acetonitrile, methanol, N,N-dimethylformamide, dimethyl sulfoxide, formamide) and deionized water were investigated for their ability to support electroosmotic flow (EOF) without electrolytic additives. In general, flow was found to be equal to or greater than flow with typical CE buffer systems. The magnitude of EOF was determined for each solvent by open tubular capillary electrophoresis (CE) and related to viscosity (eta), dielectric constant (epsilon), and the ratio of dielectric constant to viscosity (eta/epsilon). Zeta potentials (zeta) were derived indirectly from flow data and tabulated. Comparisons of flow behavior and zeta were made between pure solvents and conventional CE buffers, and questions of equilibrium and reproducibility were addressed. Similar experiments were performed using hydroorganic mobile phases (ACN/water, MeOH/water) across the complete compositional range (100% water-100% organic), with flow characteristics and zeta reported for each mobile phase system. Packed capillary columns (5-microns ODS) were evaluated for flow and retention stability under capillary electrochromatographic (CEC) conditions. A separation of 11 polycyclic aromatic hydrocarbons was performed in under 13 min by CEC with an ACN/water mobile phase. Reduced plate heights (h) were calculated between 2.5 and 3.0 for solutes with capacity factors (k') up to 4.5 for the most retained solute.

The retention mechanism in reversed-phase liquid chromatography (RPLC) has been examined over a wide temperature range with emphasis on the role of the mobile phase. van't Hoff plot shapes were used to assess the retention mechanism, and the data showed evidence of the hydrophobic effect when water-rich and/or hydrogen-bonded mobile phases such as methanol/water were used. However, different van't Hoff plot shape was observed with acetonitrile/water mobile phases, indicating a change in the retention mechanism. These data showed that the hydrophobic effect, which had previously been proposed as the driving force for retention, is not a satisfactory explanation for the retention process in all RPLC systems.

The partitioning model of retention for reversed-phase liquid chromatography, described by mean-field statistical thermodynamic theory, asserts that one principal driving force for solute retention is the creation of a solute-sized cavity in the stationary phase. Beyond a critical stationary phase bonding density, increased grafted chain density should result in enhanced chain ordering, which will increase the energy necessary for solute cavity formation and result in decreased chromatographic partition coefficients. We have evaluated chromatographic partition coefficients over an octadecyl bonding density range of 1.6-4.1 mumol/m2 and have found a maximum in partition coefficient at approximately 3.1 mumol/m2. Retention, however, approximately plateaus due to compensating changes in the partition coefficient and stationary phase volume. This provides unequivocal evidence that partitioning is the dominant form of retention for small nonpolar solutes.