Retention mechanisms mixed stationary phases

Bonded stationary phases for NPC are becoming increasingly popular in recent years owing to their virtues of faster column equilibration and being less prone to contamination by water. The use of iso-hydric (same water concentration) solvents is not needed to obtain reproducible results. However, predicting solute retention on bonded stationary phases is more difficult than when silica is used. This is largely because of the complexity of associations possible between solvent molecules and the chemically and physically heterogeneous bonded phase surface. Several models of retention on bonded phases have been advocated, but their validity, particularly when mixed solvent systems are used as mobile phase, can be questioned. The most commonly accepted retention mechanism is Snyder s model, which assumes the competitive adsorption between solutes and solvent molecules on active sites... 

This CCC system was developed to compensate for the limitations of droplet CCC, which requires adequate droplet formation. Figure 3 illustrates the column design and mechanism of rotation locular CCC. The column is made by inserting centrally perforated disks into the tubular column at regular intervals to form multiple compartments called locules . Both retention of the stationary phase and interfacial area in each locule are optimized by inclination of the colvunn while the mixing of the two phases is introduced by the rotation of the column. 

The mixed retention mechanism described above has a parallel in the effect of exclusion on retention when using porous stationary phases. The smaller molecules can enter more pores and thus interact with more stationary phase than the larger molecules that are excluded from many pores and, consequently, interact with less stationary phase. Assuming the smaller molecules are more strongly retained, the exclusion of large molecules augments the difference in retention between molecules of different size. 

Two limiting mechanisms for solute retention can be imagined to occur in RPC binding to the stationary phase surface or partitioning into a liquid layer at the surface. In the previous treatment we assumed that retention is caused by eluite interaction with the hydrocarbonaceous surface, i.e., the first type of mechanism prevails. When the eluent is a mixed solvent, however, the less polar solvent component could accumulate near the apolar surface of the stationary phase. In the extreme case, an essentially stagnant layer of the mobile phase rich in the less polar solvent could exist at the surface. As a result eluites could partition between this layer and the bulk mobile phase without interacting directly with the stationary phase proper. 

Another problem associated with LLC is that of mixed retention mechanisms. Ideally, the solid support in LLC binds the molecules of the stationary phase with strong adsorptive forces, but it does not exert these forces on solute molecules. Clearly, this ideal situation can never be realized completely [315]. 

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... 

Unexpected pH dependencies were explained by (a) competition between negative analyte ions and OH ions for interaction with the electrical double layer and (b) a mixed retention mechanism in which reverse-phase partition or interaction with unreacted silanols from the stationary-phase base may play a significant role. 

Actually, we should separate inverse gas chromatography into inverse gas-liquid chromatography and inverse gas-solid chromatography. The obvious basis of such discrimination is the state of the column content being examined. Polymers and their mixtures, commercial stationary phases, surfactants represent liquids (at the measurement temperature) involving a mixed mechanism of the retention of the test solutes. Modified silicas are examples of solids that have been studied, and, in this case, adsorption effects predominate, while solution partition in graft chains seems to be negligible. These problems will be discussed in details by Papirer and Balard in another chapter of this book. 

Higher aliphatic amines (butylamine, pentylamine, diethylamine, etc.) had larger retention volumes and values well above 1.0. A mixed retention mechanism involving hydrophobic adsorption and steric effects was observed for these compounds. Aromatic amines were found to be retained almost solely by a reversed-phase mechanism involving interaction of the solute with the unfunctionalized regions of the stationary phase. Retention of these solutes could be manipulated most easily by addition of acetonitrile to the eluent. 

EC differs from MEKC in that it uses a real stationary phase. In essence, it is conventional LC in which the mobile phase is driven by electro-osmotic flow rather than pressure. However, as in MEKC, the mechanism of separation is mixed, with electrophoretic mobility affecting the retention of charged solutes. The stationary phase particles can be very small, as there is no pressure drop in the column. Typically, 1.5 /rm particles of Cig-modified silica are used. Promising results were also obtained with monolithic columns for EC. Capillary EC provides about twice as many plates as HPLC for the same particle size and column length. 

In coupM columns, mixed-bed or twin phase variability in solute retention occurs as a result of the physical manipulation of the stationary phase. While the overall retention of a solute molecule may have changed, the retention mechanisms of the stationary phases are the same. That is, the local chemical environment of the stationary phase, on the scale of a several square nanometers, is unaltered. 

Using these methods is similar to reconstructing a puzzle. How the retention and vaporization mechanisms can be quantitatively analyzed, and the predicted retention times improved, based on molecular properties calculated in silica, are fundamental questions in chromatography. In gas chromatography, no solvent is used except in special cases where water vapor and ionic gas are mixed with the carrier gas. The basic retention mechanisms depend on the strength of the molecular interaction with the stationary phase, and the vaporization mechanism depends on the properties of the analytes. 

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