Adsorption-Partitioning Retention Mechanism

Formation of a thick adsorbed layer of acetonitrile on the surface of reversed-phase adsorbent allows the introduction of a two-stage model of the analyte retention process. The first process is the partitioning of the analyte molecules from the bulk eluent into the adsorbed acetonitrile layer, and the second process is the analyte adsorption on the surface of the packing material. 

The binary eluent adsorption equilibrium is considered to be not disturbed by the injection of a small amount of the analyte (essentially the third component in the system). In an isocratic mode at a fixed eluent composition, the organic adsorbed layer is a stationary phase for the analyte to partition into. The analyte can partition into the adsorbed layer followed by consequent adsorption on the surface of the reversed-phase adsorbent. The overall retention is a superposition of two consecutive processes. Since the eluent component adsorption could be measured independently and adsorbed layer volume could be represented as a function of the mobile phase composition, the analyte retention also could be expressed as a function of the eluent composition. 

This model assumes the absence of any disturbance of the acetonitrile adsorption equilibrium by the analyte. This assumption limits the application of this model toward very low analyte concentrations—typical conditions for linear elution chromatography. 

The analyte distribution function in the column cross section dx could be written in the following form  

Because of the assumption made above, the solution is limited to the linear region of analyte partitioning and adsorption isotherms. The analyte distribution between two liquid phases (eluent and adsorbed phase) at equilibrium could be described as follows  

---

The selected latest LC LC studies are as follows adsorption retention mechanism [233-236] enthalpic partition retention mechanism [237] and phase separation retention mechanism [229]. It is anticipated that the LC LC procedures will find numerous applications in the different areas of the polymer synthesis/characterization. 

Activation by heating at 150-200°C removes the physically bound water. The assumption that one silica is most suitable for adsorption and another for liquid-liquid partition chromatography is questionable and, beyond that, irrelevant because pure adsorption or partition retention mechanisms generally do not occur. 

In contrast to RP chromatography, water is the stronger solvent in HILIC mobile phases, where it is usually contained in concentrations of 1-30%. Hence it forms an adsorbed layer on the surface of a polar adsorbent, which is thick enough to induce liquid-liquid partition between the bulk mobile phase and the adsorbed aqueous liquid layer. Consequently, the retention in HILIC is often due to the combination of adsorption and liquid-liquid partition mechanisms with possible additional effects of ion exchange (especially on bonded amino, zwitterionic, or weak ion exchange colunms). The transition between the adsorption and partition retention mechanisms is probably continuous as the water concentration in the mobile phase gradually increases. 

In reality, pure adsorption or partition retention mechanisms ordinarily do not occur. On the contrary, in many cases a combination of both retention mechanisms is operative. To increase... 

Principles and Characteristics Liquid chromatography is the generic name used to describe any chromatographic procedure in which the mobile phase is a liquid. It may be classified according to the mechanism of retention in adsorption, partition, size-exclusion, affinity and ion-exchange (Scheme 4.4). These mechanisms form the basis for the chromatographic modes of... 

In exclusion chromatography, the total volume of mobile phase in the column is the sum of the volume external to the stationary phase particles (the void volume, V0) and the volume within the pores of the particles (the interstitial volume, Vj). Large molecules that are excluded from the pores must have a retention volume VQ, small molecules that can completely permeate the porous network will have a retention volume of (Vo + Fj). Molecules of intermediate size that can enter some, but not all of the pore space will have a retention volume between VQ and (V0 + Fj). Provided that exclusion is the only separation mechanism (ie no adsorption, partition or ion-exchange), the entire sample must elute between these two volume limits. 

Similar to other coupled methods of polymer HPLC, for example, LC CC (Section 16.5.2), the choice of the column packing and the mobile phase components for EG-LC depends on the retention mechanism to be used. Adsorption is preferred for polar polymers applying polar column packings, usually bare silica or silica bonded with the polar groups. The eluent strength controls polymer retention (Sections 16.3.2 and 16.3.5). The enthalpic partition is the retention mechanism of choice for the non polar polymers or polymers of low polarity. In this case, similar to the phase separation mechanism, mainly the solvent quality governs the extent of retention (Sections 16.2.2, 16.3.3, and 16.3.7). It is to be reminded that even the nonpolar polymers such as poly(butadiene) may adsorb on the surface of bare silica gel from the very weak mobile phases and vice versa, the polymers of medium polarity such as poly(methyl methacrylate) can be retained from their poor solvents (eluents) due to enthalpic partition within the nonpolar alkyl-bonded phases. 

Solutes undergo retention when weak interaction occurs with the stationary phase. The analyst must select a stationary phase according to the nature of the solute (apolar, polar, ionizable). For convenience we shall consider the following different mechanisms adsorption, partition, chromatography of ionizable substances, chiral separations, exclusion. Table 1 is. a quick selection guide. 

With binary and ternary supercritical mixtures as chromatographic mobile phases, solute retention mechanisms are unclear. Polar modifiers produce a nonlinear relationship between the log of solute partition ratios (k ) and the percentage of modifier in the mobile phase. The only form of liquid chromatography (LC) that produces non-linear retention is liquid-solid adsorption chromatography (LSC) where the retention of solutes follows the adsorption isotherm of the polar modifier (6). Recent measurements confirm that extensive adsorption of both carbon dioxide (7,8) and methanol (8,9) occurs from supercritical methanol/carbon dioxide mixtures. Although extensive adsorption of mobile phase components clearly occurs, a classic adsorption mechanism does not appear to describe chromatographic behavior of polar solutes in packed column SFC. 

Regardless of the exact retention mechanism — adsorption, liquid-liquid partition or their combination — the stationary phase in normal-phase chromatography is more... 

---