Stationary phase polar active sites

In interaction chromatography the packing material or stationary phase has active sites at the surface, where interaction with the solute molecules takes place according to their polarity. In general, the stationary phase must withstand... 

In the case of NPLC, the stationary phase is polar (silica, alumina, or polar-bonded phase) and the mobile phase is nonpolar (hexane, heptane, etc.). Adsorption mechanism operates and involves adsorption of an analyte by its polar groups on the polar active sites of the stationary phase. Adsorption occurs by polar interactions and formation of hydrogen bonds. The nonpolar mobile phase cannot interact with the adsorbent-active sites and thus its adsorption practically does not occur. However, the mobile phase favors the analyte desorption interacting by dispersion forces with its nonpolar groups that are not involved in the adsorption. This is illustrated in Figure 2 for analysis of an alcohol on silica. This case will be discussed again below. Thus, analyte molecules adsorb and desorb constantly, being... 

Ethanol and IPA have gained wide read use as polar mobile phase components in chiral separations (see also niarmaceutical Analytes, Section 4.7). Here the choice of solvent is critical since the steric structure of the solvent mediates the solute interaction with the stationary phase chiral binding site through either (1) direct interaction with the chiral site, or/and (2) interaction with an achiral site near the active chiral site, thereby altering the steric configuration of the site [142]. 

Pure fluids. Carbon dioxide is often the mobile phase of choice for SFC, since it has relatively mild critical parameters, is nontoxic and inexpensive, chemically inert, and is compatible with a wide variety of detectors including the flame ionization detector (FID) used widely in GC and the UV absorbance detector employed frequently in HPLC (7). The usefulness of carbon dioxide as a mobile phase in many instances is somewhat limited, however, because of its nonpolarity (8), and many polar compounds appear to be insoluble in it. For a sample containing polar compounds, pure carbon dioxide may not be the proper mobile phase. The elution of polar compounds is often difficult and the peak shapes for these polar compounds are sometimes poor. This latter difficulty is commonly observed with nonpolar supercritical fluids and may be due to active sites on the stationary phase rather than any inherent deficiency in the fluid itself. 

Since the stationary phase is quite nonpolar, the mobile phase must be polar to assure retention. Acetic acid is used in the mobile phase to eliminate tailing of the peaks due to interaction with the residual silica active sites. This technique is referred to as ion suppression since it also suppresses the ionization of the sample components. 

Organic solvents also induce changes in the properties of surfactant-coated stationary phases, such as polarity, surface area, or pore volume. Several studies have demonstrated that /i-alcohols interpenetrate the Cig-bonded alkyl chains to form a single monolayer, structurally similar to an opened micelle (i.e., the hydroxyl group orientated toward the aqueous phase). The competition between organic solvent and surfactant molecules for the active sites on the column explains the reduction of adsorbed surfactant at increasing concentration of organic solvent in the mobile phase. For ionic surfactants, this... 

Although the exact mechanism is not very clear, the following factors may contribute to the modifier effect on chromatographic retention. Polar modifiers may cover the active sites of the stationary phase (deactivation) so that solute retention is reduced. This can be explained by the differences in retention change between packed and open-tubular columns when small amounts of modifiers were used. Open-tubular columns normally do not show the drastic changes in retention or efficiency upon the addition of small amounts (<2%) of modifier as most packed columns do. These less drastic differences were caused by the differences in the degree of deactivation of the packed column stationary phase as compared with the open-tubular-column stationary phase. An open-tubular column has fewer active sites present and, thus, fewer active sites are present for the modifier to deactivate. 

In this mode, active inorganic adsorbents (e.g., silica, alumina, or Florisil) are usually employed as stationary phases and, hence, the overall mechanism of retention is governed predominantly by the specific intermolecular interactions between the functionalities of the solutes, on the one hand, and active sites on the adsorbent surface, on the other. In adsorption TLC, aqueous mobile phases are never used, and stationary-phase activity prevails over the polarity of the mobile phase employed. 

Porous graphitic carbon (section 4.2.5) is an inert but highly retentive sorbent under supercritical fluid chromatography conditions. Supercritical carbon dioxide is a weak eluent for porous graphitic carbon and even compounds such as naphthalene are difficult to elute in a reasonable time [72]. Low molecular mass polar compounds generally have poor peak shapes, but in this case most likely due to limited solubility in tbe mobile phase rather than undesirable interactions with active sites on the stationary phase. The flat surface of porous graphitic carbon leads to preferential adsorption of... 

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