Reversed-Phase Solvents

There are several synthetic methods to produce this representative superabsorbent polymer (SAP). In general ttiey are (1) an aqueous solution polymerization method in which organic solvents are not used and (2) a suspension polymerization method in which the aqueous solution of a monomer is suspended in an organic solvent (reverse phase suspension polymerization). 

Solvent triangle for optimizing reverse-phase HPLC separations. Binary and ternary mixtures contain equal volumes of each of the aqueous mobile phases making up the vertices of the triangle. 

Reversed-phase chromatography is widely used as an analytical tool for protein chromatography, but it is not as commonly found on a process scale for protein purification because the solvents which make up the mobile phase, ie, acetonitrile, isopropanol, methanol, and ethanol, reversibly or irreversibly denature proteins. Hydrophobic interaction chromatography appears to be the least common process chromatography tool, possibly owing to the relatively high costs of the salts used to make up the mobile phases. 

In classical column chromatography the usual system consisted of a polar adsorbent, or stationary phase, and a nonpolar solvent, mobile phase, such as a hydrocarbon. In practice, the situation is often reversed, in which case the technique is known as reversed-phase Ic. 

The teehniques of membrane extraetion permit an effieient and modern applieation of elassieal liquid-liquid extraetion (LLE) ehemistry to instmmental and automated operation. Various shorteomings of LLE are overeome by membrane extraetion teehniques as they use none or very little organie solvents, high enriehment faetors ean be obtained and there ai e no problems with emulsions. A three phase SLM system (aq/org/aq), where analytes are extraeted from the aqueous sample into an organie liquid, immobilized in a porous hydrophobie membrane support, and further to a seeond aqueous phase, is suitable for the extraetion of polar eompounds (aeidie or basie, ehai ged, metals, ete.) and it is eompatible with reversed phase HPLC. A two-phase system (aq/org) where analytes ai e extraeted into an organie solvent sepai ated from the aqueous sample by a hydrophobie porous membrane is more suitable for hydrophobie analytes and is eompatible with gas ehromatography. 

MLC enables to analyse drugs and active phamiaceutical substances without using special column and lai ge quantity of organic solvents. So, from the point of view of pharmaceutical analysis ecology and green chemistry conception, assay with MLC using will be better than conventional reversed-phase chromatography. 

Alhedai et al also examined the exclusion properties of a reversed phase material The stationary phase chosen was a Cg hydrocarbon bonded to the silica, and the mobile phase chosen was 2-octane. As the solutes, solvent and stationary phase were all dispersive (hydrophobic in character) and both the stationary phase and the mobile phase contained Cg interacting moieties, the solute would experience the same interactions in both phases. Thus, any differential retention would be solely due to exclusion and not due to molecular interactions. This could be confirmed by carrying out the experiments at two different temperatures. If any interactive mechanism was present that caused retention, then different retention volumes would be obtained for the same solute at different temperatures. Solutes ranging from n-hexane to n hexatriacontane were chromatographed at 30°C and 50°C respectively. The results obtained are shown in Figure 8. 

In contrast, the alkane chains on the polymeric phase cannot collapse in an environment of water as they are rigidly held in the polymer matrix. Thus, the retention of the solute now continuously falls as the methanol concentration increases as shown in Figure 4. It should be pointed out that if the nature of the solutestationary phase interactions on the surface of a bonded phase is to be examined in a systematic manner with solvents having very high water contents, then a polymeric phase should be used and brush type reversed phases avoided if possible. 

Silica gel, per se, is not so frequently used in LC as the reversed phases or the bonded phases, because silica separates substances largely by polar interactions with the silanol groups on the silica surface. In contrast, the reversed and bonded phases separate material largely by interactions with the dispersive components of the solute. As the dispersive character of substances, in general, vary more subtly than does their polar character, the reversed and bonded phases are usually preferred. In addition, silica has a significant solubility in many solvents, particularly aqueous solvents and, thus, silica columns can be less stable than those packed with bonded phases. The analytical procedure can be a little more complex and costly with silica gel columns as, in general, a wider variety of more expensive solvents are required. Reversed and bonded phases utilize blended solvents such as hexane/ethanol, methanol/water or acetonitrile/water mixtures as the mobile phase and, consequently, are considerably more economical. Nevertheless, silica gel has certain areas of application for which it is particularly useful and is very effective for separating polarizable substances such as the polynuclear aromatic hydrocarbons and substances... 

Dionex Corporation - Manufacturers of liquid chromatography systems (IC and HPLC), chromatography software data systems, reversed-phase and ion-exchange columns, and accelerated solvent extraction systems... http //www.dionex.com. 

In most situations the eluent composition is chosen to minimize the effects of hydrophobic interaction, but these secondary effects can be used to advantage. By careful selection of a salt and its concentration, specific selectivities for analytes can be achieved without the use of organic solvents. Therefore, many separations usually run by solvent gradient reversed-phase methods can be completed with a purely aqueous isocratic eluent (13,14). 

FIGURE 13.57 NOTE The importance of Solvent/column interaction using Jordi DVB columns cannot be over emphasized. We have found that a SOySO mbc of MeOH/ACN for the strong solvent Is adequate for many reverse phase separations and is better than either alone. We have now observed that the use of THF/ACN as strong solvent is often better than MeOH/ACN. In general Lewis bases (electron donor solvents) deactivate the aromatic rings and often dramatically increase column efficiencies. 

K. Grob, Concurrent eluent evapor ation with co-solvent Capping for on-line reversed-phase liquid cliromatography-gas clir omatogr aphy. Optimization of conditions , J. Chromatogr. 477 73-86 (1989). 

T. Hyotylainen, K. Grob, M. Biedermann and M-L. Riekkola, Reversed phase HPLC coupled on-line to GC by the vaporizer/precolumn solvent split/gas dischar ge analysis of phthalates in water , 7. High Resolut. Chromatogr. 20 410-416 (1997). 

In the first version with a mobile phase of constant composition and with single developments of the bilayer in both dimensions, a 2-D TLC separation might be achieved which is the opposite of classical 2-D TLC on the same monolayer stationary phase with two mobile phases of different composition. Unfortunately, the use of RP-18 and silica as the bilayer is rather complicated, because the solvent used in the first development modifies the stationary phase, and unless it can be easily and quantitatively removed during the intermediate drying step or, alternatively, the modification can be performed reproducibly, this can result in inadequate reproducibility of the separation system from sample to sample. It is therefore suggested instead that two single plates be used. After the reversed-phase (RP) separation and drying of the plate, the second, normal-phase, plate can be coupled to the first (see Section 8.10 below). 

---