Mobile-phase selectivity

Anionic and neutral polymers are usually analyzed successfully on Syn-Chropak GPC columns because they have minimal interaction with the appropriate mobile-phase selection however, cationic polymers adsorb to these columns, often irreversibly. Mobile-phase selection for hydrophilic polymers is similar to that for proteins but the solubilities are of primary importance. Organic solvents can be added to the mobile phase to increase solubility. In polymer analysis, ionic strength and pH can change the shape of the solute from mostly linear to globular therefore, it is very important to use the same conditions during calibration and analysis of unknowns (8). Many mobile phases have been used, but 0.05-0.2 M sodium sulfate or sodium nitrate is common. 

Mobile-phase selection for cationic polymers is similar to that for the other polymers in that ionic strength and pH can change the shape of the solute from linear to globular (9). Mobile phases are often low pH e.g., 0.1% trifluo-roacetic acid, including 0.2 M sodium chloride, has been used successfully for polyvinylpyridines. Sodium nitrate can be substituted for the chloride to avoid corrosive effects. Some salt must be included so that ion exclusion does not occur (3). 

Another advantage of HdC is its generosity in terms of mobile-phase selection. The polymer size and solution properties of a polymer can be studied using HdC, especially OTHdC, in almost any solvent. In SEC, by comparison, the packing material and mobile phase have to be selected to prevent the nonsize exclusion effect. Because the instrumentation of HdC is similar to SEC, and the packing material and columns have become available commercially, this technique will gain in popularity. 

Remarks Moderate selectivity Moderate separation speed Narrow mobile-phase selection Fow selectivity High-speed separation Most generous mobile-phase selection Fow selectivity High-speed separation Generous mobile-phase selection Best for high MW polymers Thermal gradient may be programmed for broad MW separation... 

Figure 8.20 Combination of bilayer plates and multiple development teclmiques in which total solvent sti ength and mobile phase selectivity are changed simultaneously, in the first direction (a), S- and are varied in n re-chromatograpliic steps, while in the peipendicular, (second) direction (b), and are again varied in m re-clnomatographic steps, to give (c).

In many cases, the solvent systems determined by analytical TLC are directly applicable to PLC with similar results. A proper mobile phase selected for PLC should have a resolution more than 1.5 in the analytical scale. According to theory, PLC resolution, however, decreases with increasing particle size. Improved separa-... 

Mobile phases are of a greater variety than the restricted number of stationary phases. Many solvents and their mixtures are used as a mobile phase. The possibility of slight modification of solvent proportions in a mixmre permits the increase of mobile phase number and, thus, different results in the component separation of the analyzed sample. That is why the optimum mobile phase selection becomes one of the basic operations for the success of the analysis. 

In the mobile phase selection for the separation of compounds on thin silica gel layers, it is necessary to use not only eluotropic series based on the eluting capacity of the solvent but also eluotropic series of compounds established according to their interaction with silica gel. 

The separations of some nonionic tensides having biological activity and consisting of ethyleneoxide oligomer mixtures were performed in many different TEC systems (silica and alumina as the stationary phase and single solvent or binary mixtures as the mobile phase). Selectivity was higher on alumina than on the silica layer. Both... 

The microcircular technique for the mobile phase selection is illustrated in Figure 4.10a. It can be observed that the solvent A3 is the most suitable for nonpolar samples and the solvent B3 is for polar samples. 

For optimum mobile phase selection we have to consider certain chemical characteristics besides the solvent strength [15,16]. From this point of view, the chromatographic solvents can be divided as follows ... 

FIGURE 4.10 Mobile phase selection by microcircular technique, a. Sample of known composition A = nonpolar compound A1 = n-hexane A2 = acetone A3 = n-hexane-acetone, 60-1-40, v/v B = polar compound B1 = methanol B2 = water B3 = methanol-water, 70-1-30, v/v. b. Sample of unknown composition testing with solvents of different Snyder s groups and binary solvent mixture. 

On the basis of Snyder s system for characterization of solvents the PRISMA method for mobile phase optimization has been developed. This system enables the optimization of solvent strength and mobile phase selectivity and also the transfer of the optimized mobile phase to different planar chromatographic techniques, in our case the PLC. 

Emphasis has to be on choice and proper handling of the mobile phase. In Chapter 4 different approaches for mobile phase selection are discussed. General hints for selection are the avoidance of the following ... 

Mobile phase selection (1) Choose a low viscosity solvent which separates the mixture and moves the desired component to an Rf of ca. 0.35 (2) If several cospounds are to be separated which run close together on TLC, adjust the solvent strength to put their midpoint at an R value of ca. 0.35 (3) If compounds are well separated, choose a mobile phase which provides an R, value of ca. 0.35 for the least retained component. 

Kopaciewicz, W. and Regnier, F. E., Mobile phase selection for the high-performance ion-exchange chromatography of proteins, Anal. Biochem., 133, 251, 1983. 

Discussed below are various modes of separations in HPLC. Included here is brief coverage of mobile-phase selection for various modes of chromatography and elementary information on mechanism, choice of solvents and columns, and other practical considerations. It should come as no surprise that reversed-phase HPLC is discussed at greater length in this section because it is the most commonly used technique in HPLC (more detailed discussion is provided in Section 15.8). Clearly,... 

The conventional approaches to mobile-phase selection and optimization are discussed here. The primary focus is on compounds with molecular weight less than 2000. More detailed information including coverage of macromolecules may be found in some basic texts [2,5,39,103]. As discussed earlier, various modes of chromatography utilized to separate these compounds can be classified as follows ... 

You are running the Column and Mobile Phase Selection module. 

In GC, the mobile phase acts only as a carrier. In LC, solute undergoes interaction with liquids or mixtures of liquids used as the mobile phase. Selection of the mobile phase is critical. The most useful criteria are the solubility parameter concept, Snyder s selectivity triangle, and solva-tochromic parameters. 

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