Mobile phase solvents, choosing

The number of mobile phases to choose from is great, since the solvents are evaporated after chromatography and no interference with the measurement can... 

Choosing a Mobile Phase Several indices have been developed to assist in selecting a mobile phase, the most useful of which is the polarity index. Table 12.3 provides values for the polarity index, P, of several commonly used mobile phases, in which larger values of P correspond to more polar solvents. Mobile phases of intermediate polarity can be fashioned by mixing together two or more of the mobile phases in Table 12.3. For example, a binary mobile phase made by combining solvents A and B has a polarity index, of... 

Procedures used vary from trial-and-error methods to more sophisticated approaches including the window diagram, the simplex method, the PRISMA method, chemometric method, or computer-assisted methods. Many of these procedures were originally developed for HPLC and were apphed to TLC with appropriate changes in methodology. In the majority of the procedures, a set of solvents is selected as components of the mobile phase and one of the mentioned procedures is then used to optimize their relative proportions. Chemometric methods make possible to choose the minimum number of chromatographic systems needed to perform the best separation. 

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. 

Instrumentation requirements for SEC are somewhat simpler than those of other modes of HPLC, since mobile phase gradients are not used however, adequate computer support for data acquisition and processing is essential. Method development involves finding a suitable solvent for the sample and choosing a mixed bed column or, more often, a set of columns in series to match the pore size of the column(s) with the size distribution of the sample. 

There remains little more for the operator to decide. Sometimes, alternative but similar solvent mixtures that have a lower viscosity or higher solute diffusivity could be selected. For example, a n-hexane/methanol mixture might be chosen as an alternative to the more viscous n-heptane/isopropyl alcohol mixture as it has similar elution properties. However, it will be shown later, that if a fully optimized column is employed the viscosity of the mobile phase does not seem to effect the column performance as it is taken into account in the optimization procedure. The operator would, under some circumstances, be free to choose less toxic or less costly solvents for example, in reverse phase chromatography the operator could select methanol/ water solvent mixtures as opposed to acetonitrile/water mixture on the basis of lower cost or less... 

A more or less opposite goal was pursued by de Smet et al. (574], who attempted to reduce the number of stationary phases to a single one, by choosing a cyanopropyl bonded phase of intermediate polarity, which can be used in both the normal phase and the reversed phase mode (see figure 3.8). Furthermore, because of a clever choice of modifiers, the total number of solvents required was restricted to six n-hexane, dichloromethane, acetonitrile and THF for NPLC and the latter two plus methanol and water for RPLC. A variety of drug samples could be separated with a selected number of binary and ternary mobile phase mixtures. 

Considerations when choosing solvents for mobile phases Miscibility... 

CONSIDERATIONS WHEN CHOOSING SOLVENTS FOR MOBILE PHASES 243... 

For reverse-phase chromatography, the mobile phase is made by choosing one solvent in which the sample is very soluble and another solvent in which the sample is less soluble. One can then prepare a mobile phase by adjusting the amount of the strong and weak solvents to a ratio where the attraction of the solutes to the packing is in a competitive equilibrium with the attraction (solubility) of the solutes to the mobile phase. The equilibrium of the solutes in the mobile phase relative to the bonded phase determines the retention time and effects the separation. 

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