Solvent strength mobile-phase mixtures

Resolution can be mapped as a function of various proportions of acetonitrile, methanol, and THF in the mobile phase. Usually the k range or run time is held constant during the process by varying the amount of water in the mobile-phase mixture so as to compensate for small differences in the strength of the three pure organic solvents. If further improvement in separations is needed, the additives given in Table 15.8 should be considered. 

Eqn.(3.73) suggests that any mixture of two solvents with the same ° value (iso-eluotro-pic solvents) will also have the same eluotropic strength. This would allow the application of a similar strategy for the definition of iso-eluotropic multicomponent mobile phase mixtures as was used for RPLC in section 3.2.2.1. In practice, the situation in LSC has proved to be more complicated, because an effect described as solvent localization limits the validity of eqns.(3.72) and (3.73) if polar components (such as acetonitrile or methyl t-butyl ether) are present in the mobile phase. This makes it difficult to calculate the composition of iso-eluotropic mixtures for LSC with sufficient accuracy for optimization purposes [360-363]. 

Conversely, at lower temperatnres the hydrophobic effect entropically leads the adsorption of the solutes [12]. Actually the solvent strengths of aU mixtures change with temperature [13] and this influences selectivity. Also, the non-constancy of with changing temperature may also be due to the difference of the heat capacities of the analytes in the mobile and stationary phases according to the Kirkhoff equation... 

Mobile Phases An organic solvent (called co-solvent or modifier) is very often used in SFC to increase the solubility of the crude mixture in the eluent and fine-tune the eluent strength. Mobile phases on SFC use from 0 to 40% of modifier. Fresh modifier is pumped in the system vith a high pressure pump. The modifier is collected at the liquid outlet of the separators. 

Smith and Cooper [601] studied the retention of three nonpolar solutes (phenan-threne, chrysene, perylene) and four polar solutes (nitrobenzene, 1,2-dinitrobenzene, phenol, aniline) in hexane and hexane/x mobile phases (where x = chloroform, methyl r-butyl ether [MtBE], and dichloromethane at the 5%, 10%, 15%, and 20% levels) on cyanopropyl, aminopropyl, and diol columns. From this work, the solvent strength of each mixture was determined for use in predicting chromatographic retention. More importantly, complex solvent/solute/adsorbed solvent/stationary phase interactions were described highlighting important and unique selectivities offered by these combinations. For example, altering the mobile phase composition from 3% MtBE in hexane to 12% MtBE in hexane (on a cyanopropyl support) leads to a decrease in the retoition of phenol and aniline. What is imexpected is the concomitant reversal of the elution order (phenol/aniline to aniline/phenol). This type of reversal of elution order is rare in leversed-phase separations (ion-pair systems notably excluded) but may be a considerable advantage in normal-phase separations. 

Recently, Janjic et al. published some papers [33-36] on the influence of the stationary and mobile phase composition on the solvent strength parameter e° and SP, the system parameter (SP = log xjx, where and denote the mole fractions of the modiher in the stationary and the mobile phase, respectively) in normal phase and reversed-phase column chromatography. They established a linear dependence between SP and the Snyder s solvent strength parameters e° by performing experiments with binary solvent mixtures on silica and alumina layers. 

Solvent strength determines the value, but not the selectivity. The mobile phase can be established by using the polarity index P proposed by Snyder. The highest values of P represent the strongest solute adsorbed in conventional TLC but represent the weakest for the separation in reversed phases. Sometimes aqueous polar mixtures cannot totally wet the chemically bonded layer. For this reason, checking... 

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. 

---