Mobile phase binary mixtures

Figure 9 Solvent selectivity triangle approach forthe selectivity optimization in HP-RPC. First, three initial experiments (1-3) with three mobile phases (binary mixtures of ACN/water, MeOH/water, and THF/water, respectively) are performed.

The first chiral separation using pSFC was published by Caude and co-workers in 1985 [3]. pSFC resembles HPLC. Selectivity in a chromatographic system stems from different interactions of the components of a mixture with the mobile phase and the stationary phase. Characteristics and choice of the stationary phase are described in the method development section. In pSFC, the composition of the mobile phase, especially for chiral separations, is almost always more important than its density for controlling retention and selectivity. Chiral separations are often carried out at T < T-using liquid-modified carbon dioxide. However, a high linear velocity and a low pressure drop typically associated with supercritical fluids are retained with near-critical liquids. Adjusting pressure and temperature can control the density of the subcritical/supercritical mobile phase. Binary or ternary mobile phases are commonly used. Modifiers, such as alcohols, and additives, such as adds and bases, extend the polarity range available to the practitioner. 

For the elution strength of a binary mobile phase, a mixture of solvents 1 and 2, having ej and e as the respective eluent strengths and knowing that component 2 is the more strongly sorbed component, Snyder obtained the following relationship ... 

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. 

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

Practically a more convenient way of expressing solute retention in terms of solvent concentration for a binary solvent mixture as the mobile phase is to use the inverse of equation (11), i.e.. 

However, there might be exceptions if the mobile phase consists of a binary mixture of solvents, then a layer of the more polar solvent would be adsorbed on the surface of the silica gel and the mean composition of the solvent in the pores of the silica gel would differ from that of the mobile phase exterior to the pores. Nevertheless, it would still be reasonable to assume that... 

Bonded phases are the most useful types of stationary phase in LC and have a very broad range of application. Of the bonded phases, the reverse phase is by far the most widely used and has been applied successfully to an extensive range of solute types. The reverse phases are commonly used with mobile phases consisting of acetonitrile and water, methanol and water, mixtures of both acetonitrile and methanol and water, and finally under very special circumstances tetrahydrofuran may also be added. Nevertheless, the majority of separations can be accomplished using simple binary mixtures. 

Then, the examples from Reference 23, that focus on retention of the selected binary mixtures of the test analytes (one comprising carboxylic acid and ketone and the other made of alcohol and ketone), chromatographed under the deliberately mild working conditions (microcrystalline cellulose was used as adsorbent and either decalin or n-octane as the monocomponent mobile phase) will be discussed. One of the test solutes in each binary mixture (either acid or alcohol) can be viewed as... 

FIGURE 2.18 Comparison of the concentration profiles of 2-phenylbutyric acid (dashed line) and benzophenone (thin solid line) developed as single analytes and as a binary mixture (bold solid line) concentration of 2-phenylbutyric acid in the sample was 1.25 mol 1 and that of benzophenone was 0.10 mol FI Microcrystalline cellulose was used as stationary phase and decalin as mobile phase [26]. 

Equation 4.13 and Equation 4.14 were tested for a series of mobile phases on alumina [29-31] and silica gel [32]. Two eluotropic series of solvent binary mixtures for alumina (a = 0.6) and silica gel (a = 0.7) have been calculated by using Equation 4.13, and the obtained data can be used to establish many such series or series of other selectivities [13,28],... 

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. 

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... 

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. 

The polarity values of binary acetonitrile/water and methanol/water mobile phases used in RPLC were measured and compared with methylene selectivity (acH2) for both traditional siliceous bonded phases and for a polystyrene-divinylbenzene resin reversed-phase material [82], The variation in methylene selectivity for both was found to correlate best with percent organic solvent in methanol/water mixtures, whereas the polarity value provided the best correlation in acetonitrile/water mixtures. The polymeric resin column was found to provide higher methylene selectivity than the siliceous-bonded phase at all concentrations of organic solvent. 

Variations in retention and selectivity have been studied in cyano, phenyl, and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline, and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile, or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns were correlated with polar group selectivities of mobile-phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers [84],... 

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