Acetonitrile-water binary mixture solvents

Retention is reduced by increasing the solvent concentration, conversely, retention is increased by increasing the proportion of water. Binary mixtures of water and acetonitrile or water and methanol are, unfortunately, not simple binary mixtures because, as is well known, they associate strongly with one another. Thus, a nominally binary mixture of methanol and water is, in fact, a ternary mixture of water, methanol and water associated with methanol. It follows, that some discussion on aqueous solvent mixtures would be pertinent. 

Katz, Lochmuller and Scott also examined acetonitrile/water, and tetrahydrofuran (THF)/water mixtures in the same way and showed that there was significant association between the water and both solvents but not nearly to the same extent as methanol/water. At the point of maximum association for methanol, the solvent mixture contained nearly 60% of the methanol/water associate. In contrast the maximum amount of THF associate that was formed amounted to only about 17%, and for acetonitrile the maximum amount of associate that was formed was as little as 8%. It follows that acetonitrile/water mixtures would be expected to behave more nearly as binary mixtures than methanol/water or THF/water mixtures. 

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. 

Reversed-phase chromatography employs a nonpolar stationary phase and a polar aqueous-organic mobile phase. The stationary phase may be a nonpolar ligand, such as an alkyl hydrocarbon, bonded to a support matrix such as microparticulate silica, or it may be a microparticulate polymeric resin such as cross-linked polystyrene-divinylbenzene. The mobile phase is typically a binary mixture of a weak solvent, such as water or an aqueous buffer, and a strong solvent such as acetonitrile or a short-chain alcohol. Retention is modulated by changing the relative proportion of the weak and strong solvents. Additives may be incorporated into the mobile phase to modulate chromatographic selectivity, to suppress undesirable interactions of the analyte with the matrix, or to promote analyte solubility or stability. 

Solvatochromism and piezochromism of a range of pentacyanoferrates(II) have been examined in binary aq ueous solvent mixtures, " and their solvatochromism in micelles and reversed micelles. The solvatochromism of [Fe(CN)5(nicotinamide)] has been established in several ranges of water-rich binary solvent mixtures, " of [Fe (CN)5(2,6-dimethylpyrazine)] in acetonitrile-water mixtures.The solvatochromism of [Fe(CN)5(4Phpy)] and [Fe(CN)5(4Bu py)] has been proposed as an indicator of selective solvation in binary aqueous solvent mixtures. ... 

Usually, when an HPLC method is developed, an acceptable degree of separation for all the components of interest in our sample is required in a reasonable time. The mobile phases more frequently used are the classical mixtures of methanol-water and acetonitrile-water in different proportions. If a satisfactory separation cannot be achieved using a binary solvent mixture as mobile phase, a ternary composition may be used. 

Polar group selectivity also occurs in ternary solvent systems (5.10). For example, the addition of 5% to 25% of a third solvent to a water-acetonitrile mixture can alter the relative retention of peaks, and often resolve overlapping peaks. Dolan et al (11) have employed ternary mobile phases of water, methanol and tetrahydrofuran to analyze vitamin tablets where interfering peaks could not be resolved with binary mixtures. See Figure 4. 

If we limit ourselves to iso-eluotropic mixtures, a one-parameter optimization problem remains. As was described in section 3.2.2, a binary mixture of 60% methanol and 40% water corresponds approximately to 48% acetonitrile in water or 37% THF in water. We may proceed with the optimization procedure by considering these binary mixtures as pure solvents (e.g. solvent A equals 60/40 methanol/water) and refer to them as pseudosolvents [537] or pseudocomponents [538]. 

When developing a separation, the first tactic is to search for the best binary solvent combination. But sometimes the first choice for the strong solvent does not work out as well as it did in the problems addressed in Figures 5-1 and 5-2. In certain situations blending two solvents may not result in a mobile phase in which a particular separation is possible. It is, therefore, necessary to try a new binary mixture. An example of this is the separation of the steroids shown in Table 5-1, where both water/methanol and water/ acetonitrile combinations are used. Examining Table 5-1 shows that the hydrocortisone acetate and dexamethasone are particularly troublesome. 

More quantitative information about solvation equilibria resulted when the association of picric acid was studied in mixtures of acetonitrile (AN) with several hydroxylic solvents (water, MeOH, EtOH) and in water-EtOH mixtures. The dependence of In Ka on 1/e is in no case linear and with two binary solvent systems (AN—MeOH, AN—EtOH) In Ka even increases with e. Fuoss et al. introduced a... 

Cell (D) has been used several times to determine the solvent transference number A of the sparingly soluble salt Ag2S04 in the binary solvent mixtures acetonitrile-water l, dimethylsulphoxide-water and dimethylsulphoxide-methanol In Fig. 3 the solvent transference number of Ag2S04 is plotted versus Xdmso =... 

When in a mixture of two solvents, both ions of a binary salt are solvated preferably by the same solvent, the term applied is homoselective solvation (Fig. 2-lOa). Similarly, the preferred solvation of the cation by one, and the anion by the other solvent, is termed heteroselective solvation (Fig. 2-lOb) [119], Thus, in a solution of silver nitrate in the binary solvent mixture acetonitrile/water, a preferential solvation of Ag by acetonitrile and of NO by water was observed (heteroselective solvation) [121, 369] . In contrast, in solutions of calcium chloride in water/methanol mixtures, both Ca and Cl are solvated largely by water (homoselective solvation) [122], Zn (from ZnCl2) in... 

As in normal phase (see section 3.5.3), the first step in mobile-phase optimization is the determination of the solvent strength that will elute the analytes with a A value between 2 and 10 from the chosen stationary phase. It is not important which modifier is chosen to determine the initial conditions, and methanol-water (50 50, v/v) is a convenient starting place. Once the initial conditions have been established, a variety of techniques may be employed to obtain the optimum separation. Most optimization strategies involve the establishment of the isoelutropic concentrations of methanol-water, acetonitrile-water and tetrahydrofuran-water. The isoelutropic concentrations can be determined by experiment or from tables of isoelutropic mixtures (e.g. Table 3.5) (Wells, 1988). The binary solvent systems A, B, C (Table 3.5, Figure 3.7) define the isoelutropic plane, which is then explored to obtain the optimum combination of water, methanol, tetrahydrofuran, and acetonitrile required for the separation. 

The mobile phase used in the lipophilicity measurements by RP-TLC is usually a binary mixture between an organic solvent and water. The organic solvent can be methanol, acetonitrile, or acetone. The first two can also be used in reversed-phase high-performance liquid chromatography (RP-HPLC) measurements, but... 

Sections 9.4 and 10.3 have already provided the basis for optimization by attempting to work with three different solvent mixtures hexane-ether, hexane-dichloromethane and hexane-ethyl acatate for adsorption chromatography and water-methanol, water-acetonitrile, water-tetrahydrofuran for reversed-phase systems. However, this concept is not restricted to binary mixtures but a third or even a fourth component may be added in an attempt to improve the separation. An arrangement of seven different mixtures (Figure 18.11) provides the best basis for systematic evaluation. An example is outlined below. 

As part of their research in optimizing chromatographic methods, M.C. Breitkreitz and co-workers employed a mixture design to search for the MP composition that would give the best selectivity for a 10-component mixture separation (Breitkreitz et al., 2005). The organic solvents acetonitrile (ACN), methyl alcohol (MeOH) and tetrahydrofuran (THF) were used as water modifiers. Initially, the solvent strength of ACN in water was varied to produce a desirable separation power for all the solutes. Once this composition had been experimentally determined, the water contents of the other two solvents were predicted based on empirical relationships. In this manner, the pure components used to specify the design were, in fact, binary mixtures ACN H20, MeOH H20 and THF H20. 

The same linear relationship between mobile phase composition and retention was noted by Katz et al. [7] for binary mixtures in LC. However, in LC the situation is more complicated, and if strong association occurs between the mobile phase components, the relationship becomes nonlinear. Unfortunately, the technique of using the two columns procedure of Purnell is impractical for mixed mobile phases in LC. Furthermore, as the solvents most commonly used are water, methanol, acetonitrile and tetrahydrofuran, and all form strong associates, the use of the linear relationship demonstrated by Katz et al. is severely limited. An example of the linear relationship between volume fraction of one component of two binary mixtures and retention is shown in figure 3.8. The linear relationship is clearly demonstrated and it is seen that the distribution coefficient (which controls retention) can be adjusted to any selected value by choosing the appropriate mixture of the two solvents. 

---