Acetonitrile water compositions/mixtures

Elution was performed using a concentration gradient of a methanol-acetonitrile-water ternary mixture. The initial proportions of the components at the beginning of the run were 40 40.5 18.5. The concentration of acetonitrile was then decreased linearly so that it reached 0% at 25 min while its concentration in the mobile phase was replaced with methanol at the same gradient rate. Elution was completed with a linear gradient of the methanol-water mixture so that the mobile phase usually contained 90% of methanol at 60-70 min and was 100% methanol at 90 min. The elution of phenacyl esters of 6 0-22 1 fatty acids was completed within 80 min at a flow rate of 1 ml/min (the detailed composition of the mobile phase is described in Table 1, elution mode E) (Fig. 4). 

MPc complexes have been employed as catalytic components in microemulsions and as composite films for the analysis of phenols and organohalides. Rusling and coworkers reported on FePc/DDAB, CuPc/DDAB, CuTSPc/DDAB, NiPc/DDAB, NiTSPc-DDAB, ZnPc/DDAB or ZnPc/CTAB surfactant films , and Jiang et al. on CoTSPc/DDAB for use in catalyzed reduction of trichloroacetic acid (TCA) and other organohalides. The catalytic reduction of TCA was more efficient in the acetonitrile/water solvent mixture than in the microemul-sions . Table 7.1 shows that lower potentials were observed for catalysis of TCA on surfactant films containing NiPc complexes than for the corresponding CuPc species. 

Selectivity for the methylene group has been mostly the focus of interest and was determined by a host of investigators. Data obtained by Karger et al. (148) we shown in Fig. 26. It is seen that the selectivity is a linear ftinction of the solvent composition, when water-methanol mixtures are used as the mobile phase, and decreases with the water content of the eluent. The linearity characteristic of methanol mixtures is absent in the data for water-acetonitrile and water-acetone mixtures but again... 

Figure 7.5 The acetonitrile-[C4CiIm][PF ]-water ternary mass phase diagram at room temperature. Closed symbols compositions belonging to the 50 50 w/w [C4CjIm][PF6]-water initial mixtures (dotted line) that separate in two phases whose compositions are listed in Table 7.2 and located by the corresponding open symbols and tie-lines. (Adapted from Berthod, A. and Carda-Broch, S., /. Liq. Chromatogr. Rel. TechnoL, 26,1493-1508, 2003.)...

The purpose of this study is to examine the structural features of acetonitrile-water mixtures over the whole composition range using the heats of solution and dilution of lithium perchlorate as a probe. The effect of water on thermodynamic properties such as heats of solution is also of interest. 

The authors have evaluated different C18 and C8 columns, studying the variation in capacity factors with the proportion of water in the mobile phase for all six acid derivatives. The detector was a fluorescence detector equipped with a 360-nm excitation and a 420-nm emission filter. They were unable to resolve derivatives of the six acids on three different C18, 5-/zm columns, despite using a wide range of solvent compositions. Table 3 shows capacity factor values for three critical acid derivatives on a number of columns. When a C8,5-/un column was used with an isocratic solvent system [acetonitrile/water (98 2)], they observed a marked improvement in the resolution of the six derivatives. However, it was not possible to achieve complete resolution of the six compounds under isocratic conditions, despite numerous experiments with a range of organic solvent mixtures consisting of acetonitrile, methanol, tetrahydrofuran, and water. 

Modification of the isocratic solvent composition shown in Fig. 8 by increasing the percentage of water did not improve matter, since this caused the C20 4 acid derivative to be eluted with the C14 0 derivative. This led to an investigation of the variation in capacity factors of all six derivatives with different acetonitrile/water 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. 

These very simple relationships can be verified experimentally as is shown in figure 3.16. The iso-eluotropic compositions of binary mixtures of THF and acetonitrile with water have been plotted against the binary methanol-water composition. The thin straight lines indicate the theoretical relationships from solubility parameter theory (eqns. 3.50 and 3.51). The thick lines correspond to average experimental data over large numbers of solutes [335]. An (average) experimental data point can be found as follows. 

In RPLC retention varies exponentially with the composition of the mobile phase, i.e. approximately straight lines are obtained in a plot of In k vs.

retention behaviour of each individual solute, then the optimum conditions (LSS gradient, see section 5.4) correspond to a linear gradient (figure 6.2b). Linear gradients will indeed be optimal when acetonitrile-water mixtures are used as the mobile... 

Next, the concept of iso-eluotropic mobile phases is used to determine the binary acetonitrile-water and THF-water mixtures that correspond to the initial and the Final composition. For example, 20% methanol corresponds [627] to 17% acetonitrile and to 12% THF, whereas 100% methanol corresponds to 84% acetonitrile and to 59% THF. 

In a simplihed form, it is generally accepted that the logarithm of the retention factor shows linear variation with the volume fraction of the eluent composition [45] similar to expression (4-5) above. This statement has to be taken only as a first and very rough approximation, since many deviations from this rule have been reported [46, 47] especially for acetonitrile/water mixtures as shown for n-hexanol and n-octanol in Figure 4-8 [47] and phenol and toluene in Figure 4-9. 

The 5 is a constant for each mobile-phase composition and type of buffer system employed. The 5 term determined for various acetonitrile/water and methanol/water mixtures ranging from 10 to 60 v/v% by Roses, Espinosa, and co-workers [73,74] are shown in Table 4-4 and Table 4-5, respectively. 

NH4TNH3 ( pH 9) and BUNH3VBUNH2 (wpH 10) show a decrease in their pK values with increasing organic content [74]. These basic modifiers have an average pH decrease on the order of -0.05 to -0.1 pH units per 10 v/v% acetonitrile. The minimum of the pH values as a function of acetonitrile composition for basic modifiers is reached at approximately 30-50 v/v% MeCN. Upon further increase in MeCN concentration the pH of the basic modifier will increase. For example, ammonium/ammonia basic modifier pH values in acetonitrile/water mixtures are 0% MeCN 9.29, 10% MeCN 9.27, 20% MeCN 9.21, 30% MeCN 9.17, 40% MeCN 9.19, 50% MeCN 9.21, 60% MeCN 9.34 [64]. For BUNH37BUNH2 (wpH 10), basic modifier pH values in acetonitrile/water mixtures are 0% MeCN 10.00, 20% MeCN 9.78, 40% MeCN 9.65,60% MeCN 9.79 [64]. For basic modifiers a decrease in pH is also observed with increase of methanol content on the order of 0.1 pH units per 10 v/v% methanol. 

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