Selecting the solvent

For proton and carbon spectroscopy, the chemical shifts of the solvent resonances need to be anticipated as these may occur in a particularly unfortunate place and interfere with resonances of interest. In proton spectroscopy, the observed solvent resonance arises from the residual protonated species (NMR solvents are typically supplied with deuteration levels in excess of 99.5%). For routine studies, where a few milligrams of material may be available, the lower specification solvents should suffice for which the residual protonated resonance is often comparable in magnitude to that of the solute. Solvents with 

Proton shifts, SH, and carbon shifts, 8C, are quoted relative to TMS (proton shifts are those of the residual partially protonated solvent). The proton shifts of residual HDO/H2O vary depending on solution conditions. 

The solvent may also result in the loss of the resonances of exchangeable protons since these will become replaced by deuterons. In particular this is likely to occur with deuterated water and methanol. To avoid deuterium 

Solvent fin/ppm (HDo/ppni c/ppm Melting point/°C Boiling point/°C 

The nature of the solvent can also have a significant influence on the appearance of the spectrum, and substantial changes can sometimes be observed on changing solvents. Whether these changes are beneficial is usually difficult to predict, and a degree of trial 

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There are several systems which can be used to select the solvents of the mobile phase. The number of selected solvents and the solvents which are selected not only depend on the chromatographic problem but also on the method which will be used to optimize the system. With response surface methodology it is appropriate to use a minimum number of solvents. For reasons stated below this minimum number of solvents was four. The second question is, which solvents will be selected, is more difficult to answer when a small number of solvents is used because the consequences of a wrong selection are large. Several approaches are possible to select the solvents. The most simple method is comparison with common solvent systems for the solutes under investigation. A more general approach is to use the selectivity triangle of Snyder [4] in the selection of the solvents. 

In order to be successful with only four solvents it is necessary to use solvents which have all different properties. The most important properties are solvent strength and solvent selectivity. The solvent strength is the overall elution power. The solvent selectivity is the way in which different components are eluted over different distances. Two solvents with the same solvent strength and different selectivity have the same average elution distance on the plate but the solutes are eluted in a different pattern. By choosing solvents with extremely different properties, intermediate... 

This reaction showed unusual selectivity. The solvent has an important effect on the reaction. If water was switched to dimethylformamide (DMF), tetrahy-drofuran, dimethylsulfoxide, or toluene, no isomerization product was ob-... 

The first connmercially available continuous-flow synthesizers were manual instruments the PEPSYNthesiser I from Cambridge Research Biochemicals (CRB) and the Model 4175 from LKB Biochrom. In these instruments, one manual valve switches between the flow and recirculate modes and another manual valve selects the solvent, reagent or the annino acid injection port. CRB later released the semiautomatic PEPSYNthesiser II before selling the rights to the instrument to MilhGen, who ultimately released the fully automated 9050 PepSynthesizer. Pharmada-LKB Biochrom also developed an automated continuous-flow synthesizer, the Biolynx Model 4170. Several years later, the rights to this instrument were obtained by Novabiochem. Most of the automated, semiautomated, and manual continuous-flow peptide synthesizers are fisted in Table 3. 

Select the solvent(s) in which the sample can be completely dissolved. As CPC has a preparative goal, the final solvent mixture must be able to dissolve large amounts of sample, and thus it should contain at least one of the best solvents that make the sample freely soluble. 

We use rigorous simulation to determine feasible separations using water as a solvent. For a theoretical ten-stage liquid/liquid extraction process, we find that rather little water is needed to recover virtually all methanol from the pentane. At higher solvent flowrates the water-rich extract contains more and more acetone, but it cannot produce a complete separation of acetone and pentane. Thus, we select the solvent flow at which the methanol-pentane separation is sufficiently sharp. Figure 35 gives the separation selected. 

Additional criteria for selecting the solvent in liquid-liquid extraction are ... 

Based on the observed chromatographic profile, the start and end conditions for a preparative gradient run are selected. The solvent composition that corresponds to the solvent mixture where the product of interest just leaves the point of application is used as the starting composition for the gradient. The solvent mixture moving the product of interest closely to the solvent front is the final composition of the gradient. 

Preliminary runs with solvents such as dioxane and THF proved to be far less effective than a mixture of 3 1 acetic acid (AcOH) and ethanol (EtOH). Both AcOH and EtOH effectively couple with microwave irradiation and had the advantage that the starting materials were soluble under the reaction conditions at elevated temperatures, while the DHPM products 35 were comparatively insoluble at room temperature facilitating product isolation. Having selected the solvent, the next... 

Water is taken here as the reference component. It is advantageous to select the solvent S in any electrolyte as the reference component, because one then does not have to be concerned with its activity. Also, one can sometimes argue that the quantities (Xi/Xs)Fs are negligibly small, so that measured relative surface excesses can be regarded as absolute surface excesses. This assumption is not rigorous, of course, but may be sound from a practical viewpoint in many experimental situations involving dilute solutions. 

At variance with the endo/exo selectivity, the solvent effects of Table 4 do not explain the observed increase of the B-regioadducts due to the increased solvent polarity. We show in Table 5 that the inclusion of electron correlation in the evaluation of the solvent effect succeeds in reproducing the experimental trend of reaction rates, of endo/exo selectivities and of regioselectivities as a function of the increasing dielectric constant of the solvent. 

This section will provide you with the information that will allow you to select the solvent and the supporting electrolyte for a particular analysis. The nature and role of this supporting electrolyte will be explained. The effect of temperature is mentioned and together with the knowledge gained from Sections 1.2 and 1.3 you will be in a position to design a dc polarography experiment. 

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