Solvents of chromatographic interest

TABLE 9.6 Solvents of Chromatographic Interest (arranged in order of increasing solvent strength)... 

A convenient way to classify solvents of chromatographic interest in terms of their polarity and the specific chemical interactions is the empirical scheme proposed by Snyder [214,215]. This scheme is based on experimental (gas chromatographic) distribution coefficients for three test solutes ( probes ) on a large number of stationary phases, which were published by Rohrschneider [216]. The probe compounds are ethanol (e), 1,4-dioxane (d) and nitromethane (n). The experimental values for the distribution coefficients undergo several empirical modifications ... 

Table 2.8 shows the P and x values for a number of solvents of chromatographic interest. The table first shows the values for the three probe solutes. It is clear that the definitions applied for the x values do not imply that the probes show a value of unity (or 100) for one of the parameters, as was true in the scheme of Rohrschneider (see section 2.3.2). It is therefore wrong to conclude that a compound with a high value for xd closely resembles dioxane, because in that case dioxane would not resemble itself more closely than 24% ... 

Using previously reported values (7) for the area required by different adsorbed molecules, we can calculate values for various solvents per unit area of adsorbent surface area. For most solvents of chromatographic interest these values of are constant within 20% of an average value, and is related to adsorbent surface area by means of ... 

At present, the purification by chromatographic processes is the most powerful high-resolution bioseparation technique for many different products from the laboratory to the industrial scale. In this context, continuous simulated moving bed (SMB) systems are of increasing interest for the purification of pharmaceuticals or specialty chemicals (racemic mixtures, proteins, organic acids, etc.).This is particularly due to the typical advantages of SMB-systems, such as reduction of solvent consumption, increase in productivity and purity obtained as well as in investment costs in comparison to conventional batch elution chromatography [1]. 

UV spectra of aromatic hydrocarbons are characterized by three sets of bands (El, E2, and B bands) that originate from ti ti transitions. Generally the E2 and B bands are of most interest to chromatographers, since the solvent cutoff for most mobile phases is <200 nm. 

There are two procedures for using reference standards in the first method a weighed amount of the standard can be added directly to the sample and the area or heights of the peaks of interest compared with that of the standard. This procedure is called the internal standard method. In the second procedure a weighed amount of the standard can be made up in a known volume of solvent and chromatographed under exactly the same conditions as the unknown sample, but as a... 

As can be seen from Fig. 6.9, dynamic pressurized hot solvent extraction (DPHSE) has evolved similarly to ASE however, as noted earlier, DPHSE has been the subject of many fewer reports, primarily as a result of the lack of commercially available equipment for implementation. In any case, the relatively scant reported applications of DPHSE are of especial interest as regards automation of the analytical process in fact, the dynamic nature of the system facilitates its coupling to other dynamic systems with a view to accomplishing preconcentration [39,42,45,145], filtration [42,45], chromatographic separation [145,146], derivatization [46,57] and detection [44,147], among others, and the partial or total automation of the analytical process. 

Supercritical fluid extraction (SFE), usually with carbon dioxide and, often, with a modifier, has become of increasing interest in the last few years because of its selectivity, preconcentration effect, efficiency, simplicity, rapidity, cleanness, and safety, mainly concerning the extraction of organic compounds prior to separation and detection by chromatographic techniques. It has several advantages over classical solvent extractions, in comparison with recent extraction techniques. Approaches to obtain quantitative extractions, including fluid choice, extraction flow rate, modifiers, pressure, and temperature, are presented, as well as the potential for SFE to extract polynuclear aromatic hydrocarbons (PAHs) from soils, sediments, and biota. Improvements and new environmental applications are also reported. 

The product is often purified further after the recovery phase, particularly when the desired product is a protein of pharmaceutical interest. Precipitation is a widely used method that can be induced by the addition of salts, organic solvents, or heats. It can both purify and concentrate a particular protein fraction, and is frequently accomplished by the addition of salts for a salting out effect. The addition of a salt precipitates proteins because increasing salt concentrations reduce the solubility of a protein in a solution. Even though precipitation is an effective and relatively inexpensive method, it is also a fairly crude step and is often followed by chromatographic separations. ... 

It is clear that as the internal diameter of the column increases, so does the mobile-phase volume, and then the volume of mobile phase that is necessary for any given chromatographic analysis. This affects the cost of the analysis, both directly in increased usage of chromatographic solvents, and also through increased solvent disposal cost. This cost factor is one of the primary reasons driving the interest in small-diameter columns, which will be discussed in more detail shortly. 

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