Organic phase composition factors

A key factor in doing a successful suspension polymerization is the composition of the aqueous phase or stabilizer. Too much stabilizer results in emulsion polymerization, which produces small particles (less than 1 /cm). Too little stabilizer results in bulk polymerization. For the production of GPC gels, the ratio of aqueous phase to organic phase should be about 2 1. 

Because a chemical step is imposed on top of the physical distribution process of partition, there is a great potential for selectivity, as noted by Schill et al, (49>50), Such factors as pH, type and composition of the organic phase, and ionic strength of the aqueous phase can be used to control relative retention. The concentration and type of counterion mainly control the absolute retention. 

A very minor effect on shape selectivity has been observed for changes in mobile phase composition [109]. Shape selectivity increased slightly with an increase in percent organic modifier for water-organic mobile phase systems and increased in order of methanol acetonitrile < ethanol. Changes in the shape selectivity factor... 

In summary, it can be stated that the stationary phase and the mobile phase (buffer) pH are the most important factors determining the generic selectivity of a CS. The organic modifier composition and the column temperature can influence the selectivity locally, i.e., when separating a specific mixture of rather similar compounds, e.g., a drug impurity profile. 

An unknown mixture can be screened on a set of orthogonal systems as a first step in the method development procedure. The chromatographic and/or electrophoretic system, on which the best separation was achieved, can then be retained for further method optimization. Sequentially, the pH and the organic modifier composition of the mobile phase can be adjusted to improve the separation on the CS. If necessary, also the temperature can be modified, while for gradient methods the gradient slope can be considered. For CE methods, the optimization steps will be different from RP chromatography methods. Other factors will be optimized depending on the type of CE method, e.g., CZE and MEKC. However, for the development of CE methods, we would like to refer to Chapter 4 of this book. 

The first results of optimization in chromatography were published in 1975 Since then a growing number of optimization experiments in HPLC using the Simplex procedure has been reported (table 9). The examples are mainly reversed-phase separations, in which the composition of the ternary or binary mobile phase composition is optimized. The factors optimized are usually a selection from flow rate, column temperature and length, the eluents constitution (e.g. organic modifier content, buffer concentration and pH), the gradient shape. Seven years after the first applications of Simplex optimization had appeared, the first fully automated optimization of HPLC separations was published by Berridge in 1982. This development coincid-... 

Among chemometrical approaches, the Simplex algorithm [82] involving an evolutionary movement in the factor space to improve the response of interest was applied to analyze antibiotics in pharmaceutical formulations. The selected control variables included the concentrations of the organic modifier and the IPR in the eluent. The response variables were the peak area, resolution, asymmetry factor, and total number of chromatographic peaks. The mobile phase composition that gave optimum results was adopted [83]. A Simplex algorithm was successfully used to maximize the resolution of 14 cosmetic preservatives [84]. 

Figure 10 Optimization of isocratic elution. Two chromatograms obtained for 19 and 14% (v/v) of organic solvent modifier in the mobile phase (corresponding to = 0.19 and 0.14, respectively) can be used to plot the resulting retention factors versus the volume fraction of the organic solvent modifier in order to identify the mobile phase composition with optimal peak spacing.

It must be stressed that factors such as the hydration (or solvation) of the metal ion and anion effects on the extracted complex often make it difficult to predict the order of extractability for such systems. Such factors may even influence the stoichiometry of the extracted species. Thus, the simple match of the metal to the whole concept is only of limited utility. For example, potassium, rubidium and sodium nitrates are extracted in the presence of dibenzo-18-crown-6 (2) as 1 1 1 complexes. On the other hand, cesium forms a 1 2 1 sandwich complex with this crown (metal crown nitrate) in the organic phase and this affects the extraction order for the above metal ions, with the order being dependent on ligand concentration. In contrast, for picrate as the anion the composition of the extracted cesium complex is 1 1 1 (Fig. 4.8) [27]. 

The variation of wastewater quality is related to the human activities (and thus time dependent as shown before) but also to some physico-chemical factors occurring along the sewer. Fresh organic matter composition can vary from the source (houses, for example) to the treatment plant because of physico-chemical phenomena (solids settling, phase transfer, oxidation, etc.). 

From the raw retention values, retention factors can be calculated, which are dimensionless numbers correcting for variations in instrumental instabilities such as variation in flowrate. The retention factor is defined as the ratio of the net retention time (tT - tu) and the retention time of an unretained peak (tu), where tr is the retention time of the solute. This is standard practice in liquid chromatography [Christian O Reilly 1986]. Moreover, taking the logarithms of these retention factors ideally linearizes the data, e.g., with respect to fractions of organic modifier in the mobile phase composition [Schoenmakers 1986],... 

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