Organic modifiers high concentration

Generally, Ultrahydrogel columns are compatible with aqueous mobile phases from pH 2 to pH 12. The addition of organic modifiers such as acetonitrile is recommended up to 50%, but most common applications do not require such a high concentration of organic modifier. These columns have been tested in applications from 10 to 80°C using aqueous mobile phases. 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

One way in which cobalt dispersion can be increased is the addition of an organic compound to the cobalt nitrate prior to calcination. Previous work in this area is summarized in Table 1.1. The data are complex, but there are a number of factors that affect the nature of the catalyst prepared. One of these is the cobalt loading. Preparation of catalysts containing low levels of cobalt tends to lead to high concentrations of cobalt-support compounds. For example, Mochizuki et al. [37] used x-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) to identify cobalt silicate-like species in their 5% Co/Si02 catalysts modified with nitrilotriacetic acid (NTA). The nature of the support also has... 

The main difficulties in CE analysis of cationic surfactants arise from their strong adsorption to the capillary wall and their ability to form micelles at low concentrations. The addition of organic modifiers in high amounts or separation in absolutely non-aqueous media disrupt micelle formation within the sample and also effectiveness of the organic modifier to disrupt micelles of alkylbenzyl dimethyl ammonium... 

Comparison of the retention behaviour in 50% aqueous DMF and acetonitrile (Figure 4.3D) indicated that DMF is a selective modifier for the separation of alkanols ( , 1-6) and PAHs (11-16) and of aromatic and aliphatic compounds. Unfortunately, DMF is too aggressive as a solvent and attacks organic polymer piston seals in the pump and, therefore, highly concentrated solutions cannot be used as an eluent. 

Figure 4.7 Effect of high organic modifier concentration on peptide retention. Data obtained using an octadecyl column and 20 mM ammonium acetate acetonitrile mobile phases. (Reproduced from C.T. Wehr and L. Correia, LC at Work LC-121, Varian. With permission from Varian Associates.)...

The amount of modifier required to prevent third phase formation can be determined in the following way. The aqueous and solvent phases are first contacted, and once the three phases have separated, the lower aqueous phase is drawn off and discarded. The modifier to be considered is then added from a burette in small increments to the two organic phases, and the mixture shaken after each addition. The amount of modifier required to produce a single organic phase is then used to calculate the amount required to be added to the solvent. Generally, about 2-5 vol% of modifier is needed, but more may be necessary if high concentrations of extractant are used in the solvent. Any effects of modifiers on the kinetics and equilibria of metal extraction and stripping can be determined by shakeout tests. 

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