Impurities enhancement

One of the purification procedures involves air-oxidation followed by washing with HCl to remove metals. Metal nanoparticles that are covered with carbon coating are difficult to dissolve in an acid treatment. In order to break-up the carbon coating, air-oxidation is usually done at an optimal temperature for a specific duration. The presence of metal impurities enhances the oxidization of CNTs at higher temperatures and, hence, the oxidation is done by a step-by-step manner. In this procedure, the sample is heated first in air at a moderate temperature (Ti 225-300°C) for a certain duration (tj)... 

Some substances, including impurities, enhance the decomposition of azides. The impurities can be present in the course of preparation of azides or formed during their storage. It is known that the presence of carbon dioxide in air may produce a decomposition of lead azide. Also water vapour in air even at rcoin temperature may accelerate (he decompr>sition. fhis problent was tackled by. Reitzner (115]. He found that the induction period was the result of the re-laction of water vapour with lead. 

Saturated solutions of two salt forms of a development candidate, hydrochloride salt and citrate salt, were created in acetonitrile. The hydrochloride salt (Figure 6) was readily soluble (>10mg/mL) in acetonitrile, whereas the citrate salt (Figure 7) was only slightly soluble (0.04mg/mL). The impurity enhancement factors were markedly different, 5x for the hydrochloride salt and 1000+ for the citrate salt. Even an increase in the impurity-to-drug ratio of 5x can make a significant difference in terms of... 

FIGURE 7 Citrate salt impurity enhancement example. 

Impurity enhancement techniques such as fraction collection and phase equilibrium purification can be used to provide enriched samples for use in the method development process.23 When using the fraction collection approach, one or more cuts (fractions) of the chromatographic separation of a bulk lot or mother liquor are isolated. The excess solvent in these fractions is then evaporated to achieve the desired concentration enhancement. These fractions typically contain extraneous peaks because of the presence of salts in the mobile phase or sample degradation during the concentration step. The salts can be removed by extraction and/or a LC cleanup step. To insure that these extraneous peaks/artifacts are not identified as key peaks for separation, the original bulk lot or mother liquor should be included in the method development sample set. The same holds true for phase-equilibrium-purification supernatants. 

It is necessary to note, that except of possible strengthening of intergranular strength due to segregation of an "useful" impurity enhancing a... 

Silicates—These chemically inert synthetic amorphous silica adsorbents have an affinity for polar contaminants. The surface area, porosity, and moisture content of the silica adsorbents provide them the capability of adsorbing secondary oxidation products (aldehydes, ketones), phosphatidic compounds, sulfur compounds, trace metals, and soap. Moisture functions to hold the pores open and aid in the attraction of the polar contaminants. Most of the synthetic silicas do not have significant direct adsorption capabilities for carotenoid or chlorophyll compounds, but the removal of the other impurities enhances the efficiency of the bleaching earths (Young, 1990). 

The regulations say that impurities are not manufactured or processed for distribution in commerce as chemical substances per se. i This means that the impurities are essentially hitchhikers that travel along with the chemical substance that is intentionally present. This definition leaves open substantial questions about what is intentionally present. For example, if an impurified product functions for its purpose, but the purified product does not, then the impurities are necessary for the success of the product and can not be considered unintentionally present. The EPA has taken the position in a Q A document that if an impurity enhances the product or is manufactured itself for a commercial purpose, it would no longer be viewed as an impurity. Similarly, if a company learns that something it had thought was an impurity in fact has some coimnerdal value, then that substance must be put on the Inventory. ... 

I. Tanaka, G. Pezzotti, K. Matsushita, Y. Miyamoto and T. Okamoto, Impurity Enhanced Cavity Formation in Si3N4 at Elevated Temperatures , J. Am. Ceram. Soc., 74, 1992, 752-59. 

It has been reported that the thermal degradation of PLA predominantly consists of random main-chain scission and unzipping depolymerization reactions. The random degradation reaction involves hydrolysis, oxidative degradation, c/s-elimination, and intramolecular and intermolecular transesterification. Almost all the active chain-end groups, residual catalysts, residual monomers, and other impurities enhance the thermal degradation of PLA. As a consequence... 

Abstract The corrosion of metals (copper, zinc, and plain carbon steel) in dialkylimidazolium tetrafluoroborates depends on the presence of traces of impurities in the ionic liquids. Water, chlorides, and especially simultaneous presence of the both impurities enhances the pitting corrosion of metals. The products of corrosion contain the metal of interest as well as nitrogen, boron, chlorine, fluorine, carbon, and oxygen. 

Phosphoric acid-doped (PA) poly-benzimidazole (PBI) films attain proton conductivities >0.25 S cm at temperatures above 150°C (Xiao et al 2005). The shift to higher temperatures of fuel cell operation, enabled by the use of these membranes, improves the fuel tolerance to carbon monoxide impurities, enhances the electrode kinetics, and alleviates humidification requirements. The problem is that the higher temperature not only accelerates the kinetics of desired electrode reactions, but also those of degradation processes. Moreover, PBI membranes function poorly under ambient conditions. PEFCs using PBI PEMs are suitable for residential applications, but they are currently not envisaged for automotive applications. 

Terasaki, L, 1. Tsukada, and Y. Iguchi, Imprrrity-induced transition and impurity-enhanced thermopower in the thermoelectric oxide NaCo2 xCuxOd. Physical Review B, 2002. 65(19) p. 195106. 

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