Weakly light paraffins

The acidity probes discussed above are the most commonly used. However, the use of many different probes has been reported in the literature. This list includes nitriles, alkanes, amines, water, di-hydrogen, deuterium, isotopically labeled molecules, benzene, etc. Probe molecules can also be used to measure basicity on zeohtes. In this case, weakly acidic molecules such as CO2, pyrrole, acetic acid and halogenated light paraffins have been used. Space does not permit discussion of these in any detail, but information about these probes and their applications can be found in the following references [87, 127-130]. 

As a result of its highly polar character, silica gel is particularly useful in the separation of polarizable materials such as the aromatic hydrocarbons and polynuclear aromatics. It is also useful in the separation of weakly polar solute mixtures such as ethers, esters and in some cases, ketones. The mobile phases that are commonly employed with silica gel are the n-paraffins and mixtures of the n-paraffins with methylene dichloride or chloroform. It should be borne in mind that chloroform is opaque to UV light at 254 nm and thus, if a fixed wavelength UV detector is being used, methylene dichloride might be a better choice. Furthermore, chloroform is considered toxic and requires special methods of waste disposal. Silica gel is strongly deactivated with water and thus, to ensure stable retentive characteristics, the solvent used for the mobile phase should either be completely dry or have a controlled amount of water present. The level of water in the solvent that will have significant effect on solute retention is extremely small. The solubility of water in n-heptane is... 

Asphalt contains many different compounds that vary not only in molecular, or particle, size but also in UV absorptivity or refractive index. Figure 17 shows the relation between detector response per unit mass and apparent molecular size for some asphalts (12). Neither detector is uniform, as a mass detector would be. The UV detector is much less uniform than the RI detector. This is mainly because paraffinic hydrocarbons, known as saturates, which comprise roughly 10-20% of a typical asphalt, are very weak absorbers of UV light, and the aromatic components in the asphalt are strong UV absorbers. Consequently, a... 

From the time of transferring the tissue in silver nitrate, all following steps were carried out In the dark with only a weak red safety light. The sections were then stained by adding crystal violet In absolute alcohol to the xylene and at the same time removing the paraffin. 

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