Mobile phase solvent selectivity effects

With the demonstration of supercritical fluid extraction, an obvious extension would be to extract or dissolve the compounds of interest into the supercritical fluid before analysis with SFC.(6) This would be analogous to the case in HPLC, where the mobile phase solvent is commonly used for dissolving the sample. The work described here will employ a system capable of extracting materials with a supercritical fluid and introducing a known volume of this extract onto the column for analysis via SFC. Detection of the separated materials will be by on-line UV spectroscopy and infrared spectrometry. The optimized SFE/SFC system has been used to study selected nonvolatile coal-derived products. The work reported here involved the aliphatic and aromatic hydrocarbon fractions from this residuum material. Residua at several times were taken from the reactor and examined which provided some insight into the effects of catalyst decay on the products produced in a pilot plant operation. 

Although there is evidence that complexation with silver ions is the governing interaction in Ag-TLC, other factors should also be considered. Thus sihca gel, which is the most widely used supporting material, possesses appreciable polarity and adsorption activity. Therefore, in many cases, an impact of mixed retention mechanism on migration, geometry of spots, and selectivity of resolution is to be expected. Also, the mobile-phase solvents are active elements of the chromatographic system and interactions both with the supporting material and FA is possible this may also have a serious effect on the whole separation process. 

Analytical screens are performed with both reverse-phase RP-HPLC and SFC isolation techniques. Analytical SFC should be screened first unless instrumentation availability or project background specifics dictate otherwise. Screening achiral column bonded phases varying in polarity and functionality against different mobile-phase solvent choices is effective for identifying analytical methods for the purpose of impurity isolation. There are currently many unique achiral SFC bonded phase column choices commercially available (2-ethyl pyridine, diethyl amino, dinitrophenyl, pyridine urea, diol, cyano, etc.). SFC column choice provides the most impact in manipulation of relative selectivity for individual... 

This is an oversimplified treatment of the concentration effect that can occur on a thin layer plate when using mixed solvents. Nevertheless, despite the complex nature of the surface that is considered, the treatment is sufficiently representative to disclose that a concentration effect does, indeed, take place. The concentration effect arises from the frontal analysis of the mobile phase which not only provides unique and complex modes of solute interaction and, thus, enhanced selectivity, but also causes the solutes to be concentrated as they pass along the TLC plate. This concentration process will oppose the dilution that results from band dispersion and thus, provides greater sensitivity to the spots close to the solvent front. This concealed concentration process, often not recognized, is another property of TLC development that helps make it so practical and generally useful and often provides unexpected sensitivities. 

Another advantage of HdC is its generosity in terms of mobile-phase selection. The polymer size and solution properties of a polymer can be studied using HdC, especially OTHdC, in almost any solvent. In SEC, by comparison, the packing material and mobile phase have to be selected to prevent the nonsize exclusion effect. Because the instrumentation of HdC is similar to SEC, and the packing material and columns have become available commercially, this technique will gain in popularity. 

Naphthalenedisulfonate-acetonitrile as the only mobile phase with a silica column coated with a crosslinked aminofluorocarbon polymer has proven to be an effective combination for the separation of aliphatic anionic surfactants. Indirect conductivity and photometric detection modes are used to monitor these analytes. The retention of these surfactants is found to depend on both the ionic strength and the organic solvent content of the mobile phase. The mechanism of retention is considered to be a combination of both reverse phase and ion exchange processes. Selective separation of both alkanesulfonates and... 

Bota et al. [84] used the PCA method to select the optimum solvent system for TLC separation of seven polycyclic aromatic hydrocarbons. Each solute is treated as a point in a space defined by its retention coordinates along the different solvent composition axes. The PCA method enables the selection of a restricted set of nine available mobile phase systems, and it is a useful graphical tool because scatterplots of loading on planes described by the most important axes will have the effect of separating solvent systems from one other most efficiently. 

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