Water solubility analytic approach

The following physico-chemical properties of the analyte(s) are important in method development considerations vapor pressure, ultraviolet (UV) absorption spectrum, solubility in water and in solvents, dissociation constant(s), n-octanol/water partition coefficient, stability vs hydrolysis and possible thermal, photo- or chemical degradation. These valuable data enable the analytical chemist to develop the most promising analytical approach, drawing from the literature and from his or her experience with related analytical problems, as exemplified below. Gas chromatography (GC) methods, for example, require a measurable vapor pressure and a certain thermal stability as the analytes move as vaporized molecules within the mobile phase. On the other hand, compounds that have a high vapor pressure will require careful extract concentration by evaporation of volatile solvents. 

Samples were analyzed by gas chromatography (GC). Water solubility was determined by equilibration of analytical grade material with water at constant temperature. Equilibrium was approached from both under and super saturation conditions and samples were analyzed by GC. Vapor pressures were determined by the Knudsen effusion method. 

The SP procedure of water-soluble vitamins from multivitamin tablets is particularly challenging due to the diverse analytes of varied hydrophobicities and pfC. Water-soluble vitamins (WSVs) include ascorbic acid (vitamin C), niacin, niacinamide, pyridoxine (vitamin B ), thiamine (vitamin Bj), folic acid, riboflavin (vitamin B2) and others. While most WSVs are highly water soluble, riboflavin is quite hydrophobic and insoluble in water. Folic acid is acidic while pyridoxine and thiamine are basic. In addition, ascorbic acid is light sensitive and easily oxidized. The extraction strategy employed was a two-step approach using mixed solvents of different polarity and acidity as follows ... 

The Food and Drug Administration [FDA] Pesticide Analytical Methods Manual QJ and the Environmental Protection Agency [EPA] Manual for Environmental Analysis (2) describe procedures that have been used for many years. Two of the commonly applied techniques are liquid-liquid partitioning and column adsorption chromatography. These approaches are used to isolate lipohilic and moderately polar residues for primary identification and quantitation with GLC. An evaluation of the number of pesticide residues that were satisfactorily analyzed by this approach was published by McMahon and Burke (3). When one looks at the data it can be seen that the highly polar and water soluble residues do not fit into the analytical scheme very well. In an attempt to rectify this problem, FDA is modifying the multiresidue method... 

The direct determination of some major elements (Ca, K, Mg, Na, and P) and Zn by ICP-AES was performed in powdered milk [14]. Samples were diluted with a 5 or 10 percent (v/v) water-soluble, mixed tertiary amine reagent at pH 8. This reagent mixture dissociated casein micelles and stabilized liquid phase cations. No decrease in analyte emission intensities was observed. Reference solutions were prepared in 10 percent (v/v) mixed amine solution, and no internal reference element was needed for ICP-AES. The direct technique is as fast as slurry approaches, without particle size effects or sensitivity losses. 

Several pollution control technologies at the refinery promote cross-media transport, the transfer of pollutants from one medium to another. Wastewater, for example, may contain hydrocarbons that volatilize into the air at the refinery, the wastewater treatment plant converted these waterborne hydrocarbons into 2400 tons/year of sludge, which were recycled to the coker. Cross-media transport from air to water is not significant for hydrocarbons or chemical that are only slightly soluble in water (Allen et al, 1989). Studies performed by the National Center for Intermedia Transport at the University of California, Los Angeles, for instance, showed that most hydrocarbons released into the air do not transfer rapidly into other media. Therefore, ignoring intermedia transfer when examining air quality impacts is a reasonable analytical approach. Water-soluble compounds, such as methanol and MTBE, can transfer from air into water and soil media under certain conditions (Cohen et al., 1991). 

The solubility-parameter approach provides a useful framework for the choice of the mode of liquid chromatography that is best suited for a given analyte. However, solubility parameters of solutes are often unavailable and the solubility of an analyte in water, methanol and hexane provides a more pragmatic basis for the choice of chromatographic system. Figure 3.4 shows that low molecular weight analytes (RMM < 1000) may be divided into those that are water soluble and... 

The analytical approach to follow is a modification of the United States Pharmacopoeia method. The technique involves titrating a solution of aspirin with standard lead perchlorate solution in the presence of a Pb-ISE and reference electrode couple. Because of the PbS04 formed has a shght but significant solubility in water, the titration is carried out a solution of 37 mL acetone and 3 mL water. This solution lowers the solubility of PbS04 critically. The following solutions are required ... 

Analytical Significance of Metabolites. The classification of metabolites as major or minor is a convenient approach from an analytical standpoint but may have little relation to toxicological significance. Metabolites can also be classified as organosoluble, water soluble, nonextractable, and releasable by acid, base or enzyme action. Most studies are done on organo-soluble metabolites because suitable analytic techniques are available. Development of analytic methods for water soluble and nonextractable "metabolites" may demonstrate that many are simply endogenous biochemical compounds into which the radioactive label has been incorporated. The limits for identifying minor metabolites should be consistent with the state of the art in... 

As detailed in Chapter 3 by Terabe, micellar electrokinetic chromatography (MEKC) is a useful technique in the retention analysis of water-soluble compounds. The separation and analysis of lypophilic analytes, however, may be difficult in MEKC due to the strong affinity of lypophilic compounds to the micelle resulting in long separation times and poor resolution. An interesting approach for the simultaneous analysis of water- and fat-soluble vitamins by microemulsion electrokinetic chromatography (MEEKC) was proposed by Sanchez. The separation of both water- and fat-soluble vitamins (Bi, B2, B3, Be, B12, C, A palmitate, D, E acetate, and K) was obtained when the microemulsion was prepared with sodium dodecyl sulfate (SDS) as the surfactant, octane as the nonpolar modifier, butanol as the cosurfactant, and propanol as the second cosurfactant. Complete separation of all vitamins was carried out within 55 min however, this approach was tested only in multivitamin formulation. 

Most compounds that dissolve in nonpolar solvents are nonpolar and may have significant vapor pressures. Thus, they can be introduced into an IMSs by the direct injection of the solvent or by deposition of the sample solution on a surface where the solvent is evaporated and the analyte is then thermally desorbed into the IMS. While this approach was acceptable for semivolatile compounds dissolved in nonpolar solvents, it still excludes important biological, environmental, and industrial samples dissolved in water. Water is the ubiquitous solvent on Earth. Thus, our living systems have evolved around the use of water to transport chanicals through our environment and within organisms. Most compounds important to chemistry or life on Earth have polar moieties that reduce or eliminate vapor pressure while increasing water solubility. Thus, application of IMS to aqueous samples significantly expands its utility as an analytical tool. 

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