Nonpolar analytes example

The most critical decision to be made is the choice of the best solvent to facilitate extraction of the drug residue while minimizing interference. A review of available solubility, logP, and pK /pKb data for the marker residue can become an important first step in the selection of the best extraction solvents to try. A selected list of solvents from the literature methods include individual solvents (n-hexane, " dichloromethane, ethyl acetate, acetone, acetonitrile, methanol, and water ) mixtures of solvents (dichloromethane-methanol-acetic acid, isooctane-ethyl acetate, methanol-water, and acetonitrile-water ), and aqueous buffer solutions (phosphate and sodium sulfate ). Hexane is a very nonpolar solvent and could be chosen as an extraction solvent if the analyte is also very nonpolar. For example, Serrano et al used n-hexane to extract the very nonpolar polychlorinated biphenyls (PCBs) from fat, liver, and kidney of whale. One advantage of using n-hexane as an extraction solvent for fat tissue is that the fat itself will be completely dissolved, but this will necessitate an additional cleanup step to remove the substantial fat matrix. The choice of chlorinated hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride should be avoided owing to safety and environmental concerns with these solvents. Diethyl ether and ethyl acetate are other relatively nonpolar solvents that are appropriate for extraction of nonpolar analytes. Diethyl ether or ethyl acetate may also be combined with hexane (or other hydrocarbon solvent) to create an extraction solvent that has a polarity intermediate between the two solvents. For example, Gerhardt et a/. used a combination of isooctane and ethyl acetate for the extraction of several ionophores from various animal tissues. 

The polarity index is a measure of the polarity of the solvent, which is often the most important factor in the solvent choice for the particular application. In extraction processes, the tenet that like dissolves like (and conversely, opposites do not attract ) is the primary consideration in choosing the solvent for extraction, partitioning, and/or analytical conditions. For example, hexane often provides a selective extraction for nonpolar analytes, and toluene may provide more selectivity for aromatic analytes. 

Membrane extraction with sorbent interface (MESI) is an interesting example of an extraction device, which is the most useful system for interfacing with GC. In this approach, the donor phase is a gas or a liquid sample, and the acceptor phase is a gas. The volatiles are continuously trapped on sorbent and then desorbed into GC [112]. Another solution is a combination of off-line GC-MESI through a cryogenic trap, which allows preparation of environmental samples in the field and performance of GC analysis after transportation to the laboratory [113,114]. MESI allows the extraction of volatile and relatively nonpolar analytes. 

In order to enhance or modify the chemical selectivity of an SERS substrate, it is possible to chemically derivatize the metal surface. For example, covalent bonding of a hydrocarbon to a silver island film should selectively adsorb nonpolar analytes from an aqueous solution. The general approach is shown schematically in Figure 13.24 for the case of metal ions detected by a surface-bound complexing agent (47). Field enhancement is provided by the substrate, while adsorption selectivity results from the chemistry of the derivatized... 

With nonaqueous samples (part B of Fig. 3.8), the decisions for sorbents are somewhat reversed. For example, an analyte that is polar and ionic is best recovered with ion exchange. This example is similar to that of an aqueous sample. If the analyte is polar but nonionic, then the sorbent choice could be reversed phase or normal phase. The choice depends on the organic solvent, either polar or nonpolar, respectively. Finally, if the analyte is nonpolar, the sorbent choice is reversed phase. The second step in methods development is to execute the SPE experiment. Lastly, one has to optimize and troubleshoot the SPE method. 

For example, for the most nonpolar compounds, both the C-18 and C-8 columns will work well to sorb the analytes from aqueous solution by reversed phase. However, it has been found that C-8 gives equally good recoveries for the most nonpolar analytes, probably due to the fact that the nonpolar compounds elute somewhat easier from C-8 cartridges because of decreased van der Waals interaction. The elution solvent for the nonpolar compounds must be one that solubilizes the compounds, such as hexane/acetone, or hexane/ethyl acetate, or in many cases just ethyl acetate. Finally, the sorbent... 

Some examples will help to illustrate the effects of polarity on selectivity. To be effective as a stationary phase, the liquid chosen should interact with the components of the sample to be analyzed. The chemist s rule of thumb like dissolves like suggests that a polar liquid should be used to analyze polar analytes and a nonpolar liquid for nonpolar analytes. Figure 4.1 shows the separation of a pesticide mixture on two columns a nonpolar SE-30 and a more polar OV-210 . Clearly, the selection of the proper stationary liquid is very important in this case a polar column worked well for the polar pesticides. The nonpolar SE-30 is a good column (high efficiency) but it is not effective for this sample (small separation factor, a see the next section). 

The absorptivities of fundamental bands in the condensed-phase spectra of most samples vary by well over an order of magnitude, but the strongest band in the spectra of typical neat liquids or solids usually has an absorbance of between about 0.5 and 2 AU if the thickness of the sample is 10 pm. This is only a rule of thumb, however. The absorptivity of strong bands in the spectra of polar analyte is almost invariably greater than that of the stronger bands in the spectra of nonpolar analytes. For example, the four strongest bands in the spectrum of a 5-pm film of poly (ethylene terephthalate) are more intense than the strongest band in the spectrum of a 20-pm film of polystyrene. 

We have shown a new concept for selective chemical sensing based on composite core/shell polymer/silica colloidal crystal films. The vapor response selectivity is provided via the multivariate spectral analysis of the fundamental diffraction peak from the colloidal crystal film. Of course, as with any other analytical device, care should be taken not to irreversibly poison this sensor. For example, a prolonged exposure to high concentrations of nonpolar vapors will likely to irreversibly destroy the composite colloidal crystal film. Nevertheless, sensor materials based on the colloidal crystal films promise to have an improved long-term stability over the sensor materials based on organic colorimetric reagents incorporated into polymer films due to the elimination of photobleaching effects. In the experiments... 

Gas-Liquid Chromatography. In gas-liquid chromatography (GLC) the stationary phase is a liquid. GLC capillary columns are coated internally with a liquid (WCOT columns) stationary phase. As discussed above, in GC the interaction of the sample molecules with the mobile phase is very weak. Therefore, the primary means of creating differential adsorption is through the choice of the particular liquid stationary phase to be used. The basic principle is that analytes selectively interact with stationary phases of similar chemical nature. For example, a mixture of nonpolar components of the same chemical type, such as hydrocarbons in most petroleum fractions, often separates well on a column with a nonpolar stationary phase, while samples with polar or polarizable compounds often resolve well on the more polar and/or polarizable stationary phases. Reference 7 is a metabolomics example of capillary GC-MS. 

The analytical solution of the Smoluchowski equation for a Coulomb potential has been found by Hong and Noolandi [13]. Their results of the pair survival probability, obtained for the boundary condition (11a) with R = 0, are presented in Fig. 2. The solid lines show W t) calculated for two different values of Yq. The horizontal axis has a unit of r /D, which characterizes the timescale of the kinetics of geminate recombination in a particular system For example, in nonpolar liquids at room temperature r /Z) 10 sec. Unfortunately, the analytical treatment presented by Hong and Noolandi [13] is rather complicated and inconvenient for practical use. Tabulated values of W t) can be found in Ref. 14. The pair survival probability of geminate ion pairs can also be calculated numerically [15]. In some cases, numerical methods may be a more convenient approach to calculate W f), especially when the reaction cannot be assumed as totally diffusion-controlled. 

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