THE ANALYSIS OF MIXTURES

A 400 MHz NMR spectrum of a mixture of ethanol (C2H6O) 5 1.24, 5 1.78, 5 3.72 and bromoethane (C2HsBr) 6 1.68 and 5 3.44 is given below. Estimate the relative proportions (mole %) of the 2 components from the integrals in the spectrum. 

A 400 MHz NMR spectrum of a mixture of common organic solvents consisting of 

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The general method to be adopted for the analysis of mixtures of organic compounds is to separate them into their components and to... 

Spectrophotometric titrations are particularly useful for the analysis of mixtures if a suitable difference in absorbance exists between the analytes and products, or titrant. Eor example, the analysis of a two-component mixture can be accomplished if there is a difference between the absorbance of the two metal-ligand complexes (Eigure 9.33). 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

The generalized standard addition method (GSAM) extends the analysis of mixtures to situations in which matrix effects prevent the determination of 8x and 8y using external standards.When adding a known concentration of analyte to a solution containing an unknown concentration of analyte, the concentrations usually are not additive (see question 9 in Chapter 5). Conservation of mass, however, is always obeyed. Equation 10.11 can be written in terms of moles, n, by using the relationship... 

This experiment demonstrates the application of the generalized standard additions method for the analysis of mixtures of K2Cr207 and KMn04. 

The procedure of simultaneous extracting-spectrophotometric determination of nitrophenols in wastewater is proposed on the example of the analysis of mixtures of mono-, di-, and trinitrophenols. The procedure consists of extraction concentrating in an acid medium, and sequential back-extractions under various pH. Such procedures give possibility for isolation o-, m-, p-nitrophenols, a-, P-, y-dinitrophenols and trinitrophenol in separate groups. Simultaneous determination is carried out by summary light-absorption of nitrophenol-ions. The error of determination concentrations on maximum contaminant level in natural waters doesn t exceed 10%. The peculiarities of application of the sequential extractions under fixed pH were studied on the example of mixture of simplest phenols (phenol, o-, m-, />-cresols). The procedure of their determination is based on the extraction to carbon tetrachloride, subsequent back-extraction and spectrophotometric measurement of interaction products with diazo-p-nitroaniline. 

In summary, examples of the successful use of silica gel as a conventional stationary phase are in the analysis of mixtures containing polarizable and relatively low polarity solutes typified by mixtures of aromatic hydrocarbons, polynuclear aromatics, nitro compounds, carotenes and vitamin A formulas. 

C Nuclear Magnetic Resonance Spectra. The sohd-state CNMR spectra of 6,7-dimethyl-2,3-di(pyridin-2-yl)quinoxahne (130) and its salts have been used to complement X-ray information (see above) on fine structure. A study of the spectra of mono- to tetramethylquinoxalines has made possible the analysis of mixtures of such methylated quinoxalines obtained from ambiguous primary syntheses. 

Pullen, F. S., Swanson, A. G., Newman, M. J. and Richards, D. S., On-line liquid chromatography/nuclear magnetic resonance spectrometry — a powerful spectroscopic tool for the analysis of mixtures of pharmaceutical interest, Rapid Comm. Mass. Spectr., 9, 1003, 1995. 

DOSY is a technique that may prove successful in the determination of additives in mixtures [279]. Using different field gradients it is possible to distinguish components in a mixture on the basis of their diffusion coefficients. Morris and Johnson [271] have developed diffusion-ordered 2D NMR experiments for the analysis of mixtures. PFG-NMR can thus be used to identify those components in a mixture that have similar (or overlapping) chemical shifts but different diffusional properties. Multivariate curve resolution (MCR) analysis of DOSY data allows generation of pure spectra of the individual components for identification. The pure spin-echo diffusion decays that are obtained for the individual components may be used to determine the diffusion coefficient/distribution [281]. Mixtures of molecules of very similar sizes can readily be analysed by DOSY. Diffusion-ordered spectroscopy [273,282], which does not require prior separation, is a viable competitor for techniques such as HPLC-NMR that are based on chemical separation. 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

Carter 12), of the Bureau of Entomology and Plant Quarantine, has adapted the total-chlorine method to the analysis of mixtures of DDT and benzene hexachloride in the following manner After determination of total chlorine in the mixture, the DDT is estimated by the Schechter-Haller colorimetric method 47), half this value is subtracted from the total chlorine (because DDT contains 50% of chlorine), and the difference is calculated as benzene hexachloride. This procedure gives no indication of the amount of the gamma isomer of benzene hexachloride. 

Cornell, J. A., "Experiments with Mixturess Designs, Models, and the Analysis of Mixture Data", John Wiley Sons, New York, 1981. 

Techniques are presented for the analysis of mixture design responses and for the optimization of... 

Other established techniques to aid in the analysis of mixture models include the use of gradients to measure the rate of response change (10), graphical analysis of the response change versus individual component changes (11), and the determination of component effects within constrained regions (6). Each of these techniques, while very useful in the interpretation of component effects do not lend themselves to the determination of the optimum composition, the most important point in the formulation space. 

Equations (9-11) are dependent on three parameters to, and Zj. All may be exploited in the analysis of mixtures of fluorophores however, co is the only one that can be systematically varied without altering the sample. 

Table 5.8 gives an indication of the range of elements that may be determined. Most procedures will require an analyte concentration of 10-3 mol dm 3 or more, although with special conditions, notably potentiometric end-point detection, the sensitivity may be extended to 1(H mol dm 3. The analysis of mixtures of metal ions necessitates masking and demasking, pH adjustments and selective separation procedures. Areas of application are spread throughout the chemical field from water treatment and the analysis of refined food and petroleum products to the assay of minerals and alloys. Table 5.10 gives some selected examples. 

The use of visible and UV spectrometry for quantitative analysis by comparing the absorbance of standards and samples at a selected wavelength is one of the most widespread of all analytical techniques. It is also one of the most sensitive. The analysis of mixtures of two or more components is facilitated by the additivity of absorbances. This has been discussed earlier (p. 356). Other applications include measurement of the absorption of complexes as a function of solution conditions or time to establish their composition, and to determine thermodynamic and kinetic stability for analytical purposes or for more fundamental studies. 

Used in conjunction with infrared, NMR, UV and visible spectral data, mass spectrometry is an extremely valuable aid in the identification and structural analysis of organic compounds, and, independently, as a method of determining relative molecular mass (RMM). The analysis of mixtures can be accomplished by coupling the technique to GC (p. 114). This was formerly done by using sets of simultaneous equations and matrix calculations based on mass spectra of the pure components. It is well suited to gas... 

NMR spectroscopy is an excellent tool for the analysis of mixtures - if a sample contains more than one compound then the areas of the signals belonging to each species in the NMR spectrum will reflect the relative concentrations of the species in the mixture. 

It is not usually possible to integrate routine C spectra directly unless specific precautions have been taken. However with proper controls, NMR spectroscopy can be used quantitatively and it is a valuable technique for the analysis of mixtures. To record C NMR spectra where the relative signal intensity can be reliably determined, the spectra must be recorded with techniques to suppress the Nuclear Overhauser Effect and with a long delay between the acquisition of successive spectra to ensure that all of the carbons in the molecule are completely relaxed between spectral acquisitions. 

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