Analysis of a Mixture

When there is more than one absorbing species in a solution, the absorbance at a particular wavelength is the sum of absorbances from all species at that wavelength  

We normally choose wavelengths at absorbance maxima. The absorbance of the mixture should not be too small or too great, so that uncertainty in absorbance is small. 

To see why there is an isosbestic point, we write an equation for the absorbance of the solution at 465 nm  

But the spectra of pure HIn and pure In at equal concentration cross at 465 nm, so 

Enin niust be equal to Setting = Ef - = we rewrite Equation 19-6 in the form 

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Spectral searching and stripping in the analysis of a mixture of mannitol and cocaine hydrochloride, (a) IR spectrum for the mixture (b) Library IR spectrum of mannitol (c) Result of subtracting mannitol s IR spectrum from that of the mixture ... 

The following table lists the molar absorptivities for the Arsenazo complexes of copper and barium at selected wavelengths. " Determine the optimum wavelengths for the analysis of a mixture of copper and barium. 

The methods just noted tell something about the physical characteristics of atmospheric particulate matter but nothing about its chemical composition. One can seek this kind of information for either individual particles or all particles en masse. Analysis of particles en masse involves analysis of a mixture of particles of many different compounds. How much of... 

Figure 11.15 Cation-exchange mia O-LC analysis of a mixture of model proteins (a) the original sample consisting of myoglobin (M), cytochrome C (C) and lysozyme (L) (b) and (c) proteins adsorbed on to and then released from the polyaaylic acid coated fibre with exti ac-tion times of 5 and 240 s, respectively. Reprinted from Journal of Microcolumn Separations, 8, J.-L. Liao et al., Solid phase mia O exti action of biopolymers, exemplified with adsorption of basic proteins onto a fiber coated with polyaaylic acid, pp. 1-4, 1996, with permission from Jolm Wiley Sons, New York.

Column coupling proves to be a rapid screening approach in identifying chiral selectivity in the most efficient and economical way. In addition to the potential for the simultaneous analysis of a mixture, the coupling practice offers the advantages... 

High-pressure pumps operating at up to 6000 psi are required to force solvent through a tightly packed HPLC column, and electronic detectors are used to monitor the appearance of material eluting from the column. Alternatively, the column can be interfaced to a mass spectrometer to determine the mass spectrum of every substance as it elutes. Figure 12.18 shows the results of HPLC analysis of a mixture of 10 fat-soluble vitamins on 5 jam silica spheres with acetonitrile as solvent. 

Figure 12.18 Results of an HPLC analysis of a mixture often fat-soluble vitamins.

ANALYSIS OF A MIXTURE USING THE INTERNAL NORMALISATION METHOD 9.7... 

To obtain an accurate quantitative analysis of the composition of a mixture, a knowledge of the response of the detector to each component is required. If the detector response is not the same for each component, then the areas under the peaks clearly cannot be used as a direct measure of the proportion of the components in the mixture. The experiment described illustrates the use of an internal normalisation method for the quantitative analysis of a mixture of the following three components ethyl acetate (ethanoate), octane, and ethyl n-propyl ketone (hexan-3-one). 

Figure 3.31 TIC traces for (a) a constant-neutral-loss scan of 42 Da, and (b) a pre-cursor-ion m/z 68 scan, obtained from the LC-MS analysis of a mixture of atrazine and its degradation products. Reprinted from J. Chromatogr., A, 915, Steen, R. J. C. A., Bobeldijk, I. and Brinkman, U. A. Th., Screening for transformation products of pesticides using tandem mass spectrometric scan modes , 129-137, Copyright (2001), with permission from Elsevier Science.

For the regression analysis of a mixture design of this type, the NOCONSTANT regression command in MINITAB was used. Because of the constraint that the sum of all components must equal unity, the resultant models are in the form of Scheffe polynomials(13), in which the constant term is included in the other coefficients. However, the calculation of correlation coefficients and F values given by MINITAB are not correct for this situation. Therefore, these values had to be calculated in a separate program. Again, the computer made these repetitive and Involved calculations easily. The correct equations are shown below (13) ... 

Example Optimization of an Eleven Component Glass Formulation. Piepel (6) discussed the generation and analysis of a mixture design consisting of eleven oxides used to prepare glasses for waste vitrification. Although many responses must be considered for the end use of this composition, the intent of Piepel s study was to minimize the response of leachability subject to the compositional constraints of ... 

A concentration of 0.15 pg/ml gave a fluorescence of 10% full-scale deflection at maximum instrumental sensitivity. These authors explored analyzing folinic acid in the presence of folic acid and found that excitation at 290 nm effectively shifted the emission band of the compound of interest to 370 nm, thus enabling analysis of a mixture. 

To record a mass spectrum it is necessary to introduce a sample into the ion source of a mass spectrometer, to ionize sample molecules (to obtain positive or negative ions), to separate these ions according to their mass-to-charge ratio (m/z) and to record the quantity of ions of each m/z. A computer controls all the operations and helps to process the data. It makes it possible to get any format of a spectrum, to achieve subtraction or averaging of spectra, and to carry out a library search using spectral libraries. A principal scheme of a mass spectrometer is represented in Fig. 5.2. To resolve more complex tasks (e.g., direct analysis of a mixture) tandem mass spectrometry (see below and Chapter 3) may be applied. 

Fig. 2.3.9. Analysis of a mixture of SDS (0.2 g L ) and Ci2En (2 g L ) on C Ultraspher column (250 x 4.6 mm2, d 5 pm) under the following operating conditions (a) mobile-phase CH3CN-water (58 42, v/v) + (C3H7)NBr 2x 10 2molL 1 NaCl 4x10 mol IT1, flow-rate 1.12 mL min-1 and temperature 29°C (b) mobile-phase CH3CN-water (55 45, v/v) + (C3H7)NBr 5 X 10 mol L 1 + NaCl 10 2 mol L-1, flow-rate 1 mL min-1 and temperature 26 0. Reprinted with permission from Ref. [86] 2000 Elsevier.

The results obtained from the LC-MS analysis of a mixture of standards containing ca. 4 ng xl-1 are shown in Figure 5.64. Note that because it does not contain a chromophore, TG is not observed in the UV trace and that because... 

S Weinstein, MH Engel, PE Hare. The enantiomeric analysis of a mixture of all common protein amino acids by high-performance liquid chromatography using a new chiral mobile phase. Anal Biochem 121, 370, 1982. 

DSC analysis of a mixture of PBT cyclic oligomers containing the stannox-ane 3 showed only the melting endotherm (AH = 68 J/g), with no exotherm evident. Apparently, polymerization starts as the cyclics melt. Cooling showed the crystallization of the polymer, while the second heating stage displayed only the melting point of the PBT polymer at 213 °C (A H = 54 J/g) the polymer had an Mw of 117 000, and was 97 % polymerized (from GPC analysis). 

The analysis of a mixture, for instance, gives information about its composition, but the mixture already existed before. We have not created anything new we have only deduced its composition. However, if we synthesise a new compound -a drug, for instance- then we have really created something new that did not exist a priori. 

The selectivity is demonstrated by analysis of a mixture of the relevant compounds, e.g., a selectivity batch/sample solution. The resulting electropherogram is compared with the selectivity electropherogram provided in the method description. The resolution between the critical peak pair(s) is determined. The target resolution should be >2.0 (the calculation... 

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