Three-component mixtures separability

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

This type of chromatographic development will only be briefly described as it is rarely used and probably is of academic interest only. This method of development can only be effectively employed in a column distribution system. The sample is fed continuously onto the column, usually as a dilute solution in the mobile phase. This is in contrast to displacement development and elution development, where discrete samples are placed on the system and the separation is subsequently processed. Frontal analysis only separates part of the first compound in a relatively pure state, each subsequent component being mixed with those previously eluted. Consider a three component mixture, containing solutes (A), (B) and (C) as a dilute solution in the mobile phase that is fed continuously onto a column. The first component to elute, (A), will be that solute held least strongly in the stationary phase. Then the... 

As was reported by Soczewinski, a three-component mixture containing 5 mg of each of the ingredients can be completely separated using a 90 X 100 X 0.5 mm layer of silica [26]. This indicates that the capacity of the chromatographic system can be considerably increased by the apphcation of wider and thicker layers of adsorbent on the chromatoplate [44]. 

If there is a three-component mixture to be separated into three relatively pure products and simple columns are employed, then the decision is between two sequences, as illustrated in Figure 11.1. The sequence shown in Figure 11.1a is known as the direct sequence in which the lightest component is taken overhead in each column. The indirect sequence, as shown in Figure 11.1b, takes the heaviest component as bottom product in each column. 

Figure 13.1 A shows a conventional high performance reversed-phase separation of a three-component mixture of aromatic acid esters obtained with a standard 4.6 mm x 250 mm octadecyl column and methanol water as the eluent. From the view of chromatographic resolution and ruggedness, this is an excellent separation. However, from a practical standpoint, an assay based on this particular separation would not be satisfactory since it wastes large amounts of time between elutions of the individual components.

FIGURE 11.12 Hypothetical displacement separation of a three-component mixture containing two trace components loaded at 0.1 mg each and a major component loaded at 20mg. Panel a, main component band and displacer front panel b, peaks of the impurities panel c, peak of the second component under analytical (linear elution) conditions. (Reprinted with permission from Elsevier from Viscomi, G.C. et ah, J. Chromatogr., 440, 157, 1988. Copyright.)... 

Under these conditions, the elution period is a maximum of 35 minutes, and the separation results are said to be satisfactory. Although bis-2-ethylhexyl, bis-2-ethylbutyl, and dibutyl phthalates in mixtures are not distinguishable by infrared spectroscopy, these three components are separated sharply by gas chromatography, which can even provide quantitative analyses of such mixtures. Thus, gas chromatography is useful in plasticizer analysis, however, exact analyses and a detailed compilation of the retention periods still remain to be worked out. 

As described above, microemulsions feature the disadvantage that the reaction product can be separated only with difficulty from other components of the system. This disadvantage can be compensated by employing highly viscous to solid liquid-crystalline phases which consist of the same components as microemulsions (oil, surfactant, water). Many enzymes display activity in several phases of a three-component mixture (Figure 12.6) (Martinek, 1986). 

Tedder and Rudd (1978) present the results of a study of the economics of separating a variety of three component mixtures using a variety of thermally coupled and ordinary columns. Their goal was to expose trends. They show which feed characteristics favor which column structure. 

Molecular fluorescence spectrometry has long been regarded as a useful technique for the determination of polycyclic aromatic hydrocarbons (PAHs) and related materials, due to the very high sensitivities which can be achieved. However, molecular fluorescence spectra measured in liquid solution usually are broad and relatively featureless hence, spectral interferences are common in the liquid-solution fluorometric analysis of multicomponent samples. Moreover, the fluorescence of a particular component of a complex sample may be partially quenched by other sample constituents if quenching occurs to a significant extent, the fluorescence signal observed for a particular compound present at a particular concentration will also depend upon the identities and concentrations of other substances present in the sample. Under these conditions, it is virtually impossible to obtain accurate quantitative results. Therefore, it is generally observed that molecular fluorescence spectrometry in liquid solution media is useful for quantitative determination of individual components in complex samples only if the fluorescence measurement is preceded by extensive separation steps (ideally to produce individual pure compounds or, at worst, simple two- or three-component mixtures). 

The combinatorial possibilities for separating a three-component mixture ABC into two product streams in which the most volatile component and the least volatile component do not distribute between the top and bottom product are... 

Figure 6 Distribution of component concentrations as a function of column height during separation of a three-component mixture (a) frontal separation (b) displacement separation.

Secondly, a separation of phases was observed (Fig. 3b,d). The feature of three component mixtures was small circular domains (Fig. 3b), probably, raft domains. Indeed, the domains that are essentially different in fiiction have been observed by AFM in the contact mode, which were practically indiscernible in the topography image. 

When [he feed mixture contains more than two components, (he separation is generally leimed multicomponent, even (hough a three-component mixture Is also known as a ternary. a four-component mixture as a qnertemary, and so on. The key point is that when (here ate more than two components, (he simple procedures such as McCabe- Thiele cannot be used with reliability and it is necessaty to use analytical rather than graphical approaches. However, the same principles are used, and die MESH equations are applied at each theoretical stage. 

A single distillation column for separating a three-component mixture into three products is shown below. Are the design variables shown sufficient to specify the problem completely If not, what additional design variable(s) would you select ... 

Figure 21.1. Hypothetical separation of a three-component mixture Component A A Component B Component C O. The dotted area represents the original solvent in the column, which is being displaced during elution.

Figure 3.67a-c shows the single chromatograms of a three-component mixture on three different stationary phases a highly unpolar RP-18e phase, a combined reversed phase/weak anion exchange phase (RP/WAX), and a cyano-modified phase. All three stationary phases have been prepared on the same base silica, a 12 pm, 100 A LiChrospher . While on the RP-18e-phase the mixture can be separated, the retention on the other two phases is shorter and insufficient for complete resolution. Through the combination of the different stationary phases the retention time of a single run with sufficient resolution can be reduced from 5 to 3 min which is equivalent to a capacity increase of the system of 40%. This can be achieved by a selection of RP-18e and RP/WAX-phases in a 2 3-combination (Figure 3.67d) (Horn, 2004).

The separation of an industrial feed mixture of fructose-glucose (Cfeed = 300 g 1 ) containing 6% sucrose as an impurity is displayed in Figure 6.44. Once again, good agreement between experiment and simulation is found. The individual concentration profiles of this three-component mixture cannot be determined by two-detection systems. Therefore, additional samples (two per shifting interval) were taken and analyzed by HPLC. 

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