Cross-over problems

With this one important step, we are now in a position to realize that this problem can be thought of as a cross-over problem. The starting material is a carboxylic acid, and we need to turn it into a ketone ... 

Now let s get some practice with some more cross-over problems. In order to do these problems, you will need to review this chapter and the previous chapter (ketones and aldehydes)— you will need to have all of the reactions from both chapters at your fingertips. 

At first, you might find it difficult to identify the following problems as cross-over problems, but hopefully, you will start to see some trends as you solve these problems. The hope is that you will train your eyes to locate problems that involve cross-over reactions. 

Another eommon eombustion problem eoneerns the erossover tubes. Cross-over tubes are used in ean-annular eombustors to assure eombustion in all ehambers and to equalize pressure. Many times the flow of hot gases through the erossover tubes is inereased due to bloeked fuel nozzles, whieh ean lead to tube failures as shown in Figure 21-17. 

The emission spectra of BODIPY derivatives normally display narrow bandwidths, providing intensely fluorescent labels for biomolecules. Unfortunately, they also have very small Stoke s shifts, typically on the order of only 10-20 nm. Excitation at the optimal wavelength may cause some interference in measurements at the emission wavelength due to light scattering or cross-over from the wide bandwidth of the excitation source. The dyes usually require excitation at sub-optimal wavelengths to prevent this problem. 

There is the possibility of carry-over effects. This is more crucial in Latin square and other cross-over designs. Knowledge of pharmacokinetics and metabolism of a compound under study generally helps in avoiding this problem. 

Direct-methanol fuel cells (DMFCs) have attracted considerable attention for certain mobile and portable applications, because of their high specific energy density, low poison emissions, easy fuel handling, and miniaturization [129,130], However, the methanol permeation through electrolyte membranes (usually called methanol cross-over) in DMFCs still is one of the critical problems hindering the commercialization [131,132], Nafion , a... 

Another problem of this approach is that it is not clear if some spot crossover occurs when the temperature or relative humidity change. This information is, however, available. At mobile phase composition DEA=0.05, MeOH=0, CHCl3=0.475, EtAc=0.475 the spot order at 20 °C, 40% RH is noscapine, papaverine, heroin, caffeine, strychnine, quinine. At 20 °C, 80% RH strychnine and caffeine have crossed. Considerably more has happened for a mobile phase composition of DEA=0.4, MeOH=0, CHCl3=0, EtAc=0.6. At 20 °C, 40% RH the order is noscapine, papaverine, heroin, caffeine, strychnine, quinine, while increase to 80% RH results in the spot order noscapine, quinine, papaverine heroin, strychnine, caffeine. It is not clear what happens at environmental circumstances which are not tested. The possibility of spot cross-over is prominent in TLC, and resolution is not a response which can easily be interpolated. 

One of the more important criteria in the estimation of robustness is the occurrence of different spot orders at different temperatures or relative humidities. This is called spot cross-over. A spot cross-over clearly implies a lack of resolution and an interpretation problem (which spot is which compound). The number of spot cross-overs should be minimal and preferably zero. The number of cross-overs can easily, be determined when at fixed settings the elution order on the plates is examined. 

Let me pass now to some problems of co-polymerization of lithium salts of living polymers. With Dr. Zdenek Laita we studied the rate of cross-over reaction converting lithium poly-styryl into 1,1-diphenyl ethylene", D end-groups in benzene(23). The stoichiometry of the reaction is... 

Beachamp [Maher and Beauchamp, 1990] to be an improvement over fixed filter hank methods for the analysis of vibrato since it is possible to track changing frequencies thereby avoiding the inter-band cross-talk problem. 

The third problem that needs to be controlled is that of spectral cross-over and the possibility of incorrect instrument compensation. As an example of a case in which controls for nonspecific staining, autofluorescence, and compensation are all critical, let us look at the staining of B lymphocytes for the CD5 marker present with only low density on their surface. As well as the problems created by nonspecific staining and by autofluorescence, the problem of spectral cross-over between fluorescein and phycoerythrin can particularly confuse the interpretation of results from this kind of experiment. Look at Figure 6.5. What we are interested in is the number of B lymphocytes that possess the CD5 surface antigen. These cells will appear in quadrant 2 of a contour plot of fluorescein fluorescence... 

In these calculations, you will be converting either from moles of the given substance to grams of the unknown or from grams of the given substance to moles of the unknown. Either way, you will need to use the mole ratio as the mechanism for crossing over from one substance to the next. Other than the step with the mole ratio, these calculations are the same as the calculations from earlier in the chapter that converted mass to moles or moles to mass for a single substance. We ll use the same equation from the sample problem to continue with this example. 

In addition to the slow methanol oxidation kinetics, methanol that crosses over from the anode to the cathode side through the membrane can react with 02 at the cathode catalyst, leading to a mixed potential at the cathode side and thereby reducing cell performance. To solve this problem, methanol-tolerant catalysts as well as membranes with low methanol permeability have been investigated. However, these materials are still in the research stages and commercial applications have not been developed. 

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