Difficulties in resolving

Nitrones arc generally more stable than nitroso-compounds and arc therefore easier to handle. However, the nitroxides formed by reaction with nitrones [e.g. phenyl /-butyl nitrone (109)]483 484 have the radical center one carbon removed from the trapped radical (Scheme 3.86). The LPR spectra are therefore less sensitive to the nature of that radical and there is greater difficulty in resolving and assigning signals. Nitrones are generally less efficient traps than nitroso-compounds.476... 

In addition to the above calculations, our computer programs also separate unresolvable sulfur and hydrocarbon components (10). This step is carried out before calculating the concentrations. It is required because of the inherent difficulties in resolving certain hydrocarbon/ sulfur doublets even with an instrumental resolving power of ca. 100,000. 

In traditional vibrational methods snch as Raman or IR spectroscopy (see Vibrational Spectroscopy), only vibrational modes allowed by selection rules will appear in the spectra. To help assign the modes, isotope difference spectra are used. But owing to selection rules, some modes do not appear in the spectrum. Spectral congestion also raises difficulty in resolving modes with close frequencies, particularly for macromolecules containing thonsands of atoms. 

The spectra reported by Grenier-Loustalot et al. [43, 44] highlight the difficulties in resolving peaks due to the acid and amide in many poly(amic acid)s. They reported a difference in chemical shift of <1 ppm between the peaks due to amic acids (167.3 and 167.0 ppm) and amide (166.3 and... 

The plasmid preparations used in these studies contained some level (<20%) of open circle forms prior to irradiation their number increased after low doses of radiation as they are produced from damaged supercoiled molecules. The increase in open circle forms ceases on exhaustion of the supply of parent supercoiled molecules. Further radiation exposure resulted in damage to the open circle molecules revealed as an exponential decrease in open circle forms (Fig. 2). Analysis of the amount of open circle forms is described by the difference of two exponentials - one due to the production and the other due to the destruction of the open circle form. The complexity of these curves presents difficulties in resolving the two exponential components (9) since only some of the data points are used in each fit. Combining data from several experiments increases the number of data points, enabling more accurate target determinations. 

However, Szulczewski and Higuchi °" were successful in using silica gel. They also had no difficulty in resolving nitrous oxide and CO2 on a column 6 ft in length. Helium was used as a carrier gas and the column was operated at room temperature for this later separation, but at the... 

These time-resolved data illustrate the experimental difficulty in resolving distance distributions. The fits to a fixed distance and to the Gaussian distibution are barely resolvable within the linewidth of the figure (Figure... 

This fact, coupled with the difficulty in resolving racemic sites on a heterogeneous catalyst, stimulated a series of indirect but elegant investigations on the polymerization of chiral monomers. 

TLC resolution and identification of PTH-amino acids on silica or polyamide layers as discussed above showed difficulties in achieving discrimination between derivatives of Leu/Ile (106) and resolution of complex mixtures without two-dimensional chromatography (113). Also, difficulties in resolving combinations of PTH-Phe/VaiyMet/Thr (114-115) and PTH-Asp and Glu were observed. Use of chloroform-acetic acid (27 3, v/v) and chloroform-methanol (30 4, v/v) has been found extremely satisfactory for the discrimination between PTH-Asp and PTH-Glu, as the difference in their /i) y values was around 10 units (116). The difficulties, previously posed and as noted above, in resolving and identifying various combinations of PTH amino acids can be overcome by the use of certain solvent systems (30a,30b) given in Table 9. 

Interest in countercurrent chromatography was revived in an effort to overcome problems frequently encountered in adsorption or exclusion chromatography, which uses a solid support. Most prominent among these problems was the difficulty in resolving mixtures of sensitive or highly polar compounds. Irreversible adsorption onto the solid phase, solid phase catalyzed decomposition, especially with silica gel, and severe tailing are obstacles to the purification of such molecules. 

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