Band spreading

The local equilibrium curve is in approximate agreement with the numerically calculated profiles except at very low concentrations when the isotherm becomes linear and near the peak apex. This occurs because band-spreading, in this case, is dominated by adsorption equilibrium, even if the number of transfer units is not very high. A similar treatment based on local eqnihbrinm for a two-component mixture is given by Golshau-Shirazi and Gniochou [J. Phys. Chem., 93, 4143(1989)]. 

Column dispersity (band spreading) causes the measured molecular weight distribution to be broader than the true molecular weight distribution (Fig. 3.5). Because Zorbax PSM columns exhibit very low band-spreading characteristics, these columns have historically provided better molecular weight distribution accuracy than many gel-type columns. 

Although SynChropak GPC supports have excellent efficiencies for small molecules at various flow rates, macromolecules, because of their low diffusion constants, exhibit band spreading when linear velocities are increased. This effect increases with molecular weight, as seen in Fig. 10.11 (4). It should be noted that proteins are usually homogeneous in size and thus yield better efficiencies than polymers, which are usually heterogeneous. For preliminary analy-... 

Elevated temperatures may be used with SynChropak GPC or CATSEC supports if necessary for solubilization, speed, or reduction of band spreading. Such conditions are not recommended for routine analyses, however, because column degradation is enhanced as temperatures are raised. 

The band spreading of peaks in SEC is described by a form of the van Deemter equation (5) ... 

Spot in a contour plot. The very many eomponents present in oil fraetions give rise to bands spread aeross the plane. As a result of the separation meehanism of GC X GC, the resulting ehromatograms show a great deal of strueture, whieh ean be used to assign the (eomponents in) different bands with high analytieal eertainty. 

The effects of various pore-size distributions, including Gaussian, rectangular distributions, and continuous power-law, coupled with an assumption of cylindrical pores and mass transfer resistance on chromatographic behavior, have been developed by Goto and McCoy [139]. This study utilized the method of moments to determine the effects of the various distributions on mean retention and band spreading in size exclusion chromatography. 

Tne derivation of eq 10 was undertaken with the assumption that the sample effluent was monodisperse in molecular weight at any V. In reality this is never the case. First, if the frequency of branching varies with molecular weight, species of different molecular weight (but equivalent hydrodynamic volume) may elute at the same V (1 ). Second, molecular weight polydispersity at V results from assorted hardware mixing volumes and band-spreading of the SEC columns 0,11). 

A separate program was developed on tfie LDS-2 to correct the SEC/LALLS Rj, for band-spreading (Appendix). 

In the tails of the distribution, however, decreased resolution and inapplicability of the band-spreading correction serve to make g behave anomalously. ... 

Since the SEC/LALLS technique always yields a weight-average molecular weight (l )y for the slightly polydisperse fraction at V, a small overestimation of the sample Rn is expected (, 1 ). As noted previously (Results) a 1% to decrease in the narrow MWD polystyrene Mp values (Table I) accompanied application of the band-spreading correction ... 

Table II shows that for SRM 706 good agreementis obtained between SEC/LALLS and conventional SEC sample My, and Rp values when the band-spreading correction was used. However, the NBS 706 polydispersity index (Ry/Rp) given by the supplier (ca. 2.1) does not agree with that 1.°) found here using the SEC/LALLS and conventional SEC techniques. Insensitivity of the LALLS detector to a small amount of low molecular weight material may account for a larger sample R however, this is not supported by the conventional SEC data. The reason for the discrepancy remains unclear.

The absorption and emission spectra of a fluorophore are bands spread over a range of wavelengths with at least one peak of maximal absorbance and emission that corresponds to the So-Si and Si—S0 transitions, respectively. There are several vibrational levels within an electronic state and transitions from one electronic to several vibrational states are potentially possible. This determines that the spectra are not sharp but consist of broad bands. The emission spectrum is independent of the excitation wavelength. The energy used to excite the fluorophore to higher electronic and vibrational levels is very rapidly dissipated, sending the fluorophore to the lowest vibrational level of the first electronic excited state (Si) from where the main fluorescent transition occurs [3] (see Fig. 6.1). 

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