Correction, band-spreading

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.

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

The apparent dispersion coefficient in Equation 10.8 describes the zone spreading observed in linear chromatography. This phenomenon is mainly governed by axial dispersion in the mobile phase and by nonequilibrium effects (i.e., the consequence of a finite rate of mass transfer kinetics). The band spreading observed in preparative chromatography is far more extensive than it is in linear chromatography. It is predominantly caused by the consequences of the nonlinear thermodynamics, i.e., the concentration dependence of the velocity associated to each concentration. When the mass transfer kinetics is fast, the influence of the apparent axial dispersion is small or moderate and results in a mere correction to the band profile predicted by thermodynamics alone. 

Detectors are not limited to solo use they can be hooked in series to get more information from the same sample. In a serial operation, be sure that the refractive index detector or electrochemical detector is the last in the line. Their flow cells are more fragile than UV and fluorescence cells and won t take the increased back-pressure. Keep the tubing diameter fine and as short as possible to avoid band spreading. You must correct for connecting tubing volume (time) delay in comparing chromatograms from the two detectors. 

The material and information on molecular weight data were kindly supplied by R. H. Burr, Phillips Petroleum Co. Mw = 129,000, Mn = 107,000, Mw/Mn = 1.21, styrene content 40% by weight. (Our own GPC measurements showed styrene content 39% and Mw/Mn = 1.38 without correction for band spreading.) SBS and SB materials caused problems because of thermal (and possibly mechanical) degradation. 

The sample, dissolved in a volatile solvent, is applied to the paper as a drop or spot by means of a syringe or micropipette. To minimize band spreading, the spot should be restricted to about 2 mm in diameter. Sample sizes are normally 10-50 /ig, and the total quantity of sample should not exceed 500 /Ag. Larger spots and larger sample sizes lead to poorer separations. Figure 21.9 illustrates the correct... 

This effect can be corrected by injecting a narrow MWD sample and measuring the variance of the peaks in each detector. Because the peak shape is nearly Gaussian, it should, ideally, be the same for all detectors. If it is not, the additional variance can be calculated for one of the detectors. In subsequent data analysis, the narrower peak can be digitally broadened using Gaussian band spreading to correct... 

Extra-column band spreading affects the measured performance of columns packed with smaller particles, especially for columns with an internal diameter smaller than the classical standard of 4.6 mm. Several pubUshed reports that characterize the performance of very efficient columns have identified extra-column effects as a major factor that negatively impacts the column performance (55-59). These reports used either UPLC7UHPLC systems with low band spread or conventional HPLC systems with corrections for extra-column effects. In other reports, sub-2 (Jim particle columns with a small internal diameter have been compared to columns packed with 2-3 (xm particles in larger diameter columns without taking into account the detrimental effects of extra-column band spreading (60-62). 

Since these surface states are spread out in a very wide band, it is only possible to determine the density of states per electron volt in the vicinity of the Fermi energy, ft is no longer experimentally possible to determine the total number. One criterion which has always been applied in the past is that in order for the observed states to be true surface states their number must be equal to the number of surface atoms. If the present model is correct this criterion may never be proved. 

For correcting band broadening, the main difficulty is in obtaining precise mapping of the spreading function of the system. Normally, this needs very high quality standards and TGIC offers new possibilities in that area. Computational techniques are now sufficiently efficient... 

---