Band-broadening thermal effects

There are several points to consider when choosing a derivatizing scheme, such as the stability of the derivative to hydrolysis, solvolysis, and thermal decomposition. In addition, if precolumn derivatization is chosen, the derivatization process will alter the chromatographic properties of the analytes, which may result in the need to adopt a different chromatographic mode. Finally, if postcolumn derivatization is chosen, the effects of band broadening and sample loss caused by adsorption and dilution effects should not be overlooked. 

Figure 18-3. Band-broadening due to thermal effects, (a) Ideal case, no thermal effects ...

Fig. 2 Thermal FFF elution profiles before (original) and after (corrected) removing the effects of band broadening. With the poly disperse sample (NBS 706), which was analyzed at a flow rate of 0.4 mL/min, the effect of band broadening on the elution profile is minimal. The polydispersity values listed were determined using thermal FFF.

From the beginning of free solution electrophoresis, band broadening from thermal effects was foreseen as the main problem restricting its development [6]. The energy generated as heat by application of an electric field to an electrolyte solution in a column raises the solution temperature, but more critically, results in a radial... 

The sfff technique is versatile it can be used to study both soUds and solutions, no standards are required for cahbration, and it is a nondestmctive technique. Low shear forces are employed and minimal absorption occurs. Hence, sample degradation does not take place. The technique cannot, however, be routinely used for molecular weight characterization of small molecules. Newer variations of sfff include the development of thermal fff (73), whereby a temperature gradient is placed between two parallel metal blocks to allow the particles to migrate toward the wall. This approach is said to allow for the molecular weight characterization of polymeric materials which are much smaller in molecular size. Advantages of this technique over gpc He in the absence of conventional band broadening effects. 

Thermal equilibrium between adsorbate and surface means that both subsystems exhibit the same temperature. Infrared spectra from CO adsorbed on a Ru(0 001) surface represent a convenient example for monitoring the temperature of the adsorbate. Figure 3.12 shows a series of IR spectra from CO adsorbed on a Ru(0 001) surface at different temperatures [47]. With increasing T, there is a continuous shift of the band toward lower wave numbers together with a continuous line broadening. This effect... 

Separations should initially be attempted with the capillary thermostatted at close to ambient temperature. The capillary temperature can be increased on most commercial CE units to as high as 60 C without substantially increasing current with most buffers. When this is done using the same applied voltage, decreased buffer viscosity leads to an increase in analyte electrophoretic mobility, thus, decreasing separation times. Also, it is important to note that, when sample introduction is hydrostatic (same pressure/vacuum and time), increased capillary temperature will lead to an increase in the injected sample volume as a result of decreased buffer viscosity. " Sensitivity may not necessarily be increased. However, Undesirable effects include concurrent changes in buffer pH, band broadening due to increased diffusion, and possible thermal denaturation of the sample. 

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