Fractionation 271, field flow capillary hydrodynamic

There are some special cases in FFF related to the two extreme limits of the cross-field driving forces. In the first case, the cross-field force is zero, and no transverse solute migration is caused by outer fields. However, because of the shear forces, transverse movements may occur even under conditions of laminar flow. This phenomenon is called the tubular pinch effect . In this case, these shear forces lead to axial separation of various solutes. Small [63] made use of this phenomenon and named it hydrodynamic chromatography (HC). If thin capillaries are used for flow transport, this technique is also called capillary hydrodynamic fractionation (CHDF). A simple interpretation of the ability to separate is that the centers of the solute particles cannot approach the channel walls closer than their lateral dimensions. This means that just by their size larger particles are located in streamlines of higher flow velocities than smaller ones and are eluted first (opposite to the solution sequence in the classical FFF mode). For details on hydrodynamic chromatography,see [64-66]. 

Another separation technique of particular application for proteins, high-molar-mass molecules, and particles is the general class known as field-flow fractionation (FFF) in its various forms (cross-flow, sedimentation, thermal, and electrical). Once again, MALS detection permits mass and size determinations in an absolute sense without calibration. For homogeneous particles of relatively simple structure, a concentration detector is not required to calculate size and differential size and mass fraction distributions. Capillary hydrodynamic fractionation (CHDF) is another particle separation technique that may be used successfully with MALS detection. 

The specific, particle sizing method chosen depends on the type of. size information needed and the chemical and physical properties of the sample. In addition to the three techniques discussed here, molecular sieving, electrical conductance, microscopy, capillary hydrodynamic chromatography, light obscuration counting, field-flow fractionation, Doppler anemometry, and ultrasonic spectrometry-are commonly applied. Huch of the particle sizing methods has its advantages and drawbacks for particular samples and analyses. 

Particle Size and PSD. According to the basic principles that they are based on, the techniques for measuring these important characteristics of the latexes are classified into four major groups [196] (i) microscopy, (ii) light scattering, (iii) particle movement (e.g., capillary hydrodynamic chromatography and field flow fractionation methods), and... 

The techniques that are able to perform the on-line evaluation of PSDs include fiberoptic dynamic light scattering (FODLS), turbidimetry, size fractionation techniques (such as capillary hydrodynamic fractionation chromatography, CHDF and field-flow fractionation. 

Hydrodynamic chromatography reUes on different particle velocities in laminar flow through capillaries or packed columns. Larger particles move faster with the flow than do fine ones because they are, on average, further away from the capillary wall. The operation and the equipment are the same as in liquid chromatography colloidal particles are injected into a column packed with beads and a suitable detector (ultraviolet light detector or a spectrophotometer) monitors the flow from the column. Both field flow fractionation and hydrodynamic chromatography are most suitable for nearly mono-sized particle systems. 

Currently, the only practical way of accurately determining an MWD is via a separation technique. The most widely used methods include SEC, field flow fractionation (FFF), capillary hydrodynamic fractionation, and gel electrophoresis. Since knowledge of C M) furnishes the most complete description of a polymer distribution, there is intensive development and application of these techniques currently in progress. SEC is covered in great detail in Chapter 9. 

Whether DLS, DWS, Mie scattering, or other applications in which unfractionated samples are analyzed, the resulting distributions produced by modern instruments, while frequently facile to obtain and neat in appearance, must be treated with caution, as there is usually a large amount of data smoothing, fitting, and assumptions applied in using inverse Laplace transform and several other commonly employed methods. The best means of finding distributions of size and mass continue to be fractionation methods, such as SEC [32-34], field flow fractionation (FEE) [35-37], capillary electrophoresis [38], capillary hydrodynamic fractionation [39], and so on. 

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