Capillary hydrodynamic fractionation chromatography

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. 

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]. 

Problems with resolution in hydrodynamic chromatography have been shown to result from radial dispersion. In ordCT to minimize this, Dos Ramos [40] has developed a column based on parallel capillaries. The technique, called capillary hydrodynamic fractionation eliminates the possibility of radial dispersion, and produces chromatograms of much higher resolution. An instrument based on this technology is being marketed by Matec Applied Sciences of Hopkinton, Massachusetts. An on-line version is currently under development at Lehigh UnivCTsity. 

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... 

In the first approximation, the column packed with small particles can be considered an array of capillaries. Hydrodynamic processes augment the exclusion based separation (see section 11.7.2, Retention Mechanisms and Accompanying Processes in SEC) and form the ground of a liquid chromatography-like method called hydrodynamic chromatography, HDC. HDC found application in separation of veiy large macromolecules, particles and dispersions. Its separation selectivity is rather low and it was partially substituted by the group of methods termed fieldflow fractionation (see seetion 11.3.3, Molar Mass Dispersity). 

Figure 5.22. Results for Capillary Hydrodynamic Chromatography of a mixture of colloidal fmeparticles fractionated over a period of eight minutes [55].

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. 

Hydrodynamic Chromatography (HOC) - Fractionation and Sizing (packed-column HDC 0.03-2 pm, capillary HOC 0.02-50 pm)... 

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