Hydrodynamic fractionation

The elution of such gels is an example not of size exclusion but rather of hydrodynamic fractionation (HDF). However, it must be remembered that merely being able to physically fit an insoluble material through the column interstices is not the only criterion for whether the GPC/HDF analysis of an insoluble material will be successful. A well-designed HDF packing and eluant combination will often elute up to the estimated radius in Eq. (5), but adsorption can drastically limit this upper analysis radius. For example, work in our laboratory using an 8-mm-bead-diameter Polymer Laboratories aqueous GPC column for HDF found that that column could not elute 204 nM pSty particles, even though Eq. (5) estimates a critical radius of —1.5 jam. 

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]. 

The theoretical separation capabilities of Th-FFF have also been compared with those of capillary hydrodynamic fractionation (CHDF) [116]. Th-FFF was found theoretically to have the highest separation potential (also compared with SEC) and high selectivity which, however, is not fully accessible due to experimental restrictions. For CHDF, low selectivity but high efficiency as well as a very high analysis speed was predicted for samples with lower M but, experimentally, capillaries with very small tube diameters are not available in sufficient quality. In addition, such capillaries are very sensitive to clogging with minor amounts of impurities, e.g. dust. SEC was found to reach selectivities between Th-FFF and CHDF and had a high separation speed for lower molar masses M<105 g/mol. 

Our understanding of miniemulsion stability is limited by the practical difficulties encountered when attempting to measure and characterize a distribution of droplets. In fact, most of the well-known, established techniques used in the literature to characterize distributions of polymer particles in water are quite invasive and generally rely upon sample dilution (as in dynamic and static laser light scattering), and/or shear (as in capillary hydrodynamic fractionation), both of which are very likely to alter or destroy the sensitive equihbrium upon which a miniemulsion is based. Good results have been obtained by indirect techniques that do not need dilution, such as soap titration [125], SANS measurements[126] or turbidity and surface tension measurements [127]. Nevertheless, a substantial amount of experimental evidence has been collected, that has enabled us to estabhsh the effects of different amounts of surfactant and costabihzer, or different costabilizer structures, on stabihty. 

Sasaki S. and Nakazawa K. (1988) Origin of isotopic fractionation of terrestrial Xe hydrodynamic fractionation during escape of the primordial H2-He atmosphere. Earth Planet. Set Lett. 89, 323-334. 

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 measurement of particle size and molar mass distributions are typically carried out off-line with relatively expensive instruments. Techniques used for particle size include transmission electron microscopy, photon correlation spectroscopy, or capillary hydrodynamic fractionation and molar mass measurement with GPC. 

Silebi CA, Dosramos JG. Axial dispersion of submicron particles in capillary hydrodynamic fractionation. AIChE J 1989 35 1351-1364. 

Silebi CA, Dosramos JG. Separation of submicrometer particles by capillary hydrodynamic fractionation (CHDF). J Colloid Interface Sci 1989 130 14-24. 

In the particular case of a bimodal particle size distribution where the second size is very small, detection can be difficult This is illustrated by the data in Table 12.13, which shows that the bimodal characteristic was measured by transmission electron microscopy (TEM), capillary hydrodynamic fractionation (FlowSizer), and by SFFF, but not by quasi-elastic light scattering (NICOMP 270 or Brookhaven B 1-90). While QELS (or PCS) instruments are capable of... 

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 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. 

CHDF (capillary hydrodynamic fractionation) Semibatch emulsion polymerization of styrene, and VAc/BA [49, 123] PSD directly measured/Invasive, dilution or sampling loop required, non-robust for industrial environment, time delay Emulsion polymerization... 

Venkatesan and Silebi [6] used capillary hydrodynamic fractionation to monitor an emulsion polymerisation of styrene monomer as a model system. A sample taken from the reactor at different time intervals is injected into the capillary hydrodynamic fractionation system to follow the evolution of the particle size distribution of the polymer particles formed in the emulsion polymerisation. After the colloidal particles have been fractionated by capillary hydrodynamic fractionation they pass through a photodiode array detector which measures the turbidity at a number of wavelengths instantaneously, thereby enabling the utilisation of turbidimetric methods to determine the particle size distribution. The particle size measurement is not hindered by the presence of monomer-swollen particles. The shrinkage effect due to the monomer swelling phenomenon is found to be accurately reflected in the particle size measurements. 

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