Conclusion

HDC is a complementary method for particle characterization in the micron size range. It is rapid, low cost, easy to carry out and very sensitive. It allows separation and size measurement of organic colloids, up =

soft and rigid polymers and various materials (spheric or elongated) in about 10 min, without special sample preparation. Moreover, it has been the subject of many theoretical studies, assuming different mechanism models and explaining the effect of parameters on separation. Effort has been directed to column design, detection sensitivity and fit of chromatogram for PSD. Some conditions must be obeyed to obtain reliable results, whatever columns are used. Firstly, there is the need for accurate calibration, with frequent adjustment of parameters and choice of a column set giving a low slope. Secondly, there is the need to adjust the detector response factor (exponent n of diameter) to the sample and to the chosen detector. Thirdly, axial dispersion must be corrected using a simple equation. As a consequence of the combined analysis of the effects of n and Pe, the proposed value for exponent n was 3.5 with UV detection and a Peclet number of 500. Results have been compared on eluted fractions with those of direct methods. With proper use of HDC interpretation parameters, PSD and average diameters are in agreement with those obtained by other methods, for instance photon correlation spectroscopy and sedimentometry, this last technique has high resolution as its main characteristic.

In packed columns, packing size, ionic strength, I, nature and amount of surfactant have very large effects on RF and other elution parameters, such as percentage recovery, which decreases for large particles. RF decreases when I or packing size or amount of surfactant increases. Porous packings may add a SEC separation effect, enhancing RF. Improvements in packing material (5 |im) and methodology tend to use a 0.5 m long column.

In capillary HDC, the effect of additives is low; surfactants are the most useful. The separation factor RF is fairly constant in water but is strongly affected by flow rate in methanol. The high number of plates in tetrahydrofuran is not accompanied by a high maximum RF value. This separation factor may be higher with a 60 m than with a 120 m column, but the resolution, RS, is not so high. This emphasizes the fact that RF, RS and N may vary independently.

Micro-equipment (diameter of the capillary in the micron range, short column (10 m), on-column injection and detection) enhances resolution and RF as it lowers axial dispersion. Peak capacity remains limited at about 10, but for rheological properties and size range, this is an interesting alternative to packed columns.

See also: I/Particle Size Separation. N/Chromatography: Liquid: Mechanisms: Size Exclusion Chromatography. Particle Size Separation: Electrostatic Precipitation;

Field Flow Fractionation: Thermal. III/Polymers: Field

Flow Fractionation. Proteins: Field Flow Fractionation.

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