Advantages and Limitations

The main advantages of HDC are that it is a rapid and convenient method for the separation of particles. It allows one to obtain a fingerprint of the size distribution with an easy-to-operate instrument, similar to those used in liquid chromatography, at room temperature. Direct analysis of the original colloid medium and the use of high dilutions avoid modification of a sample which may be observed in a dry state (e.g. shrinking under the beam in transmission electron microscopy), and any effect of interactions.

Existence of a unique, universal calibration curve allows calculation of Dp and PSD for any sample, since there is no effect of sample nature, surface charge and density on elution volume. At low ionic strength, no effect has been found of the chemical nature of the sample for vinyl copolymer colloids, even for those of low glass transition temperature, Tg. On the other hand, at high ionic strength, sample chemistry may present an additional parameter for separation. With this technique, there are none of the limitations encountered in liquid chromatography (solvent nature, stability and availability of stationary phase and temperature range).

However, there are some difficulties related to the proper choice of operating variables. Elution has been shown to depend on size and porosity of packing beads, eluent flow rate, ionic strength, pH and additives such as surfactants. It is evident that column length and diameter play a role on plate numbers, resolution and domain of measurement. Particle size may affect the total recovery of material (with packed columns). Moreover, it may act on the detector response. In consequence, the PSD might be affected by the incomplete recovery of particles, due to adsorption effects, mainly for larger particles. For example, total recovery is observed up to 200 nm only with 20 |im packing.

Intrinsic limitations include a low plate number, N, and low resolution, RS, so that generally the number of peaks (peak capacity p) in a chromatogram is low (about 5-10). Quantitative interpretation for determination of particle size distribution needs calibration in order to establish correspondence between sample size and elution volume and the relationship between signal intensity and amount of particles. Moreover, band broadening, common to every chromatographic process, has a larger influence on PSD because of the low resolution. The interpretation of data assumes that particles are spherical, although an equivalence has been found for elongated structures (1 |im particles appear as spheres of 0.153 |im diameter). Finally, soft materials may be deformed under the high rate of shear in packed column and orientation in flow may affect the apparent size.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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