Fractionating Sizing Techniques

Fractionating sizing techniques combine a size-related classification process with the measurement of particle quantities. The classification may yield a physical separation of difl erently sized particles (e.g. by sieving, cf. Fig. 2.1) or it may successively deplete the disperse system of the coarsest or finest particles (e.g. in a sedimentation column). Accordingly, the measured quantities, which can be absolute amounts or concentration values, represent either a density or a cumulative function of the size distribution. Anyhow, the type of quantification (e.g. weighing) determines the type of quantity of the measured size distribution (e.g. mass), whereas the classification defines the probed particle property. The classification process should be ideal (i.e. with maximum selectivity) and well-defined by a monotone correlation between the particle property (e.g. settling velocity) and the 

This section addresses two basic principles of classification—sedimentation and field-flow fractionation—and the corresponding sizing techniques. Additionally, a chromatographic technique is briefly introduced. The focus lies on sedimentation or centrifugation analysis, which corresponds to its practical relevance for the characterisation of coUoidal suspensions. 

Analytical sedimentation counts among the most popular techniques of particle sizing. To a major extent, this can be attributed to the good comprehensibility of the employed classification principle Isolated particles settle in a quiescent liquid with a stationary velocity, which solely depends on their individual size, shape, and density. The corresponding changes in local particle concentration, which are observed along the setding path and over time, therefore reflect the size distribution of the particle system (Edelmaim 1962, pp. 74—86 Bernhardt 2010). 

In order to quantify the changes in the local particle concentration, several techniques can be employed. For instance, one can quantify the growth rate of the sediment by means of a sedimentation balance. This indirect way of concentration measurement was proposed by Oden (1916) in one of the earliest papers on analytical sedimentation. In contrast, modem analytical centrifuges directly probe the concentration in the suspension phase. Four important principles of detection are discussed below. 

The extinction of light is a characteristic behaviour of particle systems, which is governed by particle size and concentration—apart from material properties. For dilute, weakly scattering particle systems, the extinction obeys Lambert-Beer law, which states a linear dependency between turbidity r and particle concentration  

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The sieve test is probably still the most commonly used sizing technique both to characterize particulate distributions and separate them into fractions on the... 

Techniques which seem less suitable for routine size analysis are (1) analytical ultracentrifugation combined with a Schlieren optical system (Mason and Huang, 1978 Weder and Zumbuehl, 1984) (2) the sedimentation field flow fractionation (SFFF) technique to separate heterogeneous dispersions (e.g., Kirkland et al., 1982). 

Supercritical C02 extraction coupled with a fractional separation technique is used by producers of flavours and fragrances to separate and purify volatile flavour and fragrance concentrates. Like any solvent, supercritical C02, it allows processing chemicals by predpita-tion or recrystallisation, obtaining partides of controlled size and shape, without excessive fines without thermal stresses and controlling the shape of a polymorphic substance. 

A method for on-line monitoring of particle size distribution and volume fraction in real time using frequency domain photon migration measurements (FDPM) has been described. In FDPM the time dependence of the propagation of multiply scattered light provides measurement of particle size distribution and volume fraction. The technique has been applied to a polystyrene latex and a titanium dioxide sluny at volume concentrations in the range 0.3 to 1% [341]. 

A further means of obtaining information on the distribution of molecular weights in povidone is fractionation. This technique is very imprecise and gives only the proportions above and below a particular molecular weight. It is based on the difference in solubility of molecules of different sizes in certain solvents and their mixtures, e.g. water and isopropanol or ether. 

An integrated module for on-line sample pre-analytical treatment and/or clean-up could be also included in the device. With this respect, field-flow fractionation (FFF) techniques, which can separate analytes based on their morphological characteristics (size, shape and superficial properties) can be exploited to develop pre-analytical modules for cells or macromolecules (e.g., proteins, protein complexes or adducts) fractionation, thus providing a selectively enriched fraction for the analysis. 

FFF methods are certainly developing into important techniques for colloidal particles. They compare well with other particle sizing techniques for resolution and accuracy, and usually calibration standards are not necessary. The fact that separation is employed helps in obtaining high quality particle size data for polydisperse samples and enables fractions to be examined using other physical and analytical techniques. A drawback is the fact that the separation is truly microscale, with typically 10-100 pg being processed in a single run. 

Here, we treat the case of PSD analysis of particulate systems of micron-size range (i.e., with a size distribution extending above 1 xm). Since 1994, in our laboratories, this topic has been dealt with by means of a low-costsubset of sedimentation FFF (SdFFF), the gravitational field-flow fractionation (GrFFF) technique. GrFFF had aheady... 

Yamazaki H, Nishiyama K, Tanaka E et al (2006) Radiotherapy for early glottic carcinoma (TlNOMO) results of prospective randomized study of radiation fraction size and overall treatment time. Int J Radiat Oncol Biol Phys 64 77-82 Yaromina A, Krause M, Thames H et al (2007) Pre-treatment number of clonogenic cells and their radiosensitivity are major determinants of local tumour control after fractionated irradiation. Radiother Oncol 83 304-310 Zhang T, Lu W, Olivera GH et al (2007) Breathing-synchronized delivery a potential four-dimensional tomother-apy treatment technique. Int J Radiat Oncol Biol Phys 68 1572-1578... 

Fractionating (ensemble) techniques include the two steps of fractionation and detection. The former can either result in a physical separation of the different size classes or in the depletion of coarse or fine particles in the measurement zone. In the case of colloidal suspensions, the fractionating effect is usually related to the mobility of the particles (e.g. settling velocity). The detection system monitors the fractionation process and, thus, serves for evaluating the class frequencies. It frequently employs the phase shift, extinction, or scattering of some radiation (e.g. X-rays). The applied detection system determines the type of quantity in which the size fractions are intrinsically weighted (e.g. extinction of X-rays is mass proportional Q. ... 

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