Dry sieving method

The best method of the determination of the particle size distribution of the different crospovidone types is the light diffraction measurement (e.g. Malvern Master Sizer, see Fig. 63 and 64). The particle size distribution of normal crospovidone also can be measured by sieving or air jet screening. In this dry sieving method more than 50 % of the particles are coarser than 50 pm without any swelling. Therefore even with this dry method it corresponds clearly to the definition of Type A of the Ph.Eur. monograph. 

Generally, the dry sieving method yields a relatively larger value of pore size than that obtained from either wet sieving or hydrodynamic sieving (Faure et al., 1986 Van der Sluys and Dierickx, 1991 Bhatia et al., 1994). This circumstance must be taken into account when design retention criteria are used. 

Dry sieveing is an alternative method if wet sieving (Section 3.2.3.1) cannot be used, as is the case of seasonal streams in Mediterranean countries. 

Wet sieving is recommended whenever it is possible. Instructions for sampling with wet sieving method are presented in section 3.2.3.1. If it is not possible to wet sieve the stream sediment sample in the field, the collected stream sediment material should be dry sieved. Instructions for sampling and dry sieving are given in section 3.2.3.2. 

The different methods of particle size analysis can be grouped into several categories size range analyses (sieve methods), wet or dry methods, and manual or automatic methods (laser-light). All these methods are described elsewhere. 

Average soil sample was collected from live different points (method of diagonals), was dried, sieved through 2 mm sieve and homogenized. All reagents and solvents were of analytical reagent grade (Merck). The used analytical vessels were rinsed with hexane before use. A soil sample (10 g) was spiked with PCB 30 and PCB 204 (0.25 pg each). Another aliquot of the sample (10 g) was spiked with OCPs - y-HCH (0.05 pg), ppDDE (0.2 pg) and pp-DDT (0.45 pg). The sample was covered with 10 g anhydrous sodium sulfate and extracted (18 hours) in Soxhlet apparatus with acetone/hexane (1 /1). 

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