PARTICLE ANALYSIS EQUIPMENT

2.3.1 Light diffraction

The Fraunhofer diffraction Malvern 3600E (Malvern Instruments Ltd., Malvern, Worcs.) produced a 2 mW He-Ne laser beam with a wavelength of 0.6328 jxm which was expanded to 9 mm (section 1.2.3). The particulate suspension was contained in a magnetically stirred cell having two quartz glass windows. Diffracted light was focused on to a 31 concentric ring detector. A photodiode behind a hole in the centre measured the transmitted light intensity and was used during alignment. The maximum angle of diffraction detectable was approximately 11°. The lens with the smallest focal length of 63 mm was installed corresponding to a size range of 1.2-118.4 /xm evaluated by the model independent algorithm.

The sample was required to be diluted, limited by there being sufficient light diffracted to make the effects of optical noise insignificant and that multiple scattering did not occur. This was continually indicated by the instrument.

O b s c u r a t i o n = 1 - I n t e n s i t y p a r t i c l e s p r e s e n t L i g h t i n t e n s i t y a b s e n c e o f p a r t i c l e s

[ 2 . 3 ]

Following a background measurement the suspension was added into the cell until the obscuration was about 0.3 and definitely did not exceed 0.5.

The buffer was again 0.07 M sodium acetate solution at pH 4.8. However it was prepared in 20 1 batches using Analar grade NaOH in deionised water. The buffer was stored for a couple of days to allow air introduced whilst mixing to escape. It was filtered under gravity when needed by Sartobran capsules (Sartorius AG, Gottingen, Germany) having a 0.2 /xm pore diameter and < 50 m^ filtration area.

Alternatively 0.1 ixm membrane filters (Whatman Laboratory Products Ltd., Maidstone, Kent) were used for the batch. A variable speed peristaltic pump (503S/R, Watson-Marlow Ltd., Falmouth, Cornwall) was operated at a flow rate of 20 ml/min owing to the high pressure. The sample cell was washed with filtered deionised water. Before every run the windows were cleaned by alcohol.

2.3.2 Electrical sensing zone

Particle size and concentration were determined using the Elzone 280 PC (Particle Data Ltd., Hereford). Either a 18 or 30 ixm orifice tube was fitted though 12 and 24 /xm orifices were also tested (section 1.2.3). There was a 20 /xl volumetric piece. In addition to sodium acetate electrolyte considered in section 2.3.1, 5% w/v sodium chloride was prepared in deionised water and 0 .1 /xm filtered for some latex samples (Particle Data Ltd. and Poly sciences Ltd., Northampton).

The 18 /xm tube was calibrated with 1.09 and 2.02 /xm monodisperse latex and the 30 /xm tube with 2.02 and 10.09 /xm latex. The standards had been refrigerated and briefly ultrasonicated. Three measurements were performed for 20 s in the relevant buffer, at a controlled vacuum pressure of 160 mm Hg. Each was preceded by gentle agitation with a glass stirring device; the entrainment of bubbles that may otherwise have been counted was minimised. The state of the orifice was constantly monitored using a microscope attachment. The optimum particle concentration resulted in an average count of 22,000 for the 18 /xm tube and 5600 for the 30 /xm tube. This ensured the coincidence level was less than 1 %. 128 size channels varied logarithmically having an increasing interval. A low trigger level eliminated electrical noise. The analysis was corrected for the mean background distribution. Concentrations were measured on a volumetric rather than a time basis. Special attention was given to the cleanliness of the system which was occasionally flushed with dilute detergent or 1.5 M HNO3.

2.3.3 Disc centrifuge sedimentometer

The density difference between particles and the suspending fluid was obtained using a Brookhaven DCP-1000 (Brookhaven Instruments Corporation, New York, USA) supplied by Laser Lines Ltd., Banbury, Oxon. The centrifugal disc speed was selected from 500 to 15,000 rpm. It was monitored by a digital tachometer. The disc had been balanced over the full range to minimise vibration. Light from a tungsten-halogen lamp was collimated with two slit assemblies and measured by a silicon photodiode. A fan cooled the transmission and detector assembly reducing heat transfer to the sedimentation fluid. Parameters such as the baseline were varied manually. A modelling facility aided the choice of the operating conditions.

The preferred line start method for stable sedimentation was creating an external gradient (Holsworth et al., 1987). 15 ml of the spin fluid, filtered deionised water was drawn vertically into a syringe then 1 ml of less dense methanol, the buffer fluid. The syringe was maintained upright whilst steadily loading the rotating disc through a piece of flexible tubing. This was simpler than pumping from two identical wells into a glass holding tube (Mannweiler, 1989). The buffered line start technique had also been used by injecting spin and buffer fluids separately, followed with momentary acceleration of the disc applying cut and boost controls (Titchener, 1987). A thin layer of approximately 0.5 ml nonvolatile dodecane was introduced at higher speeds. The sample was rapidly injected on to the surface layer. The possibility of streaming was observed through the transparent disc using a built-in stroboscope. The spin time lasted for up to 120 min during which the temperature recorded by a probe was as much as 28°C. Finally the disc was washed with another syringe.

2.4 EQUIPMENT FOR ON-LINE MONITORING AND CONTROL OF