Light Obscuration
In the SPOS technique, particles in liquid suspension flow through a photozone - a narrow, rectangular region of uniform light produced by light from a laser.
The particle suspension must be sufficiently dilute so that the particles pass one at a time through the illuminated region, avoiding coincidences. This is accomplished by manual predilution or an automated dilution process. As a particle passes through the photozone (sensing zone), light is either absorbed or refracted due to the physical presence of the particle or it can be scattered at some oblique angle. The magnitude of this pulse is dependent on the cross-sectional area of the particle and the physical principle of detection – either light scattering (LS) or light blocking. Light blockage is often referred to as light obscuration or light extinction (LE). Light obscuration allows for high resolution particle sizing and counting down to 1 micron. Below 1 micron light scattering is the necessary mode of detection wherein the light scattered by smaller submicron particles is detected and particle size is extrapolated. The SPOS technique used in the Accusizer 780 line of instruments employs a patented LE+LS dual detection system that allows for single particle sizing and counting down to 0.5 microns (Figure A). The illumination and detection system in the sensor is designed to provide a monotonic increase in pulse height with increasing particle diameter. The image below displays the pulses that occur when particles pass through the sensor over time (Figure B). As each successive particle passes through the sensor, a particle size distribution is created by comparing the detected pulse heights with a standard calibration curve, obtained from a set of uniform particles of known diameters (Figure C). A schematic diagram of the SPOS methodology is shown in Figure D.
Figure A Schematic of patented LE+LS technique. S and E are the scattering and extinction detectors respectively.
Figure B SPOS diagram of pulse height vs time
Figure C SPOS sensor calibration curve 0.5 to 400 microns
Figure D Schematic diagram of SPOS technique
The SPOS method is in stark contrast to the conventional ensemble particle sizing techniques (i.e. light scattering) which measure all particles in the sample at the same time. Instruments that perform particle size analysis using ensemble techniques are inherently limited in accuracy and resolution since the raw detected signal is “inverted” mathematically in order to estimate the particle size distribution. They assume a shape for the particle size distribution (usually Gaussian) using a small number of parameters, and then work toward minimizing the error between the measured data and a calculated fit. This can create inaccuracies in the particle size distribution due to limitations imposed by the imperfect signal/noise ratio of the raw data and the nature of the deconvolution algorithm. However, where ensemble methods are limited in accuracy and resolution, the SPOS method yields a more powerful technique for particle size analysis by building the particle size distribution one particle at a time. The use of a single particle optical sizing method offers the intrinsic advantages of high resolution, large dynamic range and excellent accuracy, allowing one to measure small, subtle features of the PSD that cannot be detected using ensemble techniques.