by Robert P. Donovan
 |
There are a number of differences between light scattering in a gas and light scattering in a liquid, such as the differing indices of refraction that characterize the two media.
The major difference in the design of light-scattering instruments for measuring particles in these two media is that the fluid properties of a liquid do not allow sharp focussing and directing of the particle streams, while such focussing is practical in a gas.
The particles in a liquid stream remain dispersed throughout the liquid, constrained only by the walls of the tubes or ducts through which the sample liquid flows. Hence, focussing the particles entrained in a liquid stream into a small volume for optical detection is not a design option, as it is in the design of aerosol measuring instruments.
This limitation means that illuminating the entire cross section of the particle-containing stream, as in the aerosol instrument designs, will inevitably introduce a light scattering contribution to the detector that originates from the wall-liquid interface rather than the entrained particles whose concentration is being measured. This non-particle-generated signal raises the background noise level of the detector, thereby raising its minimum detectable particle size.
While the particle spatial distribution in a liquid stream can not be effectively controlled, what can be controlled is the size and shape of the primary light beam used to illuminate the particles in the liquid stream. A designer can focus the primary laser beam to illuminate an area smaller than the cross section of the particle beam but away from the wall. This arrangement is the opposite of that typical of aerosol instruments in which the particle beam is focussed into a cross-sectional area smaller than that of the laser beam.
Thus, two design choices exist for light-scattering instruments that measure liquid-borne particles: 1.) Design the counter to have a laser beam width broad enough to illuminate the entire cross section of the liquid stream, including the wall-liquid interface; or 2.) Focus the laser beam into a narrow cross-section, which collects light scattered from only a fraction of the total particle stream but minimizes or eliminates the collection of light scattered from the wall-liquid interface.
One major manufacturer of optical particle instruments calls Design #1 a volumetric instrument and Design #2, an in situ instrument1. By this designation, aerosol instruments, with their aerodynamically focused particle beams, are all volumetric designs.
The intense light beams required for in situ designs have a Gaussian spatial distribution in intensity, meaning that a particle passing through the center of the beam scatters a higher-intensity signal than when passing through the edge of the beam. To account for this potentially confounding property, spectrometers or counters based on in situ designs use two detectors, one apertured, which, by comparing signals, enables rejection of signals originating from particles not in the “preferred” region of the beam.
Monitors count all particle signals regardless of where they cross the Gaussian beam. Large particles crossing the beam edge get counted as small particles and small particles passing through the same beam edge don't get counted at all. This mode of operation introduces errors in both particle counting and sizing; but, when measuring particle concentrations in clean fluids such as ultrapure water, monitors and spectrometers agree reasonably well1. For less-clean liquids, however, spectrometers yield more accurate measurements2. Nonetheless, monitors are now the light-scattering instrument design most commonly used for measuring liquid-borne particles.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM. His Sandia project work is developing technology for recycling spent rinse waters from semiconductor wet benches.
References
- Knollenberg, R. G. and D. L. Veal, “Optical Particle Monitors, Counters and Spectrometers: Performance Characterization, Comparison and Use,” 1992 SPWCC Proceedings, pp. 197 – 240; 1991 Proceedings of the Institute of Environmental Sciences, pp. 751 – 771.
- Fardi, B., “An Evaluation of Performance of High Sensitivity DI Water Particle Counters,” 1992 Proceedings of the Institute of Environmental Sciences, pp. 431 – 437.