Ultrafine particles, with a size and number density comparable to those of particulate impurities in ultrapure water, were counted individually by the proposed laser induced breakdown acoustic method. The detection conditions and counting efficiency of the particles were investigated theoretically, and the design conditions of the optical system for counting the ultrafine and ultradilute particles was obtained. Using 0.038 µm polystyrene ultrafine particles, which is one of the smallest standard particles on the market, particle counting at the number density of ultrapure water level, 102 particles per 1 ml, was demonstrated, and it was shown that the minimum detectable particle size was at least one order smaller than that of the conventional laser scattering method. The validity and applicability to in-line monitoring using a flow-type cell and size distribution measurements applying the pulse height distribution of the breakdown acoustic signals were also confirmed.

Basic characteristics of a particle detection method using laser breakdown were studied for a system of polystyrene standard particles dispersed in ultrapure water. The method was able to detect 0.02 µm particles. The detection sensitivity decreased with the particle size due to size dependence of the laser breakdown threshold. The plasma emission delay time from the laser pulse decreased with particle size (9.8±0.8 ns for 0.04 µm, 5.7±0.8 ns for 3.0 µm). The above results suggested the possibility of concentration and size measurement of fine particles in liquids by the proposed method.

The mechanism of acoustic signal generation from an ultrafine particle in liquids by laser irradiation was found to shift from the photoacoustic effect to optical breakdown of the particle as the power density of the excitation beam increased. The acoustic signal generated from a 0.085 µm polystyrene ultrafine particle in water increased discontinuously at the beam power density of 7.0 ×1010 W/cm2, corresponding to its dielectric breakdown threshold. Then, a novel method in which ultrafine particles were counted individually by counting the breakdown acoustic pulses was proposed, and its counting principle was verified using the polystyrene ultrafine particles.

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