(70du) Development and Uncertainty Evaluation of an Accurate Method for Counting Liquidborne Particles for Establishment of Particle Number Concentration Standards

National or international standards of particle number concentration in liquids that can be used to calibrate liquidborne particle counters has not yet been established. In the present study, a new high accuracy particle counting technique has been developed for the purpose of establishing the primary standard of particle concentration in liquids. And the uncertainty evaluation of this method was executed.

In order to count liquidborne particles with high accuracy, a method to distinguish particles from bubbles is required. We employed a flow cytometer for this purpose, and sample particles doped with fluorescence dye were used to examine the generation rate of bubbles. While both scattered light and fluorescence are detected from dye-doped particles, only scattered light is detected from bubbles, and hence we can count particles distinguishably from bubbles. With this method, conditions of sample preparation for particle counting under which the generation rate of bubbles was reduced were searched. In case of 10 µm diameter particles, a condition under which the concentration of bubbles relative to that of particles was less than 2.01 % was established.

The variation of particle number concentration caused by sampling of solutions was evaluated. Particle precipitation in solutions causes a concentration distribution. To disperse particles homogeneously, the container of the sample solution was irradiated with ultrasonic wave. Solutions in test tube each having 2 ml or 1 ml volume was then sampled from the container. While particle counting was carried out, particles still precipitate and cause the fluctuation of density distribution. In order to avoid counting errors due to particle precipitation in test tubes, all the particles contained in each test tube were counted. The variation in particle number among the test tubes was thus evaluated. In order to reduce bubbles in the tubes, the irradiation was carried out in vacuum.

The total uncertainty of the particle number concentration measurement was evaluated by combining three uncertainty components; the variation due to sampling, the effect of bubbles, and the uncertainty of measuring the volume of a solution in a test tube. The expanded uncertainty of 5.45 % (k = 2) was established.

In order to examine the validity of the uncertainty of particle counting by the flow cytometer method, the particle counts obtained by this method was compared with those by the microscopic method. The entire solution that passed through the flow cytometer was filtered through an isopore membrane filter, which had 400 nm pore size. Particles were collected onto a circular area of 4 mm diameter. Total number of particles on the filter was counted to prevent sampling errors, because the number of particles was not large enough to avoid stochastic variations. In case of 10 µm particles, the difference in counts by the flow cytometer method and the microscope method thus found was about 5 % in average.