The resistive (Coulter) method of counting and sizing particles in a conducting fluid has been extended to polystyrene spheres 900 Å in diameter, with a present detection limit near 600 Å, through the use of individual submicron pores etched in irradiated plastic sheet. The use of a nonionic surfactant and ultrasonic cleaning effectively relieves the problem of plugging. The particles may be driven through the pore by the electric field, without the use of pressure, to yield the vector sum of the electrophoretic and electro-osmotic velocities. A new theory, yielding an upper limit to the resistive pulse on passage of a sphere, agrees well with data for spheres with diameters d<0.4D, where D is the pore diameter, and complements a previous theory that gives a lower limit, valid for d>0.9D. We estimate that a detection limit near 250 Å will be attainable with the further development of current techniques.
Viruses above about 60 nm in diameter may be rapidly sized to a few nanometers in their natural hydrated state as they pass one by one through a single pore in a newly developed nanometer-particle analyzer based on the resistive-pulse technique of the Coulter Counter and the use of submicrondiameter pores made by the Nuclepore process. Size measurements for several type C oncornaviruses are: Rauscher murine leukemia, 122.3 + 2 nm; simian sarcoma, 109.7 + 3 nm; Mason-Pfizer monkey, 140.0 + 2.5 nm; RD-114, 115 ± 5 nm; and feline leukemia, 127.4 + 2 nm, relative to standard 109-nm latex spheres. The T2 bacteriophage has a volume of (5.10 + 0.15) x 10-16 cm3. Concentrations of viruses near 109 to 101/ml that are fairly clear of debris are routinely measurable in a few minutes to an accuracy near 15%. A lower practical count limit is near 5 x 107 viruses per ml.
A method for measuring the resistivity of metallic specimens is described. The measurement is made by noting the rate of decay of flux from a bar situated in an external magnetic field that has been rapidly reduce to zero. The method is suitable for specimens greater than 5×10−3 cm in diameter. For a specimen 1 cm in diameter, resistivities from 10−11 to 10−3 ohm-cm may be measured with an error of less than three percent. The method requires no contact to the specimen, and local values of resistivity may be measured. Several applications are described.
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