Electricity: measuring and testing – Determining nonelectric properties by measuring electric... – Particle counting
Reexamination Certificate
1998-07-01
2001-01-16
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
Particle counting
C324S446000, C073S061730, C073S861410
Reexamination Certificate
active
06175227
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improvements in methods and apparatus for sensing and characterizing small particles, such as blood cells or ceramic powders, suspended in a liquid medium having an electrical impedance per unit volume which differs from that of the particles. More particularly, it relates to improvements in methods and apparatus for sensing and characterizing such particles by the Coulter principle.
2. Discussion of the Prior Art
U.S. Pat. No. 2,656,508 to Wallace H. Coulter discloses a seminal method for sensing particles suspended in a liquid medium. An exemplary apparatus for implamenting such method comprises a dual-compartment dielectric vessel which defines first and second compartments separated by a dielectric wall. Each of the compartments is adapted to contain, and is filled with, a liquid medium. The particles to be sensed and characterized are suspended at an appropriate concentration in the liquid medium and introduced into one compartment through a suitable inlet port formed therein. The separating wall is provided with a relatively large opening which is sealed by a thin wafer made of a homogeneous dielectric material. A small through-hole formed in the wafer provides a conduit, which constitutes the only operative connection between the two compartments. An appropriate vacuum applied to an outlet port suitably formed in the second compartment causes the particle suspension to flow from the first compartment to the second compartment through said conduit, discussed in detail below. Each particle in the suspension displaces its own volume of the particle-suspending liquid, and the conduit provides a consistent reference volume against which that displaced volume may be compared. If the dimensions of the conduit and the concentration of particles in the suspension are appropriately selected, particles can be made to transit the conduit more or less individually. The conduit thus functions as a miniature volumeter, capable under suitable conditions of making sensible the liquid displaced by individual microscopic particles.
To enable convenient sensing of the liquid displacement occasioned by particles transiting the conduit, the particle-suspending liquid is made to have an electrical impedance per unit volume which differs from that of the particles. The contrast in electrical impedance between particle and suspending liquid thus converts the volume of displaced liquid into a proportional change in the electrical impedance of the liquid column filling the conduit. An excitation electrode is positioned in each of the two compartments and operatively connected to a source of electrical current, whereby a nominal electrical current (the excitation current) is caused to flow through the conduit simultaneously with the particle suspension. Consequently, passage of a particle through the conduit produces a pulsation in the current flowing through the conduit which is proportional to the volume of liquid displaced by the particle. An extensive art has developed whereby such particle pulsations may be sensed and monitored to provide particle-characterization information. This art has taken two forms, the first based on the original two-terminal sensing approach described in the '508 patent and the second based on four-terminal, potential-sensing approaches. The second form of Coulter art evolved from the first and shares similar limitations; both forms will be discussed.
In the '508 patent the excitation current is applied from a voltage source through the two electrodes immersed in the suspending liquid of the two compartments interconnected by the conduit. An AC-coupled sensing circuit, also operatively connected to the excitation electrodes, operates to sense the pulsations in current between these electrodes. Thus, as individual particles pass through the conduit, said sensing circuit produces an electrical signal pulse having an amplitude which is characteristic of the particle volume. Additional circuits further process the particle signal pulses to provide a count of particles exceeding some particular volumetric threshold or, via the elegant positive-displacement metering system disclosed in U.S. Pat. No. 2,869,078 to Wallace H. Coulter and Joseph R. Coulter, Jr., the particle concentration. The volumetric distribution of the particles may be conveniently characterized by causing the current source to provide a constant current and analyzing the particle pulses with multiple-thresholding sizing circuitry as described in U.S. Pat. No. 3,259,842 to Wallace H. Coulter et al. Alternatively, if the current source is caused to provide combinations of electrode excitation, including at least one source of high-frequency alternating current as discussed in U.S. Pat. Nos. 3,502,973 and 3,502,974 to Wallace H. Coulter and W. R. Hogg, an apparent volume reflecting the internal composition of certain particles may be similarly characterized. Such characterization results are displayed or recorded by appropriate devices. This method of sensing and characterizing particles, by suspending them in a liquid medium having an electrical impedance per unit volume which differs from that of the particles and passing the resulting particle suspension through a constricting conduit while monitoring the electrical current flow through the conduit, has become known as the Coulter principle. Because of their simplicity, the two-terminal sensing methods were the only ones in use for many years and still see exclusive use in commercially available apparatus incorporating the Coulter principle.
Central to the Coulter principle is the volumeter conduit which enables electrical sensing of particle characteristics by constricting both the electric and hydrodynamic fields established in the dual-compartment vessel. Due to their excellent dielectric and mechanical properties, ruby or sapphire jewels developed as antifriction bearings for precision mechanical devices were indicated for use as conduit wafers in U.S. Pat. Nos. 2,985,830 and 3,122,431 to Wallace H. Coulter et al. As shown in the enlarged longitudinal section of a traditional Coulter conduit wafer W in
FIG. 1
, a Coulter volumeter conduit comprises a continuous surface or wall
30
of length L which defines a right cylindrical opening of circular cross-section and diameter D through a homogeneous dielectric material of thickness L. (Conduit wafer W is often called an “aperture wafer”, and volumeter conduit
10
in conduit wafer W is commonly referred to as a “Coulter aperture”.) Due to material homogeneity, the electrical resistivity of conduit wall
30
surrounding the flows of particle suspension and current through the conduit is substantially axisymmetric and uniform in any longitudinal conduit section.
In practice,
FIG. 1
conduit diameter D must approach twice the maximum particle diameter to minimize risk of clogging, and conduit length L is usually made as short as possible to minimize coincidence artifacts due to two or more particles simultaneously transiting the conduit. For many medical and scientific applications, conduit diameter D ranges between approximately 0.030 mm and 0.200 mm, and the conduit length-to-diameter ratio UD ranges between 0.75 to 1.2. With these small conduits, physical clogging may limit application, especially to samples of biological origin, while their limited dynamic range in particle size may limit application where polydisperse industrial samples are involved. Critical applications can benefit from splitting such samples through a plurality of conduits and appropriately processing the multiple data streams. Parallel-conduit systems typically comprise a single entry compartment containing a first electrode, but provide each conduit with an electrically isolated exit compartment containing an individual second electrode, with all conduits being simultaneously transited by parcel streams split from the sample suspension in the entry compartment. As taught by Wallace H. Coulter and W. R. Hogg, the several conduits ma
Britton Ted
Dunstan Harvey J.
Graham Gerry
Graham Marshall D.
Harfield John Geoffrey
Alter Mitchell E.
Brown Glenn W.
Coulter International Corp.
Kurz Warren W.
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