Electrical resonance detection of particles and analytes in microfluidic channels
A conductivity counter and method of determining conductivity of a fluid sample are disclosed. The counter is suitable for high-speed, accurate counting of discrete events or items, such as cancer cells, passing through a fluid sample cell. A variable frequency current source is used to supply an excitation current to a sample cell connected in parallel with an inductance or the electrical equivalence of an inductance. This configuration can be accurately modeled as a parallel RLC circuit when the system is operated at a stable frequency. The current source frequency is tuned to the resonance frequency of the equivalent RLC circuit, which effectively eliminates the capacitive and inductive components of the impedance, leaving only purely resistive components. The output signal is due to perturbations in the fluid sample, such as passing cancer cells.
1. A method of counting small objects contained in a fluid sample, comprising:
a. filling a sample cell with a buffer solution, the sample cell being connected in parallel with an inductance and forming an equivalent RLC circuit;
b. applying a variable frequency current source to the sample cell and inductance;
c. tuning the current source frequency to the resonance frequency of the equivalent RLC circuit;
d. nulling the output of the equivalent RLC circuit;
e. initiating a flow of test fluid through the sample cell, the test fluid containing the objects to be counted; and
f. detecting changes in the conductivity of the sample cell as the objects pass through the sample cell.
2. The method of claim 1 , wherein the step of tuning the current source frequency to the resonance frequency of the equivalent RLC circuit further comprises monitoring the voltage amplitude of the output of the equivalent RLC circuit while the current source frequency is varied and detecting the frequency at which the voltage amplitude is at a maximum, such frequency being the resonance frequency of the equivalent RLC circuit.
3. The method of claim 1 , wherein the step of nulling the output of the equivalent RLC circuit further comprises adjusting a background solution offset circuit such that the output of said circuit is approximately zero when only the buffer solution is in the sample cell.
4. The method of claim 3 , wherein the step of detecting changes in the conductivity of the sample cell as the objects pass through the sample cell further comprises amplifying the output signal of the background solution offset circuit while the test fluid flows through the sample cell.