A Coulter Counter counts and sizes particles suspended in an electrolyte fluid, based on changing electrical properties. The technique does not rely on optical sources or sensors.The particles are pumped through a narrow aperture separating two electrodes, between which a weak electric current flows. The voltage across the aperture is sensitive to changes in impedance, which is proportional to the volume of particles that pass through the gap. Variant techniques that utilize the Coulter principle include Scanning ION Occlusion Sensing and CASY.
The Ken Johnson lab is developing a small Coulter Counter for potential use on profiling floats, buoys, and other small platforms.
WHOI developed an in-situ Coulter Counter in the mid-1960s for use at-sea.
Marie et al have pointed out some drawbacks to the Coulter technique:
- Difficult to quantify particles less than 1-2 microns (i.e. cannot be used for picoplankton)
- Cannot distinguish between cells and other detritus or air bubbles
- Cannot discriminate between similarly-sized microbe species
What MBARI science applications could benefit even with the above limitations?
The Millipore Scepter hand-held Coulter counter for medical applications is available for $3.5K. Note that the channeled sampling tip apparently must be replaced after each sample. Here is a blog describing its use. (pointed out by Ken Johnson)
