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So I think this is going to be some kind of sensor... :P
I'm looking for ideas on the best way to measure a dispensed volume of .1uL from the tip of a pipette
This is for a open source medical liquid handler =] ...
- Fluid will be mostly transparent
- Pipette tips can be clear or black
- Will be measuring the displaced volume of 8 lines exiting a peristaltic pump
thoughts?
I suspect that the best solution is likely in the physical design of the pipette mechanism rather than in active electronic control based on feedback from the fluid itself.
Another possibility would be to use a stepper motor on your peristaltic pump.
In general though, precise metering of fluids is something that the biotech and medical equipment people have lots of solutions for, and they are usually either based on passive fluid properties (metering pipettes), or closed loop electromechanical drivers (peri pumps, syringe pumps*), but AFAIK not on feedback monitoring of the fluid itself.
For tiny volumes you can do some interesting things with piezo elements as pumps.
Browsing cole-palmer etc catalogs should give some ideas.
Probably you want an entirely separate cross-check scheme for calibration purposes. If you don't suspect drop-to-drop variation to be an issue, you could just dispense a lot until it should match some easily measured quantity. If you are concerned about variation, a bit more creative thinking may be required.
*or a similar arrangement of a stepper or servo & screw driving lower volume metering device, such as a micro-pipette
If you really, really want to directly measure tiny quantities of fluid, I've seen video microscopy of inkjet nozzles done quite well. For some fluids you want to backlight, for others you want reflected light at some angle (also consider IR vs visible, a camera is probably fine with either). Having a known diameter of some feature on your nozzle makes it easy to calibrate the dimensions of your picture. Once you've achieve a high contrast image, even quite primitive analysis can determine the diameter of a more-or-less circular droplet and from that the volume.
I think a digital caliper's sensor may be the right tool to measure the micropipette's piston displacement. Resolution is in the order of 1/100th mm, so that should agree with 10\$^{-1}\mu\$l.
I believe it is too difficult to dispense 0.1uL from a standard pump or even a specialized one. Check the literature. The method to do this is to either use microfluidics with fine channels or piezoelectrics (such as inkjet nozzles; professional heads exist for liquid dispensing at nanoliter volumes) or other exotic methods like acoustics (done in research labs experimentally). Read the journal "Lab on Chip" to see what is being done in this area. Once it is dispensed your best bet may be to measure the diameter of the droplet on a hydrophobic surface (via microscope) where the droplet can be assumed to be spherical. Now if this were to use 1uL volumes, that might be doable reliably using very fine pipette tips or tubing.
There are commercial outfits who specialize in calibrating and validating pipettors. They may have guidance on this and be able to offer a service quote ($$).
