[DIYbio] Re: Another approach to DNA quantification

We have been discussing using absorption of UV by a cuvette as a way

of finding out how much DNA is in a sample.

But there is a much simpler (cheaper) approach to DNA quantification in

use that is very DIY hackable, and applicable to 10 microliter sample sizes.

Simply measure the capacitance of the sample.

Measuring capacitance with a microcontroller is very simple.

Two wires, or two traces on a PC board make the capacitor.

The dielectric is the sample placed across the wires.

Plain water has a different dielectric constant than water containing DNA.

The microprocessor charges the capacitor by turning on an output bit

connected to one of the wires. The other wire connects to ground.

Then, the output bit is switched to an input bit and compared to a voltage

reference, while a high speed timer counts how long it takes the capacitor

to discharge.

You can get very high resolution using this technique -- as many bits as your

timer has (usually 16) or even more if you use software to extend the timer

range to 24 or 32 bits. But even 8 bits is overkill for the DNA quantification

application, where 10% accuracy seems to be the norm, and three significant

digits are hard to find in the literature.

Calibration can be done by comparing to a Nanodrop or similar device, or

by making a pure DNA solution, removing the water, and then diluting it to

specified concentrations.

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