My physicist collaborator asked me the ionic strength of the MIV medium I'll be using for my optical tweezers experiments. This is significant because the ions in the medium may shield the DNA from surface charges on the glass slide and cover slip, and so reduce unwanted binding. I knew roughly what ionic strength is, but I had to look up the formula in Wikipedia. I then looked up the recipe for 'solution 21'(the main component of MIV), looked up the molecular weights, and did the arithmetic. The ionic strength of MIV is 0.5 M. I think this is high enough to give lots of electrostatic shielding.
While I was at it I also calculated the weight/volume concentration of MIV; it's about 3.5% (amino acids and salts), which I think is not high enough to interfere with the refraction needed by the tweezers.
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