Assume that their nanofab chamber volume is about 1 zeptoliter (10-21L). I got this number from one of Pacific Biosciences' papers, but I'm guessing the scale is quite similar. The concentration of one single hydrogen ion in 1 zL is then:
(1/6 x 1023)/(10-21) ≈ 2 mM. That is actually quite a bit! Thought about another way, though, pure water is at 55M concentration, so there are 55M x 6 x 1023 x 10-21 = 33,000 water molecules in a single reaction volume, so you're trying to detect a single proton against a background of 66,000 possible free protons (two per water molecule.) That makes it sound a bit harder. In any case, it's apparent that it works!
2 comments:
when you measure pH you see protons in much lower concentration ( 1 in 10^7 molecules of water)
Yes, I was confused by the issue as well, turns out they are not detecting a single proton afterall. They have multiple copies (it is an emulsion PCR product)of the same piece of DNA in the well. It became less cool to me knowing that they have to amplify the sample, but it is still a very powerful technology. They are undercutting the instrument cost of all existing sequencing machines with their electronic readout.
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