I'm working on some in vitro processing miRNA reactions and isolation of small fragments an or low quantities of RNA, I therefore recall some basic principles in the isolation of RNA and the more that I go into the chemical details the more I'm figuring out that there are several disagreements between different protocols for the RNA ethanol precipitation. What is my main concern is the about the incubation at low temperature of the alcohol solution in the presence of cationic ions. I thought that the low temperature increase the viscosity and increase the dielectric constant of the solution, both these aspects shouldn't be favorable for the RNA precipitation.
So my question is what are the best conditions for the RNA precipitation,
-20°C over night or few hours, -80/-70°C 30 mins, 30 min in ice or 5/10 min in dry ice?