MadSci Network: Molecular Biology |
Dear Haldun, First some basics, which I’m sure you already know. Evaporation is a phase transition between a liquid phase and a gas phase. A special case is sublimination where a compound goes from solid phase to gas phase, without transiting through the liquid phase. At any given pressure added energy will result in increased temperature except at the phase transitions where the energy is mopped up for the change in state: for water we go from ice to liquid at 0 °C and from liquid to gas at 100 °C. At any temperature, we have equilibrium between solid/liquid phase and the gas phase: this is the vapour pressure you mention. Generally one can say that the vapour pressure decreases with the increase in size of the molecules, at least when we are talking about organic molecules such as fatty acids, FA-glycerides and DNA. There are other factors of course, but let us ignore them for now. So, you will need a higher energy to get a long chain fatty acid into gas phase than you need to get a shorter chain fatty acid to evaporate. At some point the size/complexity of the molecule will be such that the needed energy will actually brake down the molecule rather than evaporate it. As an exemple of this one could mention the use of mass spectrometry to sequence DNA. In this technique the sample material is “desorbed” from a solid substrate, essentially evaporating the material, although this is a very “artifical” situation, not comparable to what you have in your PCR-cycler. The whole complexity of MS-use for sequencing lies in trying to “put the pieces together” that the MS is detecting: the DNA is more or less completely degraded by the process. I would say that any problem with contamination in your case is rather the related problem of aerosols: the heating creates nano- or micrometer-sized droplets of water that take some of the sample material with it. A classic trouble with air-displacement pipetting for instance, and the reason for using filter tips on your automatic pipette. Even if there were a way to theoretically calculate a vapour pressure of DNA, as in the case of mass spectrometry, I’m afraid that aerosols are a more realistic culprit here. Maybe you can consider a two step process: aerosol containing DNA flies off, the water in the droplet evaporates further and leaves the content of the droplet "in the air". Not really evaporation of DNA, but close! I hope this answers your question. Kind regards, Erik
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