I first dug into the manuals to try to find anything about altitude correction factors. Nothing. The closest I came was finding that they could be "safely used" at altitudes up to 2000m (~6500ft). That gets almost all the way up to Flagstaff, but says nothing about whether there are any altitude-related issues I should be thinking about.
I then called BioRad tech support and chatted with a rep for a bit on this issue. She was a little amused and quickly regurgitated the ideal gas law (PV=nRT) which would have taken me a little longer to come up with. She pointed out that there are temperature sensors in all of the blocks of the machines we use, so the temperature reported by the cycler is correct as long as you accurately enter the volume of your reaction so it can correctly calculate the sample temperature from the block temperature probes.
I then pointed out that water boils at just under 93C here in Flagstaff (assuming barometric pressure of 23 in. Hg). If we are experiencing high atmospheric pressure (most of the time), the boiling point is slightly higher (closer to 95C) or if we have a storm (low atmospheric pressure) it could go as low as 91C. What this means is that every person here at NAU who follows the prescribed protocol for their PCR is boiling their reactions at every denature step. Since the denature temperature for most programs is still 94-95C, this is just the cusp of the boiling point on most days (it is usually sunny here). It also is not above the magic point at which you will significantly degrade the half-life of your enzyme with each step (about 94-95C). The boiling can still have a significant effect on the reaction by changing the concentration of your buffer (increased concentration of all reagents). Sometimes the moisture is retained due to the heated lid preventing condensation on the upper part of the tube and the lid compression preventing any gaseous escape, but more often than not, your reaction will contain less fluid at the end of the cycle than you began with.
Example: If you perform 10uL reactions and after 35 cycles you have 8uL, your MgCl2 concentration (to pick a reagent) will increase from 2mM to 2.5mM over the course of your cycle. Excess of MgCl2 can contribute to mis-priming and production of non-specific products. Excess KCl can produce unwanted short non-specific products. Excess polymerase can result in all kinds of background (a smeared appearance). Some environmental DNA samples such as are often processed in our lab also contain a lot of PCR inhibitors (e.g. polyphenols). The evaporation can therefore increase the endogenous inhibitor concentration to a point that effectively stops the reaction somewhere along the cycle.
You get the idea. Evaporation BAD!!!(say it like Dana Carvey playing George HW Bush) Though she couldn't provide me a concrete solution to this concern short of an artificially pressurized laboratory, she offered two suggestions that sounded very good to me:
1) Persons performing PCR at high altitude should reduce their temperature of denaturation according to the local atmospheric pressure. For our altitude, she suggested 90C. I asked if this time should be extended slightly to account for the lower temperature and she said she did not know, but it probably wouldn't hurt...which takes us to her second suggestion.
2) PCR (generally speaking at all altitudes) of GC-rich regions should include a "pre-denature" step of 80C for 1 min to "slow" the denature step to allow for the disentanglement of complex secondary structures and eventual denaturation of the GC-rich region. I should point out that she offered this as a complete alternative to ever adding DMSO to your reaction which, as you may know destabilizes hydrogen bonding, thus allowing efficient denaturation of GC-rich regions, but then also complicates your annealing step. She claimed the 80C for 1 min will solve this problem without messing with your annealing and thus may also be useful for high-altitude PCR.
What I learned: At 7000ft, reduce the annealing temperature to 90C. I can probably keep my denature time at 20-30 sec, but a pre-denature step of 15-30 sec at 80C may improve overall reaction efficiency with no further changes (longer if PCR target region is GC-rich).
Happy high altitude PCRing!!
