I've been purifying RNA for RT-PCR cloning. I have been using the same type of kit (Promega Total RNA Kit). My 260/280 ratio has been between 1.9-2.1, which is good. However I just ran a new set of samples and the ratio is down to 1.7 which isn't so great. This might mean I have a lot of DNA contamination. The only difference in the protocol is the centrifuge, it is not as fast as my previous centrifuge. The protocol calls for 5,000xg for 50 minutes. The new centrifuge only runs at 3,000xg. How much time do I need to run these samples at 3,000xg to be the same as 5,000xg for 50 minutes? I thought since the radius was the same I would just run the samples 40% longer. I've looked everywhere for an equation please help.