Help RNA purification and cetrifuge time

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The discussion centers on issues encountered during RNA purification for RT-PCR cloning using the Promega Total RNA Kit. A user reports a drop in the 260/280 ratio from a healthy range of 1.9-2.1 to 1.7, suggesting potential DNA contamination. The only change in the protocol was the use of a new centrifuge that operates at 3,000xg instead of the required 5,000xg. The user seeks advice on how to adjust centrifugation time to compensate for the lower speed, contemplating a 40% increase in duration. Previous experiences indicate that insufficient centrifugation speed can lead to poor results, prompting the user to consider finding access to a centrifuge that meets the necessary specifications. Additionally, there is a reminder to verify results by repeating the process to rule out a one-time anomaly affecting yield.
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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.
 
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Increasing the time may not be sufficient. I've run into that problem in the past where I just didn't have a centrifuge available that reached the speeds required by the kits (not that particular kit, just illustrating the point here), and it just didn't work. I had to find a lab to work in that did have a centrifuge that reached the appropriate speeds.

And, while I assume you've already done so before asking here, just in case...have you tried this more than once to be sure it wasn't just a one-time fluke with bad yield?
 

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