I am receiving abnormally low yield when using the Qiagen midi prep to isolate DNA from an overnight culture. Typically, the yield for a mini or a midi prep is in the neighborhood of 400-500 ng/ul of DNA, or higher. However, I am currently receiving a maximum of 150 ng/ul, with concentrations as low as 10, 20 ng/ul, practically nothing. I've isolated similar plasmids before in this lab (this new construct that I'm trying to isolate is literally one point mutation different) and have gotten much better results than this, so I don't know what the issue could be. Another important fact is that the 260/280 ratio for these purified DNA samples is way too high: I'm getting readings of 1.91 and up when I normally see a range of 1.81 to 1.87 at the highest.
I don't know what exactly the issue is because everything seems normal...the overnight culture looks fine, I'm seeing a nice fat pellet after spinning it down. I have been following the Qiagen midi kit manual to the letter. Maybe I'm missing some subtleties that are causing my DNA to disappear, but I cannot imagine what it might be. Any advice/tips/help would be appreciated. Thanks.