Start by checking your kit reagents, anything expired? Any ethanol solutions left opened? Anyone else using the same kit and having the same problem?
If all seems good with the kit, time to think about the cells. Make sure to use a cell culture time and volume within kit parameters. Check to make sure your antibiotics are fresh and were filter sterilized to keep selective pressure on the plasmid.
Ensure that the lysis step goes well (no clumps/lumps) so that you are getting the plasmid out.
If it's a column kit, it can help to warm the elution buffer to 50C and incubate for 1 minute prior to elution.
Good luck!
In addition to the suggestions that Amy Klocko makes, you might try it one more time and take samples each step of the way to find out whether you have good concentration of plasmid at the beginning but then it is lost (and you will know at which step) or whether you never got good lysis and never started with enough plasmid.
In addition to Amy and Michael - Is your starter culture fresh and are you adding enough? You may need to add some extra lysis buffer as well and make sure the lysis buffer is fresh.
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