I am isolating Plasmid from bacteria which is more than 23kb in size. I am using 0.8% Agarose gel on 100V and lambda DNA as marker. I am always getting band at size of 23 kb. I also used supermix DNA ladder which just shows that the plasmid size is more than 23 kb as the DNA ladder is also not resolved completely. Please help me regarding this.
TAE buffer can work better on larger fragments than TBE buffer (I'm not sure what buffer you are currently using). The suggested percentage gel for something that large is 0.5%, that might help with your separation of both your sample and your ladder. Life Technologies has a fairly good page of hints for better resolution of agarose gels.
http://www.lifetechnologies.com/us/en/home/life-science/pcr/elevate-pcr-research/agarose-content-with-tips-and-tricks.html
At that size range you are better off using pulsed field gel electrophoresis. Conventional agarose gel electrophoresis is pretty much useless for anything beyond 20 kb; your plasmid could be larger than 50 kb long and you'd never know.
If you are using purified plasmids (undigested) that could also have an effect. Circular DNA especially at that size doesn't run the same as linear DNA. The two suggestions above may help a little bit better with it.
I agree, Pulsed field get electrophoresis is the best way to go with a fragment of that size.
How about digesting the DNA with restriction enzyme and adding up the cleaved fragments for the total size of DNA.
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