Ratio 260/280 of a maxi-prep<1.6 - Purify a maxi-prep (Jan/23/2007 )

Dear all,
I made 6 maxi-preps using Qiagen Kit. I did the DNA quantification by spectrophotometry and the ratio 260/280 is very low between 1.4 and 1.6. How could it happened and what can I do to obtain a ratio up to 1.8?
Many thanks for your help.
Sev

Ok, how do you manage to do this?
Do you have an idea where the contamination come from?
Thanks

Sometimes when I get the 260/280 ratio less than 1.8, i would dilute maxi by adding lets say 100ul of water or TE and then check the ratio and it would b 1.7-1.9.

The problem is that I can't dilute my DNA because I need to be at 1ug/ul to do transfection after with specific quantityof plasmid and ratio.

Do you precipiate the maxi DNA after the maxi? (we routinely do this to get better OD). If not,

I would ethanol precipitate the maxi and dissolve in appropriate volumes and check the DNA concentration.

Ok, do you have a good protocol to advice me?
Many thanks for your help.

here is my protocol for precipitation

Add to the DNA

Na Acetate - (1/10th volume of DNA vol., eg. if 100ul of DNA add 10ul) and
Ethanol (100%,-20°C) (2.5x vol. of DNA vol.)

Vortex them throroughly.

Place in Dry ice for 2-5 min or
Place in –80°C Freezer for 2 hours or

Centrifuge for 30 min. at 4°C. Remove alcohol with out disturbing the pellet.

Add 200ul of 70% ethanol (-20°C) and centrifuge for 5 – 10 min. at 4°C.

Pipette out all the remaining alcohol

Centrifuge briefly for a few seconds and pipette out all the remaining liquid.

Add appropriate amounts of TE and wait for 2-5 min. for DNA pellet to dissolve

vortex (briefly) and centrifuge.