Purifying halogenated DNA?

[Ng]

     Hi.  I am having some problems purifying bromo and iodo DNA oligomers
(10 and 12 mers).  During purification by anion exchange in 7M urea with
KCl gradient,
I usually see 2-3 significant peaks which overlap.  I don't see these
peaks when purifying non-modified DNA.  What are the components of these
peaks?  And what is a good way to identify the correct peak?  Thanks!

-- 
Ho Leung Ng
holeung at ucla.edu