May I presume that you have already tried the old classical methods?  If
I remember correctly, you can form the phosphomolybdate under highly
acidic conditions and then extract it into, for example, ethyl acetate
or butanol/chloroform.  The extraction is quite selective relative to
e.g. arsenic and silicate species.  I think you can then release the
phosphate again by going to high pH.  Does this help any?


David Pranitis
--
USDOE/Environmental Measurements Laboratory
201 Varick St., Floor 5
New York, NY 10014
(212) 620-3605