I am using a tC18 Sep Pak Vac (500mg)to eliminate a bathophenanthroline-Fe complex and the excess unreacted chelator from aqueous solution (pH about 3) containing radioactive Ga-67 (half-life 3.26 days).
I find I am losing up to 40% of the radioactive element onto the C18 column material. Ga does not form a stoichiometric complex with bathophenan-throline, so any losses beyond 10% can perhaps be accorded to 'other' physical mechanisms.
Can anyone answer to the fact that uncapped silanol groups (on the base silica particles) can act as cation exchange groups and sorb the Ga.

Silanol sites in silica based reversed phase materials definitely can act as a cation-exchanger. If it's feasible in your system (depending upon the stability of the complex at low pH) you can minimize the problem by working at an even lower pH or use a polymer based SPE cartridge.