Column for SEC-MS

I have a fairly hydrophobic polymer that is ~20,000Da and need to quantitate an impurity that is ~600 Da. Reverse-phase on C18 or phenyl doesn't provide adequate resolution and SEC-RI has some issues with the impurity being close to one of the system peaks. No chromophore and ELSD has been less than reproducible for the quantitation. Any thoughts on a column for SEC-MS? The polymer could be sent to waste and only the region with the impurity sent to the MS.

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