Antibiotics for M9 growth?

[Kevin M Jude]

A coworker in my lab is planning to grow deuterated protein in E coli
using minimal media.  The problem is that his plasmid carries Amp
resistance, and the growth is projected to take about 2 weeks.  Since 
ampicillin isn't stable over such a long time, we have been trying to 
think of alternatives to recloning his gene into a KanR vector.  Two such 
suggestions are using carbenicillin instead of ampicillin, or refreshing 
the growth with ~100 ul of ampicillin every 24 hours.  Are either of 
these viable alternatives?

Thanks
kmj