Try a variable replacement.

local lcMyScanFor
lcMyScanFor='This.colCA('+chr(34)+'AGMASTER'+chr(34)+').IsSelected(ID)'

Scan For &lcMyScanFor

endscan

I've ran into this many times. It's not just the scan and locate either. It behaves the same for just about any value you are searching for or testing. Like Calculate, If, Case, replace etc...