filtering a single quote in a query

I am trying to include this in a long case statement to say that when the first name contains a single quote return the word 'invalid': 

when contains(FirstName, ''') 

then 'Invalid' 

but I keep getting this error: 

Something’s wrong with the function at line 35, column 7. A parameter is missing, or it uses a field name that’s also a function name.

Is there a way to do this in a case statment?

Thanks in advance.