Single quotes in xlabel

Dear all,

I've just started using matplotlib for some data processing and
plotting and have ported one of my existing Matlab scripts to python.
However, I have been unable to get the following working properly in
python. In Matlab, this works:

xlabel('Normalised \psi''''_x', 'Interpreter', 'tex')

(The 'Interpreter', 'tex' bit is unnecessary if
'DefaultTextInterpreter' is set to 'tex')

However, the translation into matplotlib doesn't work:

xlabel(r"$\rm{Normalised}\ \psi''_x$")

I've also tried some other variations such as:

xlabel(r'$\rm{Normalised}\ \psi\'\'_x$') or
xlabel(r'$\rm{Normalised}\ \psi_x\'\'$') etc

The single quotes do not appear in the label. Is this just a
limitation of matplotlib, or is there some way to work around it?

Many thanks in advance,