How to export only mass matrix to an ASCII file (avoid stiffness matrix)

Dear all, 

Does anyone now how to export only the mass matrix of a structure while avoiding to write the stiffness matrix to the same file?

I am using OUTPUT,MATRIX,FULL,SPARSE, which writes the stiffness and mass matrix to the same ASCII (.full.mat) file. However, since I have a big model, it is taking ages to write the stiffness matrix, because it is consistent. It should be much faster to write only mass matrix (which is the only output I care about) since it is lumped (diagonal). If I could write only the mass matrix to a file, it would be ideal. 

Thank you very much.

Best regards,

Pedro Millan