  A beam of polar molecules can be focused and transported through an ac
electric quadrupole guide. At a given ac frequency, the transmission of the
guide depends on the mass-to-dipole-moment (m/\textmu) ratio of the molecular
quantum state. Here we present a detailed characterization of the m/\textmu
selector, using a pulsed beam of benzonitrile (C$_6$H$_5$CN) molecules in
combination with rotational quantum state resolved detection. The arrival time
distribution as well as the transverse velocity distribution of the molecules
exiting the selector are measured as a function of ac frequency. The
\textmu/$\Delta$\textmu resolution of the selector can be controlled by the
applied ac waveforms and a value of up to 20 can be obtained with the present
setup. This is sufficient to exclusively transmit molecules in the absolute
ground state of benzonitrile, or rather in quantum states that have the same
m/\textmu value as the ground state. The operation characteristics of the
m/\textmu selector are in quantitative agreement with the outcome of trajectory
simulations.
