A radiofrequency electric resonance cell has been used to modulate individual rotational states present in a state-selected molecular beam of symmetric top molecules prepared for use in a beam attenuation experiment. Tagging states in this way allows cross sections for collisionally induced Delta M transitions to be measured for single rotational states. A hexapole transmission curve attenuation method reported earlier suffered from poor rotational state resolution and the measurement of cross sections was very time consuming in comparison to the method reported here. Cross sections measured using the two different methods show remarkably good agreement.