The study of the tip-sample interaction has been achieved combining the simultaneous measurement of force, resonance frequency, oscillation amplitude and quality factor vs, distance curves. From the analysis of these experiments performed in air with cantilevers of low force constant (< 0.75 N/m), we propose that the tip jumps to contact due to the formation of a liquid neck between tip and sample. Besides, a few nanometers before the tip jumps to contact, a decrease in oscillation amplitude is detected. We observe that this decrease is mainly caused by a dissipative interaction. By selecting this interaction as feedback signal, the scanning force microscope can be operated in the non-contact regime.