We suggest an approach to mass sensing via tracing the shift of the node position as an alternative to current sensing approaches based on the detection of the frequency shift. We demonstrate the compatibility of our approach with fast and versatile in situ resonator fabrication and mass measurements by means of a scanning electron microscope. The proposed sensing mechanism is minimally affected by parasitic deposition during the measurement. Within this approach, we demonstrate the measurement of several femtogram masses for single-segmented amorphous carbon nanowire cantilevers. We use the experimental results to extract material parameters of the cantilever fabricated inside a microscope chamber. We use these material parameters to model the mass-sensing performance of the double-segmented cantilever geometry. Double-segmented cantilevers show Fano resonances originated from the coupling between top and bottom segment resonances. This coupling leads to two- to three-fold responsivity enhancement in comparison to a single-segment cantilever. Our approaches to mass sensing and sensitivity estimation are general and can be extended to other cantilever materials applied for mass and force measurements.