Mass spectrometry (MS) provides rapid and quantitative identification of protein species with relatively low sample consumption. Yet with the trend toward biological analysis at increasingly smaller scales, ultimately down to the volume of an individual cell, MS with few-to-single molecule sensitivity will be required. Nanoelectromechanical systems (NEMS) provide unparalleled mass sensitivity, which is now sufficient for the detection of individual molecular species in real time. Here we report the first demonstration of MS based on single-biological-molecule detection with NEMS. In our NEMS-MS system, nanoparticles and protein species are introduced by electrospray injection from fluid phase in ambient conditions into vacuum and subsequently delivered to the NEMS detector by hexapole ion optics. Precipitous frequency shifts, proportional to the mass, are recorded in real time as analytes adsorb, one-by-one, onto a phase-locked, ultrahigh frequency NEMS resonator. These first NEMS-MS spectra, obtained with modest mass sensitivity from only several hundred mass adsorption events, presage the future capabilities of this approach. We also outline the substantial improvements that are feasible in the near term, some of which are unique to NEMS-MS.

Nanoelectromechanical systems (NEMS) resonators can detect mass with exceptional sensitivity. Previously, mass spectra from several hundred adsorption events were assembled in NEMS-based mass spectrometry using statistical analysis. Here, we report the first realization of single-molecule NEMS-based mass spectrometry in real time. As each molecule in the sample adsorbs upon the NEMS resonator, its mass and the position-of-adsorption are determined by continuously tracking two driven vibrational modes of the device. We demonstrate the potential of multimode NEMS-based mass spectrometry by analyzing IgM antibody complexes in real-time. NEMS-MS is a unique and promising new form of mass spectrometry: it can resolve neutral species, provides resolving power that increases markedly for very large masses, and allows acquisition of spectra, molecule-by-molecule, in real-time.

Derivation of Eq. (2) in the main textPrecipitous downward shifts in the modal resonance frequencies of a nanomechanical device occur upon adsorption of individual analytes [1]. These measured frequency shifts can be used to calculate the mass, position, and molecular shape of individual analytes that adsorb upon a NEMS resonator -as described in the main text. Importantly, in the limit where the particle mass is much less than the device mass, the sequential measurement of multiple particles is unaffected by the mass loading due to previous particles.Previous analyses [1,2] have considered the analyte particles to be point masses. In this work, we model the individual particles as finite-sized objects with a spatial mass distribution that is initially unknown. We consider a general device of arbitrary composition that is loaded by an adsorbate with mass, m , which is much less than the device mass, M. We further assume that the particle size is small compared to the device dimensions (and the wavelengths of the vibrational modes) or, if not, that the particle is much more compliant than the device itself.Under such conditions, which are especially relevant for the case of soft biological molecules (of special interest to us), the vibrational mode shapes of the device are unaffected by the adsorbed analyte, and thus the strain energy of the device is also unchanged. It then follows that the maximum kinetic energy of the device, before and after mass loading, is invariant for the same oscillation amplitude, i.e.,