A nanomechanical mass sensor based on carbon nanotubes has been developed with atomic mass resolution. This device, a nanomechanical mass spectrometer, achieves a mass sensitivity of 1.3×10⁻²⁵ kg/√Hz, equivalent to 0.40 gold atoms/√Hz. It detects atomic mass shot noise, analogous to electronic shot noise in semiconductor experiments. Unlike traditional mass spectrometers, this device does not require ionization of the sample, making it suitable for large molecules. It is more sensitive to large molecules and can be integrated onto a chip. Nanomechanical resonators function as mass sensors because their resonant frequency shifts when mass is adsorbed. The sensitivity depends on the resonator's geometry and the position of the adsorbed mass. For a cantilevered beam resonator, the responsivity function R(x) relates the frequency shift Δf to the mass change Δm. The device uses a double-walled carbon nanotube, which is much lighter and more rigid than traditional resonators, enabling high sensitivity. The device's geometry, particularly a singly clamped design, increases dynamic range and quality factor, enhancing sensitivity. A transmission electron microscope (TEM) image determines the nanotube's exact size and mass. The nanotube is placed in an ultra-high vacuum chamber, with gold atoms evaporated from a distance and detected by a quartz crystal microbalance (QCM) for calibration. The device detects mechanical vibrations using a nanotube radio receiver design, leveraging the unique field emission properties of carbon nanotubes. Experiments show that the resonant frequency shifts when gold atoms adsorb, with the sensitivity of 0.13 zg/√Hz or 0.40 Au atoms/√Hz. This is the lowest mass noise ever recorded for a nanomechanical resonator at room temperature. The study also examines atomic mass shot noise, which is analogous to electronic shot noise. The noise is analyzed using statistical fluctuations in mass adsorption rate and frequency shifts. The results confirm the mass of a gold atom as 0.29 ± 0.05 zg, consistent with the accepted value of 0.327 zg. The nanomechanical mass spectrometer has significant advantages over traditional mass spectrometers, including no ionization of the sample, higher sensitivity for large molecules, and potential integration onto a chip. The device's sensitivity and stability make it a powerful tool for mass sensing at the atomic level.A nanomechanical mass sensor based on carbon nanotubes has been developed with atomic mass resolution. This device, a nanomechanical mass spectrometer, achieves a mass sensitivity of 1.3×10⁻²⁵ kg/√Hz, equivalent to 0.40 gold atoms/√Hz. It detects atomic mass shot noise, analogous to electronic shot noise in semiconductor experiments. Unlike traditional mass spectrometers, this device does not require ionization of the sample, making it suitable for large molecules. It is more sensitive to large molecules and can be integrated onto a chip. Nanomechanical resonators function as mass sensors because their resonant frequency shifts when mass is adsorbed. The sensitivity depends on the resonator's geometry and the position of the adsorbed mass. For a cantilevered beam resonator, the responsivity function R(x) relates the frequency shift Δf to the mass change Δm. The device uses a double-walled carbon nanotube, which is much lighter and more rigid than traditional resonators, enabling high sensitivity. The device's geometry, particularly a singly clamped design, increases dynamic range and quality factor, enhancing sensitivity. A transmission electron microscope (TEM) image determines the nanotube's exact size and mass. The nanotube is placed in an ultra-high vacuum chamber, with gold atoms evaporated from a distance and detected by a quartz crystal microbalance (QCM) for calibration. The device detects mechanical vibrations using a nanotube radio receiver design, leveraging the unique field emission properties of carbon nanotubes. Experiments show that the resonant frequency shifts when gold atoms adsorb, with the sensitivity of 0.13 zg/√Hz or 0.40 Au atoms/√Hz. This is the lowest mass noise ever recorded for a nanomechanical resonator at room temperature. The study also examines atomic mass shot noise, which is analogous to electronic shot noise. The noise is analyzed using statistical fluctuations in mass adsorption rate and frequency shifts. The results confirm the mass of a gold atom as 0.29 ± 0.05 zg, consistent with the accepted value of 0.327 zg. The nanomechanical mass spectrometer has significant advantages over traditional mass spectrometers, including no ionization of the sample, higher sensitivity for large molecules, and potential integration onto a chip. The device's sensitivity and stability make it a powerful tool for mass sensing at the atomic level.