Resonant Mass Measurements
A mechanically resonant structure or resonator, such as a beam, suspended at one end resonates at a specific frequency. This frequency is known as the resonant frequency of the lever and is a function of its length and mass. When an additional mass is added to the beam the resonant frequency decreases by an amount that is proportional to the added mass. Therefore, accurately measuring the shift in resonant frequency yields the additional mass to very high precision.
A resonant lever surrounded by water, or even air, dissipates a significant fraction of its vibrational energy to its environment. This energy loss has the effect of reducing the resolution to which changes in frequency, and hence mass, can be measured. To address this limitation the resonator used in the LifeScale instrument is enclosed in a vacuum environment allowing it to vibrate without dissipating energy unnecessarily. This yields a high Q or quality factor, typically greater than 10,000, allowing the resonant frequency to be measured to high precision. LifeScale is able to measure frequency shifts of around 0.01Hz (20ppb) in a 1 kHz bandwidth. This measurement accuracy equates to a mass resolution of better than 1 femtogram and gives the mass of individual microbes to better than 1%
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