Resonant Mass Measurement
ARCHIMEDES employs the principle of resonant mass measurement to determine particle mass with the highest possible resolution and accuracy. A mechanically resonant structure, such as a beam suspended at one end, resonates at a specific frequency. When a mass is added to the beam, the resonant frequency decreases:
By measuring the shift in resonant frequency, the mass can be determined to very high precision.
ARCHIMEDES extends this principle in a number of ways:
The resonator has very low mass. Using MEMS fabrication methods, the resonator has a size scale of about 100 µm, and a total mass of just a few nanograms. Such a small, light resonator is sensitive to very small amounts of added mass. - A fluidic microchannel is embedded inside the resonator (right). The microchannel allows particles suspended in fluid to be delivered conveniently through the resonator and measured one by one. And biological samples such as living cells can be measured in physiological conditions.
The microchannel resonator is contained in a vacuum environment (cutaway view, right) This allows the resonator to vibrate without dissipating energy unecessarily, leading to a very high Q or quality factor, typically > 10,000. A high Q in turn allows the resonant frequncy to be measured to very high precision. And, using advanced MEMS fabrication methods, the fluid inside the resonator can remain at ambient pressures without affecting the Q. In contrast, a resonator surrounded by water or even air dissipates a significant fraction of its energy to its environment, making precision frequency measurement impossible. - The resonant frequency can be measured very quickly and with very high resolution. Advanced detection optics, electronics, and signal processing combine with the high Q to enable measurement of the resonant frequency to a few parts per billion, in a 1 kHz bandwidth (i.e., 1000 times per second).
These characteristics allow the mass of individual particles or cells to be measured with a resolution below 1 femtogram (10-15 g) at high throughput. This is about one million times better than a high-end quartz-crystal microbalance.
The illustration below shows where ARCHIMEDES' mass resolution (gold arrow) stands in relation to various objects.
ARCHIMEDES can easily measure a variety of sub-micron particles as well as living bacteria and other cells. Although the mass of individual molecules is below the detection limit, the accumulation of sub-monolayers of molecules on the microchannel surface can be detected with this technology.
For further information please refer to this article in Nature.