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(Color online) (a) Particles flow through the suspended microfluidic channel contained within the resonating cantilever (the channel is completely enclosed within the beam). (b) A mixture of gold and polystyrene nanoparticles are measured in PBS. The differential masses of both particles are positive and the mass histograms overlap. The inset shows the time course of the frequency signal as particles flow through the device, but each peak cannot be assigned to either gold or polystyrene. The schematic indicates that both particles have a positive differential mass since their densities are larger than that of the surrounding fluid. (c) The differential mass of gold in a 44% glycerol/ solution remains positive, but is negative for polystyrene, as immediately discerned from the time course data shown in the inset. The two histograms are now clearly distinct.
(Color online) The density of polystyrene particles is determined by making frequency shift measurements in three solutions of differing densities at . Each average mass value is obtained from 300–600 individual particle mass measurements. The intercept yields the particle mass density. The densities of E. coli bacteria and of human red blood cells were also measured (inset). The sample solution densities can be quantified from the change in the baseline frequency that occurs when water ( at ) is replaced with the sample solution.
(Color online) Size histogram for a mixture of polystyrene particles of five distinct sizes. The mass histograms of all five particles were acquired during a single run counting 5398 individual events. The five histograms were fitted to Gaussians (solid green lines). The densities of the particles were determined by measuring the same mixture in . The density measurement showing the intercept is shown (inset) for the three smaller particles. Mean sizes of , , , , and were measured. Their respective densities were , , , , and .
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