Figure 3.6. Transmission electron micrograph of polyaniline nanoparticles in a polymer matrix. (From B. Wessling, in Handbook of Organic Conductive Molecules and Polymers, H. S. Nalwa, ed., Wiley, New York, 1997, Vol. 3, pp. 497.)

The sizes of particles < 2 nm can be conveniently determined by die method of mass spectrometry. The typical gas mass spectrometer sketched in Fig. 3.8 ionizes the nanoparticles to form positive ions by impact from electrons emitted by the heated filament (f) in the ionization chamber (I) of the ion source. The newly formed ions are accelerated through the potential drop in voltage V between the repeller (R) and accelerator (A) plates, then focused by lenses L, and collimated by slits S during their transit to the mass analyzer. The magnetic field B of the mass analyzer, oriented normal to the page, exerts the force F = qvB, which bends the ion beam through an angle of 90° at the radius r, after which they are detected at the ion collector. The mass m: charge q ratio is given by the expression

The bending radius r is ordinarily fixed in a particular instrument, so either the magnetic field B or the accelerating voltage V can be scanned to focus the ions of

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