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8.4 Characterization of bionanoparticles by electrospray-differential  (Page 2/4)

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Basic principle of a general DMA. Adapted from P. Intra and N. Tippayawong. Songklanakarin J. Sci. Technol ., 2008, 30 , 243-256.

In the third step, the aerosol flow (Q a ) enters through a slit that is adjacent to one electrode and the sheath air (air or N 2 ) flow (Q s ) is introduced to separate the aerosol flow from the other electrode. After a voltage is applied between the inner and outer electrodes, an electric field is formed and the charged particles with specific electrical mobility are attracted to a charged collector rod. The positions of the charged particles along the length of the collector depend on their electrical mobility (Z p ), the fluid flow rate and the DMA geometry. In the case of particles with a high electrical mobility, they are collected in the upper part of the rod (particles a and b, [link] ) while particles with a low electrical mobility are collected in the lower part of the rod (particle d, [link] ).

The electrical mobility of the particles (Z p ) is a function of the fluid flow rate (Q s and Q a ), the applied voltaje (V) and the dimensions of the DMA as described by [link] , where R 1 and R 2 correspond to the radii of the outer and inner electrodes and L is the effective electrode length. Since DMA differentiates small changes in mobility, particles with a narrow range of electrical mobility (monodisperse aerosol) exit through a small slit at the bottom of the collector rod (particle c on inner electrode, [link] ). The excess air flow contains remaining particles.

Z p = ( Q s + Q a ) ln ( R 2 / R 1 ) 2π LV size 12{ { size 24{Z} } rSub { size 8{p} } = { { \( { size 24{Q} } rSub { size 8{s} } + { size 24{Q} } rSub { size 8{a} } \) "ln" \( { size 24{R} } rSub { size 8{2} } / { size 24{R} } rSub { size 8{1} } \) } over {2π ital "LV"} } } {}

With the value of the electrical mobility, the particle diameter (d p ) can be determined by using Stokes’ law as described by [link] , where n is the number of charge units, e is the elementary unit of charge (1.61x10 -19 C), C c is the Cunningham slip correction factor and µ is the gas viscosity. C c , [link] , considers the noncontinuum flow effect when d p is similar to or smaller than the mean free path (λ) of the carrier gas.

d p = ne C c 3 πμ Z p size 12{ { size 24{d} } rSub { size 8{p} } = { { ital "ne" { size 24{C} } rSub { size 8{c} } } over {3 ital "πμ" { size 24{Z} } rSub { size 8{p} } } } } {}
C C = 1 + 2λ d p [ 1 . 257 + 0 . 40 exp ( − 1 . 10 d p 2λ ) ] size 12{ { size 24{C} } rSub { size 8{C} } =1+ { {2λ} over { { size 24{d} } rSub { size 8{p} } } } \[ 1 "." "257"+0 "." "40""exp" \( { { - 1 "." "10" { size 24{d} } rSub { size 8{p} } } over {2λ} } \) \] } {}

In the last step, the size-selected particles are detected with a condensation particle counter (CPC) or an aerosol electrometer (AE) that determines the particle number concentration. CPC has lower detection and quantitation limits and is the most sensitive detector available. AE is used when the particles concentrations are high or when particles are so small that cannot be detected by CPC. [link] shows the operation of the CPC in which the aerosol is mixed with butanol (C 4 H 9 OH) or water vapor (working fluid) that condensates on the particles to produce supersaturation. Hence, large size particles (around 10 μm) are obtained, detected optically and counted. Since each droplet is approximately of the same size, the count is not biased. The particle size distribution is obtained by changing the applied voltage. Generally, the performance of the CPC is evaluated in terms of the minimum size that is counted with 50% efficiency.

Working principle of the condensation particle counter (CPC). Reprinted from Trends in Biotechnology, 30 , S. Guha, M. Li, M. J. Tarlov, and M. R. Zachariah, Electrospray-differential mobility analysis of bionanoparticles, 291-300, Copyright (2015), with permission from Elsevier.
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Source:  OpenStax, Physical methods in chemistry and nano science. OpenStax CNX. May 05, 2015 Download for free at http://legacy.cnx.org/content/col10699/1.21
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