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Refractive index matching, pioneered by Anderson JS 1920, has long been used to determine the refractive index of small particles, such as bacteria (for example, Ross KFA and Billing 1957, Barer R and Ross 1952, Jonasz M et al 1997), bacterial spores (Ross KFA and Billing 1957), and mineral grains (for example, Hodgson RT and Newkirk 1975). It is a striking visual phenomenon that manifests itself by a significant reduction in light scattering by a dispersion when the refractive indices of particles and the surrounding medium are matched.
Refractive index matching is also employed in the optical spectroscopy of and imaging through tissue in a technique of optical clearing of tissue.
In principle, the method of immersion refractometry has an advantage of being independent of any assumptions regarding the shape of the particles whose refractive index is being measured with this method. However, in practice, there are - as usual - caveats. First, with inhomogeneous particles, the refractive index of the medium surrounding the particles cannot be matched to that of the particles by definition. Hence, the optical density (OD) of the dispersion cannot be nulled and the refractive index of the dispersion fluid corresponding to the minimum OD depends in a complicated manner on the refractive index distributions within the particles. Second, the dispersion fluid may permeate the particle and complicate the interpretation of the immersion refractometry results (for example, Gerhardt et al 1982).
In photometric (as opposed to visual) immersion refractometry, assumptions are usually made regarding the particle shape and a model of their iteraction with light (for example, Jonasz M et al 1997). This enables one to derive a relationship between the OD of the dispersion of particles and the difference between their refractive index and that of the dispersion fluid, Δn. Thus, a curve can be fitted to the OD vs. Δn data and the Δn corresponding to the minimum of that curve can be found.
With these caveats, immersion refractometry is a method of determining the refractive index of small particles that is the least dependent on assumptions regarding the particle shape and related particle characteristics that are central to methods based on modeling of light scattering by the particles.
Hänel G 1968 used a variation of the immersion refractometry technique to determine the real part of the refractive index of aerosol particles. He dispersed particles in two liquids with different refractive indices (RI), and determined the average RI, m', as well as density, ρ, of the particles by solving a system of two volume mixing rule equations. Such an equation for a single liquid can be written in terms of the masses of the dispersion components
| M m' = M m'd - ρVl (m'd - m'l ) | (1) |
where M is the mass of the aerosol particles, m'd is the RI of the dispersion, and V l and m'l are the volume and RI of the liquid, respectively.
| CITATION: Jonasz M. 2006. Immersion refractometry (www.tpdsci.com/Tpc/RIbyImmRef.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 18-Jan-2006 Modified: 05-Feb-2008 Reviewed: PENDING |
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