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Optical absorption spectra of dispersion of pigmented particles have long been observed to be different than those of the pigment solution. Even after accounting for the effect of light scattering by the particles, significant differences remain. A simple explanation was provided by Duysens (1956) who found that when a solution of a pigment is packaged into discrete particles (dispersion), such as phytoplankton cells, and the pigment volume is preserved, the optical density, ODdsp, of the dispersion of particles is related to that of solution of the pigment, ODsol, as follows:
| ODdsp = ODsol (1 - Tp) / ln(1/Tp) | (1) |
where Tp is the transmission of light by a pigmented particle (assumed to be made of pigment homogenously dissolved throughout the particle). This expression applies if the volume fraction of the particles in the sample is much less than unity.
Transmission through a particle, Tp, can be expressed formally, by using the Lambert law as follows:
| Tp = exp(-leff asol) | (2) |
where leff is an effective "length" of the particle along the incident light direction, and asol is the absorption coefficient of a solution of the pigment with concentration equal to that inside the particle. Hence,
| ODdsp = ODsol (1 - Tp) / ( leff asol ) | (3) |
The ratio of optical densities of two media with equal thickness equals the ratio of absorption coefficients of these media, a1/a2 (see Transmission, optical density, and thickness). Thus, the absorption of light by the dispersion decreases with increasing absorption (i.e. the decreasing Tp) of light by the pigment solution. Indeed, the ratio in the right-hand of Eq. 3 vanishes when asol → ∞, i.e. when Tp → 0. When the absorption of light by the pigment decreases to 0, i.e. Tp → 1, we have ODdsp = ODsol (i.e. adsp = asol) because for small asol, Tp ≈ 1 - leff asol. As a consequence, the absorption spectrum of a dispersion of pigmented particles is flattened as compared to that of the pigment solution.
The effect of pigment packaging can be also observed in other turbid media which contain light-absorbing particles, for example, tissue with red blood cells.
| CITATION: Jonasz M. 2006. Pigment packaging: Absorption spectra of dispersions (www.tpdsci.com/Tpc/PgpDsp.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 17-Jan-2006 Modified: 03-Jul-2006 Peer-reviewed: PENDING |
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