How can light scattering influence the absorbance?
It depends on many things:
1. If you are probing a gas and the molecules are much smaller than the wavelength of the probe light, then light scattering is a negligible effect and can be ignored.
2. If the molecules/particles are comparable in size to the wavelength of the probe light, then they can scatter as well as absorb. For spherical particles, you can use Mie scattering theory to model the scattering and absorbance. More complicated shapes require more elaborate modeling.
3. If you are probing bulk materials, then once again light scattering can be neglected when probing the absorbance in a transmission measurement. If making reflection measurements off of a bulk material, however, it is necessary to convert the measured reflectance spectrum to an absorbance spectrum.
If I understand you correctly, the question is if scattering can affect the measured absorbance. Note, that absorbance can be defined using the Beer-Lambert law which contains the the absorption coefficient (mu_a) of the substance that one wants to measure. Hence, the assumption is that the measured attenuation of light is due to absorption of certain chromophores only and one can easily extract mu_a from the measurements.
If however, the sample of interest exhibits scattering in addition to absorption, what is measured by absorbance is called the extinction now. The extinction coefficient is the sum of the absorption AND scattering coefficients since two effects are now responsible for attenuation of light. One can still apply the Beer-Lambert law but has to use mu_a+mu_s that allows extracting mu_extinction.
please refer to Kramers-Kronig relations and check the scattering and absorption cross section.
the book 'Bohren C.F., Huffman D.R. Absorption and scattering of light by small particles (Wiley, 1998)(K)(400dpi)(T)(545s)(ISBN 0471293407)' is a good reference.
It depends what you mean by "influence on the absorbance":
On the absorbance spectrum recorded: light scattering will appear as continuous increase in extinction as you go toward shorter wavelengths. This increase will generally follow a " x/labmda (Mie scattering) + y/[lambda^4] (Rayleigh scattering)" model for simple spherical objects. This makes it easy to remove from the actual absorbance spectrum by a simple "fit and substract" macro.
On the absorbance itself: while light scattering does not affect the molar extinction coefficient of a compound, it increase the light pathway within the medium. Since the path length is increased, absorbance appears increased as well (Beer Lambert law : A = epsilon x l x [c]). If you really need a very precise measurement, you can probably find mathematical models that calculate this increase in length and correct the absorbance accordingly. Otherwise, I would simply neglect this effect.
Light scattering causes extinction, and this can be measured experimentally via cavity ring-down spectroscopy. See our paper Naus and Ubachs, Optics Letters 25 (2000) 347.
http://www.nat.vu.nl/en/sec/atom/Publications/pdf/rayleigh.pdf
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