Does Beer-Lambert's law calculate the absorbance of a solute

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I'm currently doing a project that involves beer-lambert's law and am confused as to what it actually calculates. So far, my intuition goes as follows: The molar extinction coefficient is for the solvent at a specific concentration and wavelength. In this case, it is blood. And if you have to calculate the concentration of a solute, then you have to get a wavelength that is primarily absorbed by the solute. So the question is: if I have another wavelength that primarily absorbs something in the blood, then will the molar extinction coefficient be that of blood at that wavelength?
 

Borek

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Very roughly speaking measured absorbance is a sum of individual absorbances of all substances present. Doesn't make much sense to speak of "molar extinction coefficient" of a mixture with variable composition.

This is actually much more complicated, as it works reasonably well only for diluted solutions, but that's the general idea.
 
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How would I get the most accurate results?
 

Borek

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Most accurate results of what?
 
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Sorry for not clarifying - the absorption of the solute in the blood. I'm using an LED and a photodiode for detection. Would I just use the molar extinction coefficient of the blood?
 

Borek

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No, you need to prepare the sample so that you measure absorbance of the solute you are interested in. There are no simple rules here, sample preparation is not something where a single recipe works for everything. Sometimes you need to extract the solute, sometimes you need to add things that will destroy/precipitate other interfering substances. The more complicated the solution, the more elaborate the procedures are. Blood is quite a difficult mixture to analyze.
 

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