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

[Irfan Nafi]

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]

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.

 

[Irfan Nafi]

How would I get the most accurate results?

 

[Borek]

Most accurate results of what?

 

[Irfan Nafi]

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]

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.