Homework Help: Biochem

Posted by Liv on Sunday, September 30, 2012 at 9:50pm.

1 ml of whole blood is collected and centrifuged to obtain a plasma sample. The volume of the plasma sample is 600 microlitres. A 1 in 4 dilution of the plasma is made and analysed using a cholesterol assay like the one you performed in the laboratory. 10 microlitres of both the diluted plasma sample and a cholesterol standard are assayed using 1mL of cholesterol reagent. Following incubation at 37 degrees Celcius, the samples are measured spectrophotometrically at 500nm.
If the absorbance of the plasma sample assay was 0.5, the absorbance of the cholesterol standard assay was 0.3, and the concentration of cholesterol in the original whole blood sample was 4.5mM, what was the concentration of the cholesterol standard (in mM)?
(assume all of the cholesterol in the whole blood sample was recovered in the plasma)

Any help is greatly appreciated

Biochem - DrBob222, Monday, October 1, 2012 at 12:10am
It isn't clear from your post if the standard was diluted in a manner similar to that of the sample. I will assume it was; if not you will need to make a adjustment for the dilution.
If the sample is 4.5 mM and gave A = 0.5, it must be a higher concn than the standard at A = 0.3. So the standard sample must be 4.5 mM s 0.3/0.5 = ?

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