Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: STM on June 21, 2015, 11:55:55 AM
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Dear all,
Please I need clarification on this calculation procedure.
I did a UV-Visible measurement of Conjugated diene in lipid by measuring absorbance at 234 nm. I got an Absorbance = 3.31. Using molar absorbtivity, e, = 24000 M-1 cm-1, and path length = 1 cm.
From the formula A= eCL, C=A/eL = 3.310/ 24000x1 =0.0001379 M (137.9uM).
The weight of lipid (Oil) sample taken was 0.018g and was dissolved in 3.5 ml methanol as medium for the UV-Vis measurement.
I am expected to express the answer as umole/g (micromole/g). Is it just the 137.9 uM/0.018g or will there be need for other parameters to come into the calculation.
Please I need you advice on how to get to umole/ g. I have done some calculations already but I just need some clarifications from you all.
Thank you.
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137.9 μM is the concentration of the diene in the solution, not its amount in the original sample.
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Maybe I am missing something here and I am not an analytical chemist, but an absorbance of 3.31 is out of the linear range of most spectrophotometers, due to stray light. If stray light is a problem, there is an apparent negative deviation from the Beer-Lambert law.
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Babcock Hall has a good point.
Typically, with Beer-Larmbert, linearity has to be proven. It
can't be assumed.
If you want a quick test, do a 1:10 dilution and rerun the test.
See if the answer comes in at one tenth the value. If it does, all is
good with the world.
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Thank you all for your response. I am grateful. The points on the linearity range for Beer-Lamberts law is noted. Although, I do not intend to use the measurement for any form of calibration. Nevertheless, the result of the 10-fold dillution was still consistent.
Also, I will like to check this conversion>
If an oil sample has peroxide value of 5 milliequivalent of Oxygen per Kg, density = 0.889 g/ml. Its conversion to mmol/L is?
If 1 mmol of O2 = 2 meq, therefore, 5 meq = 2.5 mmol
5 meq/kg x 1/2 = 2.5 mmol/kg. 2.5 mmol/Kg x 1 kg/1000 g x 0.889 g/ml x 1000 ml/L = 2.22 mmol/ L (2.22 mM)
Can you please confirm this conversion?
Thank you