I'm currently estimating total protein content in cowpea seed samples using both CHNS elemental analysis (based on nitrogen content) and Lowry's colorimetric assay.
in my case, the Lowry values are noticeably higher than the CHNS-derived protein content. I have ensured proper extraction and followed standard protocols for both methods. Is there any differences in what is considered “protein" by each method? The nitrogen-to-protein conversion factor (6.25) being inappropriate for cowpea?
Is anyone had similar experiences? Can anyone clarify the doubts?
The 6.25 factor comes from the assumption that 16% (1 / 6.25) of all protein is nitrogen. A quick read of the just abstract of Krul 2019 :Article Calculation of Nitrogen‐to‐Protein Conversion Factors: A Rev...
suggests that this is a simplification for several reasons.
On the analytical side of things, have you tested a known reference material by both methods to check your procedures are accurate? There is also the question of the measurement of uncertainty in your laboratory procedure: for one thing, have you tested a sample on different occasions with different calibration preparations and determined how consistent your results are? You may find the Lowry measurements are more variable than the Dumas results or vice versa, and you may be able to explain some of the differences in that way.
It's always a problem analysing with two or more methods. Just choose one and stick with that :)
Andrew Paul McKenzie Pegman You're avoiding Segal's Law: https://en.wikipedia.org/wiki/Segal%27s_law
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