Chromatography Forum

Can someone explain the number 1.08 in the denominator for the ethyl ester determination of omega-3s in AOCS Official Method Ce 1b-89?

Thanks,
H

On first glance I thought it might be to convert the amount of ethyl esters in the product/sample to the amount of the unesterified free fatty acid available to the body. e.g molecular weights 356/328 for DHA and 330/302 for EPA.
(And the 1.04 in the preceding equation would be for methyl esters.)
But the shorthand format of the written method leaves out the background info, and I don't have access to the references given at the end. Just a thought. DH

dhdh is correct -- the 1.08 is there to convert your result from ethyl ester content (which is what you're actually measuring on the instrument) to an equivalent fatty acid content. See Note 9, which says "Please note that this calculation gives results for fatty acids and not for esters of fatty acids."

The better question is, why is the value given 1.08 and not 1.09? When I do dhdh's molecular weight math the average of the two conversion factors is 1.089.

dhdh is correct -- the 1.08 is there to convert your result from ethyl ester content (which is what you're actually measuring on the instrument) to an equivalent fatty acid content. See Note 9, which says "Please note that this calculation gives results for fatty acids and not for esters of fatty acids."

The better question is, why is the value given 1.08 and not 1.09? When I do dhdh's molecular weight math the average of the two conversion factors is 1.089.

Thanks for your reply and I thought this was an error and this 1.08 should not be in the equation. A variable such as CF should be used where CF=conversion factor converting the fatty acid ethyl ester to the free fatty acid. This is just the molar mass ratio between the two. I will try to contact AOCS to point this error out. This method is used by many for their CoAs for finished dietary supplements omega-3 fatty acids. Since the number in the equation represents a small numerical error that is probably less than the error of the measurement, probably not that of big difference in the final answer. However, if tons of product are being certified, then it would make a difference.
H

Yes, I noticed it's closer to 1.09, even when using more accurate molecular weights going out to 2 decimal places. But didn't want to speculate and get too wordy. Maybe they 'truncated' the 3rd decimal place and 'rounded down' to err on side of the manufacturer...or because whatever pre-Windows Excel spreadsheet originally used was erroneously programmed to accept only 2 decimal places?
Also notice they use the 1.08 factor in Calculation 2b for both methyl esters and ethyl esters (which doesn't seem right), at least in the '93 edition I'm looking at (in the blue covered 3 ring binder).
Like the original poster says, the overall method error is probably going to be well above +/- 1% anyway. Some of the crude USP methods allow the material to be 98 - 102 % pure, even though it's not theoretically possible to be 102 % pure. Just another thought.