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Topic: diesel purity by chromatography  (Read 3032 times)

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Offline Fadzli

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diesel purity by chromatography
« on: February 28, 2017, 10:50:12 PM »
Hi,
I have conducted chromatography testing for 2 different diesel fuel. The first one was claimed by supplier should be pure diesel, while the second one should be biodiesel (~7% mixture of palm oil). So when I look on the GC results, both are looks like same to me. How to differentiate these two results? How to confirm that the first one was pure diesel? I tried to look on GC standard for pure diesel but I still didn't found it. Anyone can help?
Thanks

Offline Arkcon

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Re: diesel purity by chromatography
« Reply #1 on: March 01, 2017, 07:18:25 AM »
OK.  Tricky.

OK, first of all, the attached chromatograms don't look exactly the same to me.  Maybe you expected more differences, but we don't know your method parameters.  With those, and expert (not me) may be able to help.

You don't have a diesel standard.  Since diesel fuel may or may not have a standard, given its likely a mixture, you may have to search more carefully.  We'll save that for later.

Do you have a palm oil standard?  You can see if that shows what you need.  You can also spike more palm oil into your unknown, to see if peaks get bigger.  That will help you identify what you're looking for.  Spiking also helps quantitation.   Example:  you think you have 7%, you add palm oil until you have 10% and 17%, not only do you see what, but you see how much response matches 7% more.

Also I'm lest wondering what you actually have.  Do you have diesel mixed with 7% palm oil, because that will work just fine, but only in a warmed up, carefully tuned diesel engine.  Or do you have diesel fuel that is 7% made of aliphatic esters derived from palm oil, because that is a separate question.

Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Enthalpy

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Re: diesel purity by chromatography
« Reply #2 on: March 01, 2017, 10:28:59 AM »
Biodiesel is derived from palm oil. It's not a triglyceride, which would be too thick and prone to freeze, but a mix of esters resembling methyl palmitate. The smaller molecule is a thinner fluid, but with a molecular mass similar to petrol Diesel.

Oxygen atoms in the mix should be a better indicator, if you have some means to quantify them.

Or would it make sense to try a saponification and observe if something happens?

[Just for fun: Firefox' UK-English dictionary ignores saponification and proposes sanctification, pornification and pontification instead.]

Offline Fadzli

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Re: diesel purity by chromatography
« Reply #3 on: March 01, 2017, 09:08:11 PM »
Thanks Arkcon and Enthalpy...
First of all, I think maybe I forgot to tell you that I'm a mechanical person, not a chemist so there are terms which I not really familiar with (#triglyceride, methyl palmitate, aliphatic esters etc...:-) ) but due to some reason on my project, I have to run the test to ensure that the diesel fuel used was pure (not a biodiesel) then that's how the question arises...
The first figure was claimed a pure diesel, compared to the second one which was claimed a biodiesel, but I don't know how to prove it. I expected that the shape or figures on graph can be used as a prove? Maybe by some chemist/experts?

Online Borek

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Re: diesel purity by chromatography
« Reply #4 on: March 02, 2017, 03:17:39 AM »
I would check the presence of a carbonyl with IR spectroscopy. While not a perfect solution, definitely there is no carbonyl in a "normal" diesel (perhaps traces) but it is by definition present in biodiesel (the "bio" part being mainly esters).

Problem with GC is that each peak you see is a separate compound - and the diesel oil is a mixture of many compounds. What matters is the behavior of the mixture, not its exact composition. Thus even when you take two good diesel samples from two different sources, their GC will be different. Without precisely assigning each peak to a compound (tedious, difficult and not trustworthy in the case of complicated mixtures) you can't say if the mixture contains esters (bio) or not.
« Last Edit: March 02, 2017, 06:40:20 AM by Borek »
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Offline Enthalpy

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Re: diesel purity by chromatography
« Reply #5 on: March 02, 2017, 11:33:22 AM »
[...] I'm a mechanical person, not a chemist so there are terms which I'm not really familiar with (#triglyceride, methyl palmitate, aliphatic esters etc...:-) ) [...]

Traditional Diesel oil is a fraction of crude mineral oil, hence a huge mess of many molecules composed essentially of carbon and hydrogen. Oxygen atoms have escaped as carbon oxides or water during the formation of the oil, sulphur is removed to reduce the pollution by vehicles, nitrogen too, and so on. While nobody even tries to know what molecules exactly compose Diesel oil, it is known that they contain only traces of oxygen, and thanks to the distillation process, they boil in some temperature range which also tells a range of molecular mass.

Vegetable oils and animal fats are triglycerides: the esters of glycerol and three fatty acid molecules
https://en.wikipedia.org/wiki/Triglyceride
these big molecules freeze easily and are too thick for a fuel (palm oil is solid at room temperature), so they're transformed into bio-Diesel
https://en.wikipedia.org/wiki/Biodiesel

The operation first breaks ("saponification") the ester into glycerine and salts of the three fatty acids, for instance palmitic
https://en.wikipedia.org/wiki/Palmitic_acid
which are then reacted ("esterification") with methanol to make molecules of proper size, for instance methyl palmitate, about one-third the original fatty acid.

The result is a broad mess too, and the hydrocarbon side of Biodiesel molecules resembles traditional Diesel. One notable difference is the carbonyl that remains in bio-Diesel but is quasi-absent from traditional Diesel
https://en.wikipedia.org/wiki/Ester
and this may be observable in the infrared spectrum
https://en.wikipedia.org/wiki/Carbonyl_group

My suggestion (opinions please!) was to mix the unknown fuel with a strong base like potassium hydroxide and observe if something happens.

Depending on the country where you get the Diesel fuel, traditional Diesel oil without any bio-Diesel can be uncommon.

Offline Fadzli

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Re: diesel purity by chromatography
« Reply #6 on: March 02, 2017, 09:35:10 PM »
Thanks Borek and Enthalpy for your answers and efforts...really appreciate it. Looks like I need to revise the method in proving the properties or purity of those diesel fuel...either make another test as suggested or maybe try to find a 'standard' chromatogram of pure diesel for comparison purposes...
Thanks

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