FAtty acid analysis

Chromatography Forum: GC Archives: FAtty acid analysis
Top of pagePrevious messageNext messageBottom of pageLink to this message  By Vivek on Tuesday, April 16, 2002 - 10:27 pm:

I am analysing C8 and C10 fatty acid using CPsil polar column ( Capillary 50 mts) and fid detector.. Individual injection of both the std. Gives 99% purity by area % but when I mix them in 1:1 ratio and inject I get 60% of C8 and 40 % of C10 (NOT as expected i should get approx 50% of each by area %). Can any one help me out as I want to know what is the response factor for both the component.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By H W Mueller on Tuesday, April 16, 2002 - 11:23 pm:

The response is, roughly, linearily and directly proportional to the number of carbons in the molecule.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Ron on Wednesday, April 17, 2002 - 02:08 pm:

Inject the pure components and determine the response factor (response/unit mass) for the individual components. The response factors should be roughly equal for the two compounds. If there is a large difference in response factor, then start looking for a problem in the intrument, the method, or the injection technique. There is not much information given above, so basic parameters such as flow rate, temperatures, injection volume, injection mode, etc. would be very helpful.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By B.Buglio on Wednesday, April 17, 2002 - 03:26 pm:

Agree w Ron-the FID should respond to the mass of
C atoms injected. Most compounds have roughly the
same sens. in terms of coulombs/g C. Therefore if
the fatty acid sample was in fact made up 50/50
there's something wrong w the system.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, April 18, 2002 - 10:49 am:

FYI, I work for a soap company, and we formerly used theoretical response factors as described for fatty acids from C8 to C18. However, we later changed to "real life" response factors by using Alltech mixture #19007 diluted to about 3 ml with petroleum ether (equal amounts of saturated C8 through C20 FAME) and found EQUAL RESPONSE to all of them, so response factors are not used. We normally use (still) a 6 ft. SS x 1/8" Supelco SP-2330 column from about 130 C to 190 C with FID detector. For C6 we use a different technique, not methyl esters.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, April 18, 2002 - 09:33 pm:

From C8 to C20 FAMEs there is little difference in response on a FID if equal concentrations by weight are injected, but one generally gets a slightly larger response for the higher carbon #.
From Caproic acid ME (C6) down to methyl Acetate (C2) there is less response on a FID, but they are still rather comparable.
Vivek, you have not indicated if you are injecting these acids as esters or as free acids or have told us the conditions of your analysis.
By not telling the members of the forum details you are making all of us guess about what you are doing and how you are doing it.
Your problem may be in the details, but your results indicate you are doing something very wrong.
I hope you find the reason.
Rod


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, April 18, 2002 - 09:40 pm:

I forgot to add that equal molar concentrations give a striking difference in FID response. See comments above by others concerning FID response by different number of carbon and hydrogen atoms in the molecule. Equal concentrations by weight do not give this striking difference.

Rod


Top of pagePrevious messageNext messageBottom of pageLink to this message  By H W Mueller on Monday, April 22, 2002 - 12:01 am:

Thatīs what I gave above: molar responses. If Vivekīs ratio is a mole ratio (1:1) than his peak areas should be in the ratio 8:10 (C-10 larger). If his ratio is a weight ratio the peaks should of course be about the same size (weight wise you have about the same number of C, the moles of C-8 being higher...). Not knowing more details one could surmise that Vivek has a discrimination in the injector: More C-18 volatilizes.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Monday, April 22, 2002 - 07:44 am:

H.W.Mueller is quite right. I was agreeing with your correct answer, Sir. Or I thought I was.
I also agree with your surmise of the problem. Too low or too high an injector temperature may be the problem. But as you are aware I am sure there could be other issues involved. But it seems to be characteristic that analysts post problems, giving too little details for a good diagnosis. Oh, well. Good luck to Vivek and my appreciation to everyone who contributes here.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By vivek on Thursday, April 25, 2002 - 03:04 am:

thanks for all your suggestions. I am injecting methyl ester of fatty acid(by treating with BF3).
MY injector and detector are at 250 degree centigrade each. Can any one tell that this might be the problem as I am now getting 48% of C8 and 52% of C10 by area % while they are mixed in exactly the same weight and then submitted for esterification. Why I am getting 2% extra for C10. As theory i should get C8 more than C10. I am very close to solve my problem it is just 2% difference.
can any solve this.Thank u
vivek


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, April 25, 2002 - 04:49 am:

You sound as though you have solved your problem.
I do not see a 2% higher FID response to the ester of the C10 as a problem.
In theory you should NOT get a higher FID response for the Caprylic ME in comparison to the Capric acid ME based on starting equal weights of the free acid.
You are very close to ideal, assuming small losses in extraction are higher for the C8 over the C10.

Don't forget each acid will have the other present in small amounts which contributes to the problem.

Rod


Top of pagePrevious messageNext messageBottom of pageLink to this message  By H W Mueller on Thursday, April 25, 2002 - 11:59 pm:

If you want to split hair: The C-10 has proportionally more carbon (proportionally less O and less H per given weight) so, theoretically, a higher response. As the the previous post stated, this will go under due to all kinds of practical factors.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By B.Buglio on Friday, April 26, 2002 - 03:32 pm:

What did you do to get the area ratio down from
60/40 to 48/52? Have you tried injecting standard
methylated esters (from a supplier) to see how the
ratio compares to the ones your getting? In any
event we need more info on the whole assay to
help.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Thursday, May 2, 2002 - 10:19 pm:

I tried premixing fatty acids in 1: 1 ratio and then subjecting to esterification.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By B.Buglio on Sunday, May 12, 2002 - 03:02 pm:

To determine if a problem actually exists you
should purchase a kit of fatty acids and methyl
ester standards. Methylate the fatty acids and
determine their area percent. Compare these to the
area percent of corresponding methyl esters in the
kit (using equivalent masses of course). There are
suppliers (Ultra Scientific for example) who
provide even carbon fatty acid /methyl ester kits
which would be perfect for this experiment.


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