Determination of Residual Hexane in cocoa Powder

[oscarBAL]

Hi, I was wondering if in place to try to dissolve your buttler don't just made an emulsion with some surfactant? think what the people use for suspend "oleous" substance in cream, they use tween 20 or things like that, I think you maybe would be able to undesrtand my spanish beter than my inglish! any way, looking out the lab, you could find any of that tensoactive substance and prepare an emulsion! I don't know if this could work?? is Just an Idea. and after your vacation you could try with SPME is less complicate!!

What hapen if you use n-Heptano like internal standard? I think is better than ciclohexane!!

I don't see any problem with the n-C6H14 that you are using because you are working at high concentration!!

Good Look!

[dcav]

Thank you oscarbald
n-epthne is also good as internal standard, but the one i have in my lab contain traces of hexane, i should diluite it or purify.

I can try to make an suspension, because powder is solid, but os quite difficult because of the huge partiles.
srry. what do you suggest to do with SPME? However i think they don't have SPME?, i aslo heard abaout it but i'd never used it.

Thank you very much and have a good holiday tii.

[oscarBAL]

hello again, I was thinking to make a suspension because I read that you have som problem to disslove both water and buttler, I dont know is this really could help, its obvious you have to increase your area to hepl the difusion or extraction, and I think that you don't have to dissolve both, just one of them and suspend the other one, any way don't give me atation.
You have HS already, and that is perfect, SPME is less usual but is so sensitive you use Head Space technic too to extrac your sample but you dont have to worry about pressure because you don't pressurize, and is cheaper!! and have a good reproducibility But, require more atetions that traditional HP.

[dcav]

Hi oscarbald
you say i should dissolve butter and make a suspension of powder?
I think that using water and surfactants at the temp of 80°C i will get to a phase separation anyway because if butter would be emulsified in water,at such an high temp the emulsion will break.
moreover i have seen that no good results were reached using butter.

My tutor want me to try with soy oil but I'd prefer to do things less complicated and easier to keep under control.

In my opinion the best method i've tried is external calibration, with standard solution of thecnical hexane in iso-octane added to a "clean" powder.

I should improve this method, to get a better precision ( now RSD is 10%).

What do you think of it?
Any idea to get better results?
Thank you very much

[chromatographer1]

As long as you do not dissolve the powder you will not be measuring the true hexane content. You are only measuring the hexane on the surface of the powder. This can be highly variable and inconsistent from sample to sample. The powder must be melted or dissolved to perform an analysis in which you can have confidence in the results.

Have you been unable to dissolve (not slurry or suspend) the powder? Is there no dilute NaOH solution or HCl solution that will dissolve it? Have you tried the DMAc-Water mixtures I proposed earlier? water-isopropanol? Carbon Disulfide? Ethylene Glycol Monoethyl ether?

I have been through this kind of problem before and although I was told in all confidence that dissolution was not necessary, the end result was that the analysis was worthless. Sorry to be so insistent but I would hate for you to spend a lot of time only to find out you wasted your time.

best wishes,

Rod

[dcav]

Dear Rod,
I really think you are right, but my tutor doesn't think so because he fears of interference.
I've been able to try with oleum solforic acid, but it doesn't wrk.
I 've been looking for Dimac in the reagentary but i didn't find it.
I tryed with ethyl acetate and it seems to work better than other as THF or ethanol.
I'll try to carry on with this trial while i'm doing what they want me to do

I would try easily with NaOH and other solvents after the 20th of August when i'll come back to work.
I want you to know that i understood what you said me and its importance, but i don't have the complete freedom in the lab!!
Thank you again and i promise you i'll let you know my results.

Dario

[chromatographer1]

Thank you for clarifying the situation.

dimethylformamide or dimethylacetamide are quite useful solvents.

It is amazing what sodium hydroxide solutions will dissolve. EGmEE and Ethylene Glycol mono methyl ether are also very useful with difficult dissolutions.

Good luck in finding a suitable solvent, that is the hardest part of headspace analysis. Remember with a sealed vial you can heat the solvent to dissolve it and cool it down for sampling later. Hexane rarely gives problems with this kind of analysis. It takes little time to equilibrate and does so linearly over a wide range of concentration in headspace analysis. That is the silver lining of the dark cloud of cocoa powder.

Rod

[dcav]

Dear mr. Rodney
I've spent a beautiful week on holiday almost without thinking on cocoa.
At my return I've finally demonstrate t the inconsistence of the data obtained in this way.

I' ve replied four real sample ( of a powder wich has been grinded in the plant, so the most homogeneus we can have) 1g of powder and 10cc H2O
and I've obtained four results completely different (rsd more than 60%).

I have some questions:
I've tried some solvents: diluted NaOH, concentrated H2SO4, ethyl acetate, ethanol, isopropanol - water, ethilic ether.
No results even keeping the vial at 80°C.
There is always a residue much more than expected ( 8% of powder are ashes).
With solforic acid maybe it is the residual of digestion, but how can i verify it?
Hexane didn't have problem in your opinion (i used oleum) ?
Any other suggestions?

The second argument is this:
Among all the sample analysed, the best were those done when i was tryng internal Standard: i mean that there was the same irreproducibility on the areas, but the proportion between hexane and ciclohexane was more or less the same.
I explain better, i wanted to determinate the relative response factor hexane-IntStd, so i added to clean powder a known quantity of this two substances in a solvent( to dilute and transfer them).
i made four sample in the same way.
Areas had a RSD of 100%(!!!) but RF of 7%.
What does this means??
Could be a problem in the reaching of equilibrium of the sistem?

Thanks a lot

[dcav]

Sorry another idea:
if it is true, as they say, that most of the hexane is dissolved in cocoa butter using a solvents able to extract butter-hexane instead of water could improve precision and confidence in the results, even if we don't dissolve all of the matrix???
I was thinking of iso-octane (they used it to extract butter in soxlet) or some alogenated.
Could work???

[chromatographer1]

isooctane could have small amounts of hexane in it.

but if you went up to dodecane that would be a good choice.

Your solvents that fail indicate that a non-polar solvent is required.

Methylene chloride chloroform carbon tetrachloride (all nasty and undesirable) ?

dimethylformamide dimethylacetamide dioxane(nasty) toluene phenol ?

DMSO?

what other solvents do you have available?

Rod

[dcav]

i've checked isooctane and it doesn't have hexane.
i could try vith dodecane or something similar.
If I will solubilise almost fats the results would be more accurate? (i think so.)

My only fear in using hydrocarbons is that they will give enormous peak, maybe requiring more time to clean the column. Any other problems?

For this thing maybe i should prefer alogenated ( that i avoid till now), they don't give any problem with FID, do they?

I'm sure we have CCl4, CHCl3, and tetracloroethilene,tricloroetihlene, Hexanol, pentanol, toluene, xilene, CS2 i'll look also for others.

Thank you very much
i'll keep contact

[dcav]

I've done it!!
I've dissolved the powder in a acqueous solution of KOH 10%!!!
i tryed with 1 g of powder and 8 cc solution.
I injected headspace with no evident interferences.
I'm really grateful to you, i hope this should solve some problems!
I'll keep in contact
Many thanks

[chromatographer1]

Now you can do headspace with confidence.

Tell me, now, in your kindness, just how much hexane is there in cocoa powder, not much I wager !

Glad you found a solution to your problem.

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com

[dcav]

Dear mr. Rodney,
as you suggested me time ago i used a smaller sample:
0,5 g powder, 10 cc KOH solution, in a vial 25 cc (i have only this type!)
then injected three real sample.(500 microliters, split ratio 60)
They give a signal more or less equal to those analised with 1,0 g cocoa and 10cc water, so the real concentration could be at least the double they thought with their method.

These three sample have been kept at 80°C for different times ( because i didn't have much time and i wanted to have an idea from wich time starting to determinate equilibration time), from 40 min to 80 min.
The RSD of the results was 15%.
It isn't very good but i should improve it analysing more samples tomorrow.
Even so is better than any other trial made before, and adeguate to their porpouse of controlling the drying process!

Tomorrow morning i will analysed maybe 9 samples, do you think that 20 min would be an adeguate equilibration time?

do you see any evident mistake in the procedure i'm following which could affect the precision?

I don't know how to thank you.
I'll say how your suggestions helped me in my final relation of course.

[chromatographer1]

With the long time you are heating the samples you may be losing hexane if the caps are not sealed perfectly, which is very easy to do, by the way.

I would reduce the liquid volume from 10cc to as little as 0.5cc if possible and reduce the heat time at 80°C to 10 min. I don't know if the present solubility of the powder: 500mg per 10mL (50mg/mL) can be increased, but you will [u]improve[/u] your RSD values by a shorter equilibration time.

The optimum time of equilibrating hydrocarbons from a 100-250 microliter sample of water is 5 minutes, alcohols require 8-10 min and dioxane requires 10-12 min. Any additional heating (with shaking) will increase not decrease your RSD results. But don't be surprised that hexane will vary from sample to sample as much as 10% or more. Especially if you are pan drying the powder in bulk. It has been seen before !

Take care in preparing your samples. Most variability in HS comes from sample preparation.

It sounds like you are making real progress in your analytical method development. Reminds me of the good old days.

Enjoy your success.

best wishes,

Rod

ps I can struggle with Italian, please send me your article. I would love to read it.