Chromatography Forum

I have been running 1,4 dioxane analysis via gc-fid using spme (headspace) and have noticed a considerable decrease in reponse for 1,4 dioxane and my internal standard in samples containing ethanol. The samples in question contain ethanol at greater than 10%. Is there any way to mitigate this?

Thanks!

Have you checked the formation of azeotropes with ethanol and dioxane?

This rings a bell for me, but I do not have any references.

You could try using less sample in your vials.

You could try adding salts to the sample.

Is your recovery linear with the decreased response?

It would be helpful if you shared the matrix involved in greater detail.

best wishes,

Rodney George
consultant

I have seen the same effect when running Vacuum Distillation (method 8261) on water samples with % levels of acetone/isopropanol/ethanol.
I have labeled 1,4-dioxane as an internal standard and with too much polar organics in the water I can lose that peak entirely along with the labeled acetophenone. That makes the calculations hard to do. The only way around it I have found is to dilute the sample down enough that I can get usable peaks. It's all about partitioning between the aqueous and gas phases.

Thanks for the suggestions. I have tried all of your suggestions. Lowering the sample size and salting out helped loads.

K

Ok-something new has come up. There is currently a huge debate going on that needs cpme to a conclusion. When I reduce the sample size...Do I have to make up the volume (ie with water the base for my IS)? Also, if so, how does this affect our calibration? Thanks for any help. We have agreed to follow whatever suggestions we get....within reason

When you reduce the sample size (total volume of dissolved sample) you change the recovery and the calibration.

IS should follow the amount of sample (not as % but as amount of volume) . If you expected 10 micrograms of solvent and have 10 micrograms of IS before the reduction, and you should have for both after a 10x reduction in sample size, 1 microgram of each.

Be aware that the peak areas may change: if you had 10000 area of solvent and 12000 area of IS, then after reduction you may have 1250 area of solvent and 1500 area of IS.

You should do a series of levels for your solvent so you know you have iinearity and your IS should be consistent over that range in a validated method.

I hope I wasn't murky, but clear in what I just stated.

Ask again if I was not clear.

best wishes,

Rod

You have an equilibrium (for your 1,4-Dioxane) between your water and your headspace, and an equilibrium between your headspace and the SPME fibre. Using the labelled Dioxane as your Internal Standard will minimise the effects changing those equilibriums will cause, but ideally, you should keep them as consistent as possible, particularly if you're trying to keep your abundances consistent. There really isn't any need for them to be consistent in an isotope dilution method, but it sure helps with diagnostics if things go wrong.

So, for calibrations and samples, use the same volume of water, the same amount of salt, and, if your samples are fairly consistent in ethanol content, about the same amount of ethanol. You want the partition between the water and the gas to be as favorable as possible (hence the salt to push the dioxane out of the water), and also want a good interaction between the headspace and the SPME fibre. Ethanol will increase the solubility of the dioxane making it harder for you to recover (hence your lower abundances), but it should equally affect your Internal Standard, so you should get the right result providing you're getting a decent amount of the dioxane onto your fibre.

Well, this is known, and may be not associated with the formation of classical azeotrope. 2 semesters of medical school, hate "physical and colloid chemistry"
"Reducing the partial pressure of vapor over the true solutions and increase the boiling temperature of each component to".

Way to fight - increase the temperature of the sample from which sorbing dioxane ( add 10- 20 C)