Chromatography Forum

Hello,

I'm new here.

I am currently faced with a problem of developing a method of analysis of TEA and MeOH in an aqueous matrix containing large levels of mineral salts. Obviously I cannot inject the aqueous solution so I make the solution up in acetone which leads to precipitation of minerals. I then syringe-fiter the mixture and do a direct injection on GC. This, however, produces irreproducible results. I suspect this is partly due to some TEA and MeOH being retained in aqueous phase containing minerals.

My instinct calls for head-space analysis. Is it the way to go or there is a way around it. My lab does not currently have a head space setup but this can be purchase if needed.

Thanks in advance for any recommendations

Regards,

Gung

Hi Gung

Welcome.

Headspace might work. What are the concentrations of the TEA and methanol ? What is TEA, - triethylamine ? What accuracy and precision do you require ?

Peter

Hi

Also have you considered the pH control of the sample solution?

Adjusting the pH to about 10 (with a carbonate buffer?)might be needed to keep the amine deprotonated.

Hi Gung

Welcome.

Headspace might work. What are the concentrations of the TEA and methanol ? What is TEA, - triethylamine ? What accuracy and precision do you require ?

Peter

Hello Peter,

Thanks for replying. Yes, TEA is triethylamine. I will be looking at levels of methanol between 0-15 %. If I can get accuracy and precision of about 20 % I would consider myself lucky!!!

The problem I currently have is that I weigh a 1.0 g sample of aqueous solution containing MeOH and TEA. This sample is also heavily loaded with minerals and other things. When I dilute this sample to 50 ml with acetone I get a cloudy mixture (due to precipitation of minerals) but later I get a separation of acetone phase and high density layer at the bottom. I can mix them and filter off solids before the analysis but I'm not getting good reproducibility .

Regards,

Gung

Hi

Also have you considered the pH control of the sample solution?

Adjusting the pH to about 10 (with a carbonate buffer?)might be needed to keep the amine deprotonated.

Thanks very much, this is a good point. Yes, I do add a few drops of alkaline solution before adding acetone.

Regards,

Gung

Hi Gung

pH control is important, you need it to be 2 pH units higher than the pKa of TEA. Before you go to the expense of a headspace sampler try extracting with acetonitrile. This is immiscible with salt solution, but should still be polar enpough to extract methanol and TEA from a basic solution. Separation on the column might become an issue - which column are you using at present ?.

Peter

Hi Gung

pH control is important, you need it to be 2 pH units higher than the pKa of TEA. Before you go to the expense of a headspace sampler try extracting with acetonitrile. This is immiscible with salt solution, but should still be polar enpough to extract methanol and TEA from a basic solution. Separation on the column might become an issue - which column are you using at present ?.

Peter

Thanks Peter. I will follow your instructions re pH. I haven't tried acetonitrile yet, but it is a good idea. I use a DB-5 60 m x 0.25 x 0.25 column. The column separates MeOH from Acetone and TEA very well. It did not separate MeOH from EtOH when I used EtOH as extraction solvent though. Hopefully will separate MeCN and MeOH! My worry now is whether TEA and MeOH will be mainly in the acetonitrile or remain in water, i.e. the degree of partition between two solvents. But I will definitely give this a go.

Thanks so much!

Gung

Hi Gung

The high salt concentration in the aqueous phase will favour partition of the organics into the organic phase.

Peter

Hi Gung

The high salt concentration in the aqueous phase will favour partition of the organics into the organic phase.

Peter

Thanks Peter!! Of course, the salt out effect! Very true!

Alex

Hi Gung

The high salt concentration in the aqueous phase will favour partition of the organics into the organic phase.

Peter

"Obviously I cannot inject the aqueous solution" - why? isn't headspace injecting just the vapors?

i have little practical experience in GC and headspace so treat my questions as novice trying to learn something useful

Headspace can perform accurate determinations of methanol and TEA. I have done 50ppm of TEA as a TFA salt solution when I converted the salt to the TEA free base.

But I don't know why you feel you cannot do direct injections of the aqueous salt solutions? Without a doubt, if you want to get accurate TEA you should adjust the pH of your solution.

After a series of injections the injection port should be cleaned but it can be done, with an accuracy of better than 2% relative, (15% +/- 0.3%, 1% +/- 0.02%).

best wishes,

Rodney George
consultant

"Obviously I cannot inject the aqueous solution" - why? isn't headspace injecting just the vapors?

i have little practical experience in GC and headspace so treat my questions as novice trying to learn something useful

Well, I don't have the headspace setup yet. Only the direct liquid injection. I would go for the aqueous injection except that the solution is full of salts and this will stuff the injector up pretty qucikly.

Thanks

Gung

"Obviously I cannot inject the aqueous solution" - why? isn't headspace injecting just the vapors?

i have little practical experience in GC and headspace so treat my questions as novice trying to learn something useful

Well, I don't have the headspace setup yet. Only the direct liquid injection. I would go for the aqueous injection except that the solution is full of salts and this will stuff the injector up pretty qucikly.

Thanks

Gung

Direct injection of aqueous sample will be my last resort before going for the headspace set-up purchase. The sample is very dirty and contains lots of mineral salts.

Regards

Gung

I thought we are talking about headspace all the time