I have a mixture of glycol, water, MDEA, and piperazine which I want to analyze with GC.
Dear Parisa Ahmadi,
In order to better answer your question, information about the expected concentration range would be useful. The equipment and configuration of the GC system is also important. For example, is there a headspace sampler, a liquid sampler or manual injection? What type of inlet system is available (I assume split/splitless)?
In principle, aqueous solutions are not suitable for injecting directly into the gas chromatograph. They also have very polar analytes with high boiling points, so that techniques such as headspace GC also reach their limits. In my experience, it would make sense to derivatise the N-DMEA. As your matrix consists of water, glycol and piperazine, you would have to derivatise everything. The reaction products should then be easy to analyse with GC. The derivatisation reagent must be present in large excess for quantitative conversion, otherwise the entire sample will not be completely derivatised and you will no longer be able to quantify it reproducibly.
My idea would be to add 50-100 µl of sample to 1 ml of reagent, then let it react at 80°C for 30-40 min and analyse it with GC. I suggest using a silylation reagent such as hexmethyldisilazane or trimethylchlorosilane. The reaction is fast and is also good for quantitative analyses.
I hope I could help you with my ideas
Best regards
Joachim Horst
Direct analysis of those highly polar and high boiling point analytes will be hard. Even with proper hardware setup, you will be dealing with broad peaks and low sensitivities (except for piperazine, which should behave relatively well). Derivatization as Joachim recommended would be the way to go.
Joachim Horst Dear Joachim,
Thanks a lot for your comprehensive answer. I am looking to buy the best GC that works for our needs, probably a liquid sampler and an FID detector will be good for detecting MDEA?
Gang Chen Thank you for your answer. Do you think LC with a UV-VIS detector will work better for this mixture?
Normally LC/MS will work, but I cannot have a mass spectrometer so I am looking for another detector. Gang Chen
This should be possible without derivatisation using a GC column designed for amines, unless concnetrations are very low. A long time ago we used Restek Rtx-35, but there are other manufacturers and they all publish application notes. The diamine piperazine might cause the most trouble. An old trick is to include in the injection solvent another amine, pure enough not to interfere and either relatively weakly or strongly retained.
I don't know if anyone has tried it, but it occurred to me that you could try ammonia as the other amine. Success or not would depend on the nature of the presumed deactivation mechanism.
Ammonia runs cleanly on Restek's Rtx-volatile amine column (thermal conductivity detection):
https://www.restek.com/global/fr/chromatogram-detail/GC_PC1250
Its advantage would be zero response with FID. Ammonia has been used as carrier for packed capillary column solvating gas chromatography (SGC) of amines. The FID baseline was high, but that could be attributed to impurities or leached material.
Shen, Y., & Lee, M. L. (1998). Solvating gas chromatography using ammonia as mobile phase. Journal of Chromatography A, 806(2), 345–348. doi:10.1016/s0021-9673(98)00053-3
Assuming that the injection solvent will be water, this would have to be well separated from the analytes, because otherwise large amounts would perturb both chromatography and detection.
To quantify N-Methyldiethanolamine (MDEA) using GC-FID, prepare your sample by diluting or extracting it to concentrate MDEA while minimizing interference from water and glycol. Consider derivatizing MDEA to enhance its volatility if necessary. Establish a calibration curve with known MDEA concentrations, then inject your sample and measure the peak area to determine the concentration using the curve. Running a known MDEA standard alongside your samples can help confirm retention times and peak characteristics.
Use a very polar column, something similar to a wax phase, which is much more polar that what Christopher Lee is talking about. The diethanolamine will come off of that type of phase with decent enough peak shape that you'll get good quantification. You need a pretty high injector temperature, say 275-300, since you have both water and glycol present. You'll pick up the glycol and the piperazine as well, obviously. A wax phase will give you quantifiable glycol as well.
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