GC-MS Carrier Agent Question

[kmh235]

I want to use a GC-MS to analyze the concentration of a dissolved acid in supercritical CO2. I figure the CO2 could act as a carrier agent in the GC-MS. However, instead of injecting the acid into the injection port, I would like to dissolve it in the supercritical CO2 before it enters the GC-MS. Could I simply do this and ignore the injection port and still yield a valid reading for the concentration of the dissolved acid?

[CE Instruments]

This just screams no to me but lacks information to make me really certain. If you are analysing in supercritical CO2 then SFC would be my technique of choice. Introducing CO2 as a slug into a GC is not IMO going to go well and surely you do not think that you can run CO2 carrier gas rather than He/N2 ? I used to do capillary SFC with home made capillary restrictors, GC type SFC will never work unless anyone can control the flow with a variable restriction.

[kmh235]

Thanks for the response. Why do you think CO2 would not act as a good carrier gas? As long as it is inert towards the sample, it should work.

[Peter Apps]

By "carrier agent" do you mean a solvent for the sample, or a carrier gas for the separation ?

CO2 is too viscous (too high MW, too low diffusivity) to be a good carrier gas unless you have lots of time to wait for your compounds to elute.

CO2 molecules will soak up the electrons in the MS source - at best you might get some chemical ionisation.

If you plan to do supercitical chromatography then the volume flow of CO2 gas through the MS would overwhelm the pumps.

If you plan to use the CO2 as a sample solvent (equivalent to say hexane or dichloromethane) how do you plan to keep it supercritical while you inject it ?

Peter

[kmh235]

I plan on using CO2 as a solvent (I would dissolve the acid in supercritical CO2 and then send it through the SFC-MS). If I were somehow able to keep the CO2 in its supercritical state, would it be feasible to measure the quantity of the acid?

Sorry if this is a silly question - I am a novice with this technology.

[kmh235]

If using SFC-MS, can I simply skip the extraction chamber step and go straight to the recovery chamber (since the acid is already dissolved in supercritical CO2)?

[Peter Apps]

An SFC-MS and a GC-MS are two different kinds of hardware. The gas flows through an SFC are too high for most GC-MS vacuum pumps, and EI-MS does not work well with high MW gasses in their source. SFC-MS uses API and ESI sources, like LC-MS.

I presume from the hints that you give that you are extracting the acid from something, using supercritical CO2. While analysing the extract directly using SFC sounds attractive, you need special hardware to do it. Evaporating off the CO2 and taking up the acid in a GC compatible solvent might be easier in the end if you have a concentional GC-MS instrument.

Some more details about what you are doing would help people give more useful replies.

Peter

[kmh235]

Thanks for the response. I am not extracting the acid from something, I already have the acid in liquid form. The question I am trying to answer is what compounds does the acid break up into when dissolved in supercritical CO2? Any thoughts on how I might be able to determine this?

[Peter Apps]

Thanks for the response. I am not extracting the acid from something, I already have the acid in liquid form. The question I am trying to answer is what compounds does the acid break up into when dissolved in supercritical CO2? Any thoughts on how I might be able to determine this?

Not unless you can share what the acid is and what you expect it to turn into.

Peter

[kmh235]

Peracetic acid - I believe it will dissolve into hydrogen peroxide and acetic acid

[Peter Apps]

You are probably right - the three compounds form an equilibrium mixture. You could conceivably detect the acids with an MS but I doubt that you can do peroxide.

Peter

[kmh235]

Thank you very much for your help, Peter. Much appreciated

[kmh235]

Sorry for the delayed question, but why exactly MS couldn't detect peroxide?

Thanks again for the help

[Peter Apps]

It seems that it can be done after all:

http://pubs.rsc.org/en/Content/ArticleL ... ivAbstract

First hit on google. Free full text into the bargain. Let us know how it goes.

Peter

[James_Ball]

It seems that it can be done after all:

http://pubs.rsc.org/en/Content/ArticleL ... ivAbstract

First hit on google. Free full text into the bargain. Let us know how it goes.

Peter

Possible, but not an off the shelf instrument by any means.