GC/MS and respiration analysis

[Clayton]

I am still trying to get a straight answer on this topic

aim: measuring soil respiration by quantifying changes in headspace O2 and CO2 using GC/MS

I am wanting to measure O2 consumption and CO2 evolution using a HP6890 GC/MS that I have. Obviously this system is capable of detecting these two compounds using the selective ion program (m/z 44 and 32) as this is basically the process it uses for the air and water check.

Has anyone done this or know of any similar work. Obviously my concern is that the MS is sensitive to oxygen and I am unsure if I am able to put small amounts of air thru the system without damaging it.

My rationale for not wanting to measure O2 and not just CO2 via infra-red or alkali trap is that wetting the soil gives off CO2 NOT from respiration.

I have been given mixed messages as to if this is possible. Do people looking at environmental contaminants in air or simarly looking at analysis of breath remove oxygen before it gets to the MS?

Any help would be greatly appreciated!

[Consumer Products Guy]

GCMS could be used for this; we routinely inject 1 ml air samples when looking for components in headspace. What about TCD, is that sensitive enough for you? Your, if you're looking at low levels, would also include background O2 and CO2; you'd have to be at levels above any O2 and CO2 in background.

[Clayton]

GCMS could be used for this; we routinely inject 1 ml air samples when looking for components in headspace. What about TCD, is that sensitive enough for you? Your, if you're looking at low levels, would also include background O2 and CO2; you'd have to be at levels above any O2 and CO2 in background.

I dont have a TCD and this is over 10 grand to put in. When you inject air you let this go through to the MS without removing or venting the O2? I am only using sample size of 250uL so this is much less O2 going through to the MS and would be less damaging would you agree?

I will be looking at reduction in O2 in my headspace from 21%, given the size of my jars and the volume of soil I think that i will pull the O2 down by 2-3% so I think I can detect that (waiting for some more gear to be able to make my standards)

My concerns were more that I will get criticised for running air samples thru the MS and that it may hinder my ability to publish the data

Thanks for your help!

[dcobice]

hi, I was the same issue a long time ago...I used the same GC/MS and when I did the air and water check i saw that when the value of oxigen was under 2% in the inonization source there was no problem to performed the analysis. In our case the sample was a teddlar markets containing air from chemical indrustry enviromental. I Remember that I had no problem with the ion source when I inyected 1 ml of air into GC inlet.The only one issue was the cuantity of air in the ion source.I used the SIM mode to performed the analysis ( 32 and 44 M/Z), because of the nitrogen that also ( was depends on the column you used) chromatographed at these conditions.The columns satationary phase was DB-Wax
Thanks and good look!

[Clayton]

Thanks Dcobice

Just to clarify what you mean. Once you have finished running your samples and do the air/water check that the detector has less than 2% in the ion source? i.e. it is retaining a vaccuum and working well even with the O2 going through it.

Can you recall what sample size you used.

Have you published any of your work? It is important that I am able to get a good reference base to support what I am doing. Plus I would be interested to read what you did.

Cheers!

Clayton

[Waterboy]

Clayton

I routinely make 2 mL headspace injections (mostly air) into my 5973 GCMS. You will not see any performance problems. Your biggest problem is going to be contaminating lower level oxygen samples with atmospheric oxygen levels surrounding everything.

If you could provide more information regarding your instrument configuration such as GC inlet type, pumping system (diff pump or turbo), column used, and how you plan to introduce the sample ie. by manual syringe, injection valve, or headspace sampler. I may be able to offer some suggestions.

What you are trying to do is analagous to analyzing a sample for water content at the bottom of the ocean.

A more appropriate instrument for your task would have been a respirometer.

[JI2002]

Injecting 1 ml of air into GC/MS shouldn't be a problem. Actually one way to measure carrier gas linear velocity is by injecting air into GC/MS and timing the ion m/z 28 or m/z 32 peak in MS. My concern is if 6890/5973 has the accuracy and precision for the task. I did some calculations, 250 ul of air has about 68.8 ug of O2 (assuming 21% O2 in the sample) and 65.6 ug of O2 if O2 is 20% in the sample. The amount of O2 injected into the instrument decreased by only 3.2 ug (5%) if oxygen decreases by 1% (from 21% to 20 %). Not sure if the instrument can generate the results with accuracy and precision in a small range like this.

[Clayton]

hi, I was the same issue a long time ago...I used the same GC/MS and when I did the air and water check i saw that when the value of oxigen was under 2% in the inonization source there was no problem to performed the analysis. In our case the sample was a teddlar markets containing air from chemical indrustry enviromental. I Remember that I had no problem with the ion source when I inyected 1 ml of air into GC inlet.The only one issue was the cuantity of air in the ion source.I used the SIM mode to performed the analysis (32 and 44 M/Z), because of the nitrogen that also ( was depends on the column you used) chromatographed at these conditions.The columns satationary phase was DB-Wax
Thanks and good look!

I am using splitless injector @250degC, helium as carrier at 1mL per minute, the column is a SGE BPX-5 30mx250uLx25uL, oven at 50degC.
direct injection into the front inlet of 250uL

to make my CO2 and O2 standards I am using nitrogen as the diluent

Does anyone have their work published? even in miscellaneous publications or have stuff for me to read?

maybe i should increase the volume of air I inject and that way i wouldnt be operating with such small differences in the amounts of O2

suggests and feedback welcomed!!!