Martin,
We did have an experience where one set of what I recall were soil organic
matter which had been treated with HF wrecked out combustion tube and
copper tube, and started the GC column trailing. It was this that had us
bake out the column and got it working again. So it is possible that some
particular samples affected the GC column.
Paul Brooks.
At 08:30 AM 3/6/2007, you wrote:
>Thanks John and Paul,
>
>I will look into your suggestions. The puzzling part about this problem
>is that the system was apparently running just fine until recently. I am
>still in the process of locating old data to see just when this all
>started, but it appears to have begun between two sample runs. I am at a
>loss as to what could have changed from one run to the next. Since we
>have never used a CO2 trap, I am going to attempt a GC bake-out, and
>verify combustion. By the way, what is the "typical" life on these
>columns (Porapak QS)?
>
>Thanks again.
>
>M
>
>At 10:13 AM 3/2/2007, you wrote:
>>Martin, I am not familiar with the Micromass system, but for a discussion
>>on using a CO2 trap and valve after the GC column on an EA to remove
>>trace CO2 from the N2 peak see:
>>
>>Brooks, P.D., H. Geilmann, R.A. Werner and W. A. Brand. 2003. Letter to
>>the Editor:Improved precision of coupled delta 13C and Delta 15N
>>measurements from single samples using an elemental analyzer/isotope
>>ratio mass spectrometer combination with a post-column six port valve and
>>selective CO2 trapping: improved halide robustness of the combustion
>>reactor using CeO2. Rapid. Commun. Mass. Spectrom. 17:1924-1926.
>>
>>It also helps to bake out the GC column in the EA regularly and possibly
>>replace the GC column.
>>
>>Hope this helps,
>>
>>Paul Brooks.
>>
>>
>>
>>At 06:28 AM 3/2/2007, you wrote:
>>>Hi Martin,
>>>VY should open a couple of seconds before the peakjump from N2 to CO2. It
>>>should be closed just before the the end of the CO2 acquisition. Switching
>>>prematurely before the introduction of the reference CO2 pulse changes the
>>>dilution factor for the CO2 still bleeding from the GC column from say 10 or
>>>20:1 to no dilution so causing an immediate and not constant change to the
>>>CO2 background levels. This will affect the reference ratio hence the
>>>importance of acquiring both sample and reference peaks on the same
>>>background.
>>>With regard to the tailing of the soil sample it sounds like a question of
>>>combustion efficiency. Have you tried adding a combustion accelerator such
>>>as V2O5 to the samples.
>>>Regarding the carry over of CO2 affecting the next nitrogen result. This of
>>>course can be a problem when analysing both N and C isotopes on the same
>>>sample however there may be a solution to it for those of you using the GV
>>>diluter. By adding an ascarite CO2 adsorbtion trap between the diluter
>>>restrictor valve and the valco switching valve on the nitrogen line you will
>>>be able to trap the CO2 affecting the nitrogen whilst at the same time still
>>>sampling the CO2 into the dilution line. Anyone requiring more details on
>>>this point please contact me direct.
>>>
>>>Regards
>>>
>>>John
>>>
>>>---------------------------------------------
>>>John Morrison
>>>GV Instruments
>>>Crewe Rd
>>>Wythenshawe
>>>Manchester
>>>M23 9BE
>>>---------------------------------------------
>>>44 (0) 1619022100 Main office
>>>44 (0) 1619022165 Desk
>>>44 (0) 1619022107 Lab
>>>
>>>
>>>
>>>
>>>
>>>-----Original Message-----
>>>From: Martin du Saire [mailto:[log in to unmask]]
>>>Sent: 01 March 2007 15:51
>>>To: [log in to unmask]
>>>Subject: [ISOGEOCHEM] CO2 carry-over
>>>
>>>
>>>Hi,
>>>
>>>I am running into carry-over issues on a EA-IRMS with an Isotech
>>>diluter for CO2. What appears to be happening is that the incoming
>>>sample stream still has CO2 levels higher than the diluted stream
>>>when VY closes. I get a jump in currents, but this in turn causes a
>>>jump in 2/1 ratio. Since VY closes just before the CO2 ref gas peak,
>>>this causes a spike in the ref gas 2/1 ratio too. With an 18 mg soil
>>>sample, the elevated 2/1 ratio can persist for more than 1000 s
>>>(essentially 2-3 subsequent samples). The effect is smaller and
>>>shorter in duration for smaller samples (plant: 3 mg), even though C
>>>peaks are larger. (I might be able to avoid the problem with
>>>the CO2 ref gas peak by closing VY afterward, but I suspect I would
>>>have CO interference with the N2 peaks.)
>>>
>>>Two questions:
>>>
>>>Is this normal?
>>>
>>>If my interpretation of what is happening with the sample stream is
>>>correct, why would the un-diluted stream have a higher 2/1 ratio?
>>>
>>>Thanks.
>>>
>>>M
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>>
>>Center for Stable Isotope Biogeochemistry
>>Valley Life Science Building Room 3060
>>Integrative Biology - MC3140
>>Berkeley CA-94720
>>Phone: (510)-643-1748
>>Fax: (510)-643-1749
>
>Center for Stable Isotope Biogeochemistry
>Valley Life Science Building Room 3060
>Integrative Biology - MC3140
>Berkeley CA-94720
>Phone: (510)-643-1748
>Fax: (510)-643-1749
|
