All,

We ran many analyses for methane on our older GCC III using the integral fused silica combustion region that I've preached about before (Keith Goodman, “Hardware Modifications to an Isotope Ratio Mass Spectrometer Continuous-Flow Interface Yielding Improved Signal, Resolution, and Maintenance”, Anal. Chem. 1998; 70(5), pp. 833-837). According to my notes, we used our typical 4 wire system (1 Pt, 1 Ni and 2 Cu) and ran the temperature up to 950C. We had at one point checked the temperature of the combustor, and found that our temperature was indeed rather accurate. For that work, accuracy and precision were only needed to 1 per mil, but I recall doing about twice that good.

As memory serves, we had much more trouble with the injection system (Valco valve and Gerstel inlet), and purging the regulator of the methane standard tank to get a true value (it may have taken several liters of flushing before the little regulator was finally fully purged. However, once the system was up and running I ran many hundred analyses over a period of several weeks, checking against my standard tank each morning.

It can be done, but like everything else in this game, it will require patience and perseverance.

Good luck,
Carl




***********************************************************
***********************************************************
Mr. Carl G. Johnson
Research Specialist
Dept. of Marine Chemistry and Geochemistry
123 Fye Laboratory, MS#4
Woods Hole Oceanographic Institution
Woods Hole, MA 02543-1543
USA

Tel: 508-289-2304
Fax: 508-457-2164
[log in to unmask]
http://www.whoi.edu/sites/MCG_OrganicMassSpectrometry

***********************************************************

Organic Geochemistry & Paleobiology Lab Manager
Dept. of Geological Sciences
Brown University
324 Brook Street, Box 1846
Providence, RI 02912
USA

Tel: 401-863-3091
[log in to unmask]

***********************************************************
***********************************************************






Wolfram Meier-Augenstein wrote:
[log in to unmask]" type="cite">
Hi Herb and Pat,


It would seem you have both overlooked that Harish is running a CuO only system, which explains the temperature setting of 850 C (he has no choice in this matter). However you are both correct in saying 850 C is far too low a temperature for quantitative combustion of methane.

That said, running a CuO only reactor at 950 C is about as useful as having a one-legged man on your team in an arse-kicking contest. CuO decomposes quite rapidly at temperatures in excess of 820 C, so running a CuO filled reactor at 950 C would result in exhaustion of the CuO bed without the benefit of combustion.

The temperature setting of 950 C give or take 10 degrees is geared up for CuO/NiO reactors in which NiO acts as oxygen sink while the oxidising agent CuO is essentially formed in situ (from oxygen provided by NiO).

That aside, combusting methane efficiently in a GC/C interface is tricky and if memory serves the solution Dawn Merritt proposed was to increase the reactor temperature to as high as it would go (ca. 1000 C) and change the reactor filling from CuO/NiO to NiO only.


Best,

Wolfram


****************************************
"Facts do not cease to exist because they are ignored."
Aldous Huxley, "Proper Studies", 1927
****************************************
Dr W Meier-Augenstein
Senior Lecturer in Stable Isotope Forensics

Environmental Forensics & Human Health Research Group
EERC, S.P.A.C.E.
Queen's University Belfast
David Keir Building
39-123 Stranmillis Road
Belfast, BT9 5AG
United Kingdom

Tel.: +44 (0)28 9097 4015
Fax: +44 (0)28 9066 3754
Email: [log in to unmask]<mailto:[log in to unmask]>
URL (Isotope Lab): http://www.qub.ac.uk/eerc/facilities/siff.htm
URL (Research): http://www.qub.ac.uk/eerc/people/academic_staff/wma/wm_a/efnhh.htm

________________________________
From: Stable Isotope Geochemistry [[log in to unmask]] On Behalf Of Patrick McLoughlin [[log in to unmask]]
Sent: 06 December 2007 14:48
To: [log in to unmask]
Subject: Re: Havier d13 C values

Harish,

I’d guess you are regenerating with oxygen. My combustion oven runs at 940C, not 850. 850C will work, but could be problematic and probably gets you into a messy region where some compounds combust fine, and others do not. I’m certain many others will weigh in, but I think the rule of thumb is that the higher the temperature, the more frequently you need to regenerate. If I go too long between regenerations, I definitely see my organics get heavier – 2 mil is on the high side, but  not “unheard of”.  You can pretty readily replace the tube in the furnace. I don’t know the exact cost, but I seem to recall it is pretty modest and pretty easy to do. Knowing when you need to do it is trickier. The easiest thing is probably to get on a regular schedule. Your reply will be something like: “OK, what is the frequency?” Of course the answer is: it depends. So for a little bit you need to play with it, then set up your schedule. You will find that as the tube ages, you have to do re-g
!
 enerations more and more frequently. I have tried to watch oxygen del’s to know when the regeneration is needed, but I have had no luck. Perhaps others can help.

You question about lower temps during idle times is an interesting one. I never tried it, but I must admit I am curious. We shall see what the responses are.

Pat McLoughlin

From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of harish pande
Sent: Thursday, December 06, 2007 7:18 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Havier d13 C values

Dear All

I have the following queries which I want to share with this forum.

1. We are using GC-IRMS for d13C determinations of hydrocarbon gases. The Combustion interface (CuO) is at 8500C.Off late we noticed that our d13C values for methane are ~2 ‰ heavier than the actual. The d13C values for pure CO2 are OK.
 Can we increase the temperature of furnace without losing on its life time?

2. What temperature of furnace is to be maintained when we are not using this interface,does it require regeneration or replacement?

Thanks & Best regards

Harish Pande

________________________________
Post free auto ads on Yello Classifieds now! Try it now!<http://ss1.richmedia.in/recurl.asp?pid=255>

  

-- 



***********************************************************
***********************************************************
Mr. Carl G. Johnson
Research Specialist
Dept. of Marine Chemistry and Geochemistry
123 Fye Laboratory, MS#4
Woods Hole Oceanographic Institution
Woods Hole, MA 02543-1543
USA

Tel:  508-289-2304
Fax:  508-457-2164
[log in to unmask]
http://www.whoi.edu/sites/MCG_OrganicMassSpectrometry

***********************************************************

Organic Geochemistry & Paleobiology Lab Manager
Dept. of Geological Sciences
Brown University
324 Brook Street, Box 1846
Providence, RI 02912
USA

Tel:  401-863-3091
[log in to unmask]

***********************************************************
***********************************************************