Thanks to all who provided help with my question regarding problems 
analysing FAMEs by GC-c-IRMS a few weeks back. The problem was fixed 
after replacing the capillary between the x-piece  in the GC oven and 
the oxidation reactor and by replacing both oxidation and reduction 
tubes. I saw right away that both tubes were spent. I also replaced all 
capillaries between the reactor and the IRMS. Since then, I've had no 
trouble and the FAMEs have run beautifully.
Thanks again

Melissa



Wolfram Meier-Augenstein wrote:
> <http://www.qub.ac.uk/eerc/people/academic_staff/wma/wm_a/efnhh.htm>
>
> Hi Melissa,
>
>
> If I understand you correctly, the IRMS chromatogram of injected n-hexane looks the same as your last FAME run.
>
> By the way, what did the FID chromatograms look like of both the last FAME and the hexane injection?
>
> Or asking outright, do the FID chromatograms continue to look OK, i.e. as expected and as before the problem started?
>
> If the answer is yes to the above your problem is most likely interface related.
>
> You see, problems at the injector (leaking septum, leaking split valve) or a leaking backflush valve (connected to the X-piece) will result in reduced peak size but usually not in a complete disappearance of all (major) peaks. Incidentally, to avoid fractionation in the injector it's a good idea to inject splitless and open the split subsequently (e.g. after 15s).
>
> So, either a teensy-weensy piece of fused silica (FS) is blocking the X-piece or the X-piece is leaking He like it's goign out of fashion. If the X-piece is a full metal Valco job leaks at the screw connectors that develop after a few runs ending at 300+ C are not uncommon. That would be option 1.
>
> Option 2, the FS connecting the X-piece to the reactor tube has become blocked reactor-side, either by "condensed" partially combusted sample material or the tip has melted and collapsed. Positioning of this piece of capillary is crucial.
>
> Option 3, the oxidation reactor is exhausted and wants replacing.
>
> Considering you have run a few samples after the problem manifested itself, my suggestion would be rebuild the interface and heat out the interface capillaries anyhow since there is a change of uncombusted sample material break-through into the interface.
>
> Step 1: Replace both ox and red reactor tubes making sure that up-stream connecting capillaries come to end within the ceramic tubes but don't touch the metal wires. Also replace the capillary between X-piece and ox reactor.
>
> Step 2: Check the X-piece for any debris before re-assembly; inspect the ends of all capillaries to make sure they are not crushed; neither should they have extraneous FS prodruding from the cut.
>
> Step 3: With the IRMS disconnected from the interface (moving capillary open split), the reactors at operational temperature (with the ox reactor oxidised) and the back-flush closed (i.e. GC connected to the interface), work from the ox reactor towards the IRMS heating the capillaries with a heatgun (carefully).
>
> Step 4: Connect the IRMS to the interface and run a magnet scan from mass 12 to mass 50.
>
> Step 5: If step 4 gives your system a clean bill of health, load your CO2 settings, set the GC injector to split mode and inject some gas (collected in a test tube from a gas tap for a bunsenburner in your lab) using you current GC standby parameters and see if within the dead volume time equivalent of your system you detect a CO2 peak.
>
> All going well you should then be back in business.
>
>
> Best,
>
> Wolfram
>
>
> ________________________________
> From: Stable Isotope Geochemistry [[log in to unmask]] On Behalf Of Melissa Bautista [[log in to unmask]]
> Sent: 20 January 2008 00:11
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] GC-c-IRMS
>
> Hi Wolfram,
>
> Thank you for replying. I have answered your questions as best I can below. Our GC is a Thermo Trace Ultra, The combustion interface is the Finnigan III, and the IRMS is the Delta V Plus.
>
>
> 1. What is the temperature setting for the injector? 260C
> 2. What's the average concentration per FAME in your FAME mix (in nmol/microL)? Unsure. Samples were extracted from both seagrass and mangrove sediments, but I only received them ready for methylation. I was told that the total lipid weight was between 1-2mg.
> 3. Have you looked what the chromatogram looks like for a dummy run (no injection or neat solvent only)? I have run hexane only with similar results (can send you the Isodat run file if that would help).
> 4. Same as point 3 but for single compound sample, say 10:0 or 12:0. I have not done this, but will do so now.
> 5. Is your system fitted with a FID and a cross-piece prior to the oxidation recator? Yes, FID and cross-piece.
> King regards,
>
> Melissa
>
>
>
>
>
> ________________________________
>
>   

-- 
Melissa D. Bautista
IRMS Research Technician
Centre for Coastal Biogeochemistry
Southern Cross University
P.O. Box 157 Military Rd
Lismore NSW 2480
Australia
+61 266 269 565
+61 431 928 278