I recently started up my Thermo TC-EA after a long idle period. When I last
worked with it I was also getting really poor results. When I started it up
again recently I baked the GC column at 300 oC for several days but teh
results didn't improve and I noticed that the peaks were tailing badly. In
the between time I had discussed the technique with Matthias Gehre at
JESIUM. He suggested that the GC column might be the culprit. I then
repacked the GC column with fresh molesieve (molecular sieve 5A, 80/100,
Chrom Specialties) and peak shapes improved immensely. I also lengthened
the runtime to ensure that the trace returned to baseline for each sample.
(I'd advise that you watch the baseline at the 10's of mV scale to monitor
Matthias suggests that any flourinated gases produced during the thermal
conversion will seriously damage the GC column. Previously we had run a lot
of AgNO3 samples which I later found out were likely contaminated with Cl
salt, so maybe my GC packing was dry but it was simply toasted and I
wasn't paying enough attention to the baseline ??
To avoid this problem he suggests putting an ascarite trap in the flow path
(I think Paul Brooks also came to this conclusion with his system). I have
followed this advise and have a put a small ascarite + magnesium
perchlorate + P2O5 w indicator trap right at the bottom of the unit. This
serves the dual purpose of also trapping any water from the frequent
repacking and reheating. Keeping the water backgrounds in the IRMS down is
critical for 18O work. We are currently running pure (well mostly pure ...)
BaSO4 samples and I use this ascarite + water trap (amongst other changes
to the TC-EA) and it is working well.
See Matthias Gehre and Gerhard Strauch, "High-temperature elemental
analysis and pyrolysis techniques for stable isotope analysis" RCM, 2003:17
:1497 - 1503
At 01:44 PM 12/16/2004, you wrote:
>I have a TC-EA system based on the Hekatech furnace with glassy carbon tube
>in ceramic tube design. It used to work fine. Now I have severe tailing
>of the CO peak which I have been unable to cure. I'm out of ideas as to
>what to try next.
>Here are some things I have already tried.
>1. The tailing is similar whether I try organic (eg sucrose) or inorganic
>(eg silver nitrate) samples.
>2. Changing the reactor temperature between 1200 and 1400 has no effect
>3. Baking the molecular sieve GC column did not help
>4. Reassembing the reactor numerous times including replacing the glassy
>carbon tube and filling (130 mm GC 'chips' sieved to 2.5-3.5 mm + 5mm
>layer nickelized carbon) had no effect.
>5. Increasing the spring loading on the glassy carbon tube did not help -
>the theory here was that the balance of gas flow through the inner and
>outer tubes was incorrect and that more spring pressure would increase the
>flow through the inner tube.
>6. The system is leak tight and has a low background and blank.
>What really gets me is that nothing I have tried so far had any effect -
>any ideas most welcome
>UC Davis Stable Isotope Facility
>Department of Agronomy
>3112 Plant & Environmental Sciences Bldg.
>Davis, CA 95616
>phone 530 754 7517
>fax 530 752 4361
Lab Manager, Isotope Science Lab,
University of Calgary, Physics & Astronomy,
2500 University Dr. N.W.
Canada, T2N 1N4
(p) 403 220 8268
(f) 403 220 7773
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