Do an Ar leak check--but I'm guessing Ar levels are low and a blowing Ar
on the couplings or up the side of the alumina tube won't show any big
peaks/leaks. There is a off chance that the He tank is slightly
contaminated, and if you have a water/O2 trap on your He line, that may
need to be replaced.
The GCT and alumina will react at temp., and the alumina tube becomes
more permeable at temp too. And if you look in the archives you'll see
discussion of micro-fractures too. And as Wolfram mentioned, different
alumina tubes behave differently.
Basically, will take days to fully bake out. If you are not below 200
mV on 28 after a couple of days at 1325--you might have other problems.
If the baseline is fairly stable, you can run with relatively high
backgrounds seen at 1450. If you do a series of standard runs, using
enriched and depleted standards, try fitting the results using a blank
correction--see if that helps. That said, the lower the backgrounds, the
On 1/11/2017 12:05 PM, Matthew Lachniet wrote:
> Hi all,
> I have a question for TC/EA users.
> Over the past four weeks I've been resurrecting from the dead my TC/EA
> for water analysis after no use for three years.
> The last major issue among a long list of them is that I've
> experienced very high CO backgrounds (28/29/30) of >500 mV that don't
> decrease over time. H2 backgrounds are very low to be essentially
> negligible. As I understand it from the ISOGEOCHEM archives, the
> glassy carbon tube (GCT) can react with the ceramic tube to produce CO
> and Al2. Indeed, when I removed the GCT, which has only seen about 20
> water injections since installation, the hot zone had large amounts of
> reaction products, and I found ash particles in my bottom feed adapter.
> My question is: how can the generation of CO be avoided? Both the
> ceramic tube and the GCT are new, and I had the same results with an
> old ceramic tube/new GCT combo. The high backgrounds are preventing
> usable data for d18O, but the d2H data are fine. Both tubes are
> straight and have no bends or deformities that would cause contact
> between them.
> Other background: the Helium supply is clean. Backgrounds are very low
> when I turn on He dilution in the ConFlo, and backgrounds disappear
> when I close the needle valve to mass spec, so I'm pretty sure the CO
> is coming from the TC/EA. I have done extensive leak testing and found
> none. I've baked out the GC at 150+ degrees many times with no
> appreciable effect on backgrounds. The high backgrounds start to
> appear when reactor temperature is above 1300 C. Though I don't have
> all the specs off the top of my head, the parameters for He flow rate
> are the same as previously successful runs. I've removed and reset the
> GCT/ceramic tube combo about three times to ensure there wasn't some
> leak, and each time the high backgrounds return.
> Any advice would be appreciated.