Cyndi,

your yellow crystalline substance, from my experience, should be
the deposition of silver vapor from your silver cup. I'm using Ag2SO4
as my working sample, and I get the same thing. In fact, if your tube
is enough hot, the silver can even diffuse through the crystal. With quartz
tube, you end up with a pretty nice quartz doped with Ag, which gives
a pale yellow color to your quartz.
Concentrate nitric acid is very efficient to dissolve Ag.

Hope this will help you

joel

At 05:58 PM 1/3/01 -0700, you wrote:
>Greg and other sulfate-oxygen people,
>
>We are using a TC/EA set up with a Delta Plus XL for running d18O-SO4 with
>reasonable success.  I am interested if any other users of the same
>equipment have experienced any of the same problems we have though.  We have
>less memory problems when we run at higher temperatures (>1400C), but, it
>appears at higher temperatures we begin to damage the ceramic tube and/or
>the glassy carbon tube after a couple hundred samples.  We have not run
>enough other "stuff" to determine if the problem is just with SO4 or if
>water, NO3, etc. might also cause problems regardless of temperature.
>Anyhow, we start to accumulate a yellowish crystalline substance on the
>outside of the glassy carbon tube at approximately the level of the
>combustion.  We scrape this stuff off when we clean out the crucible (~200
>samples).  But eventually it gets thick in spots and then the ceramic starts
>to get a clear glaze on the outside at about the level of combustion.  We've
>had fine cracks then develop and then larger cracks until the ceramic tube
>ends up breaking.  We have thought that maybe the S is poisoning the tube at
>high temperatures??  Has anyone else seen this with or without SO4 samples?
>We've mainly run pure BaSO4 precipitates, some barite minerals, jarosite,
>alunite, etc.
>
>As far as how to actually do the analysis, we basically follow the TC/EA
>manual for recommended flow rates, temps, etc. we have the reactor set
>1400-1450C and the GC at 90C.  We weigh about 0.3 mg BaSO4 into a silver
>cup.
>
>Does anyone have insight on the mystery crystalline-goo?
>thanks,
>Cyndi
>
>**************************************************************
>Cyndi Kester
>Ecologist--Stable Isotope Lab
>U.S. Geological Survey
>Denver Federal Center, Bldg. 21, MS 963
>Denver, Colorado 80225
>
>office:  303-236-0278
>lab:     303-236-7694
>fax:     303-236-4930
>email:   [log in to unmask]
>
>
>----------
> >From: "Gregory A. Wandless" <[log in to unmask]>
> >To: [log in to unmask]
> >Subject: Sulfate oxygen by on-line pyrolysis EA
> >Date: Wed, Jan 3, 2001, 11:41 AM
> >
>
> > I am looking for references or any other information on doing sulfate
> > oxygen isotopic
> > analysis by on-line pyrolysis EA. Any suggestions?
> >
> > Thanks,
> > GAW
> >
> > --
> > =======================================
> > Gregory A. Wandless
> > Chemist
> > US Geological Survey
> > MS954
> > Reston, VA 20192
> >
> > Email:[log in to unmask]
> > Phone: (703) 648-6189
> > =======================================

******************************************************
Joel Savarino
Research Associate
Stable Isotopes Lab.
Dept. Chem. & Biochem. Urey Hall -0356
UCSD
9500 Gilman Drive
La Jolla, CA 92093-0356 USA
Tel.   +1 (858) 534 6053
FAX. +1 (858) 534 7042
*******************************************************