Dear All,

Prompted by what Gerard wrote, I thought I ought to mention what our 
observations are (though they may not be applicable to the particualr 
problem Adam is dealing with).

Running solid samples on our TC/EA system, H2 peaks start to go pear-shaped 
first (while CO peaks still look fine and the numbers still make sense). 
This is predominantly caused by build-up of very fine silver 'pearls' at 
the bottom of the reactor (and silver coating the glassy carbon chips) when 
too many capsules miss the crucible.

This used to happen after 50 to 70 samples and ties in with observations 
made by Paul Brooks who as a result fitted his machine with a self-built 
'hoover' to remove silver in-situ (while switching the He in reverse flow).

The only samples leading to a deposition of ash are wood samples, which 
doesn't surprise much. By the way, the maximum samle size we run is 
0.15-0.2mg for stuff such as hair, wood, waxes.

Not living in a lab such as Paul's with perks like a centrally supplied 
vacuum line, I looked at the reactor tubes and crucibles more closely and 
noticed that depending which way manufacturing tolerances add up, there are 
tubes in which the crucibles sit 'slanted' instead of 100% up-right.

So, I ordered from a 'tame' supplier crucibles with slightly wider 
dimensions figuring should they get stuck I can always file them down 
(another case of "too much not being a problem"). With these tight fitting 
crucibles, I have managed to get 150 solid samples run before both the H2 
peak and d2H-values go south.

There is another (obvious and more elegant) solution to this particular 
problem but so far I haven't found the right people to talk to.

Changing the way the He is piped in the furnace is supposed to help too.


Cheers,

Wolfram


On Aug 1 2005, Gerard Olack wrote:

> HI Adam--
> 
> What size samples?  We were running some amino acids awhile back, ca. 
> 300 ug each run, and were only getting 40 some down to 20 runs before 
> crucible had to be changed and then the eventual cleaning of the 
> reactor.  The excess carbon filling up the crucible seemed to be the 
> problem.  We had pushed through some plant material previously, 
> basically CHO so no excess C, and got 49 runs (one autosampler load) 
> before changing crucible and the reactor itself wasn't getting too dirty 
> either.  We won't talk about mispacking the reactor, upside down 
> crucibles or that piece of kimwipe leftover from cleaning the 
> autosampler that fell into the reactor.
> 
> In case any other garbage makes it's way through the reactor, we have a 
> cold (liq N2) trap between the reactor and the GC, via a 4-way valve so 
> the trap can be vented when it's warmed, akin to Werner's set up in 
> Isotopes Environ. Health Studies, vol 39, #2, June 2003 85-104.  Don't 
> know if it helped in this case.
> 
> Smaller sample sizes and larger crucible (still in hot zone) are the 
> only things I've tried.  I haven't really looked for any particular 
> tricks of trade in the hair and feather studies (for example, Z. D. 
> Sharp, et al., J. Archeological Science 30, 2003, 1709-1716, amongst 
> others).  Nor have I tried a water or nitrate "clean up" run.  Back to 
> barite and phosphate for me--for now.
> 
> good luck
> 
> gerry
> 
> 
> Adam Zoltan Csank wrote:
> 
> > Hi Wolfram,
> >
> > Thanks for the thoughts, It is the CO peaks primarily but both do it. 
> > And the TCEA was in perfect working order (new packing on the ceramic 
> > reaction tube, baked out GC etc...).
> >
> > What seems to happen is that the little graphite crucible, that 
> > catches silver preventing it from running into the ceramic tube, seems 
> > to soften and expand preventing the sample from dropping all the way 
> > into the hot spot. Because we've never had this problem before but it 
> > has now happened twice to us and both times when running continuous 
> > samples of benzoic acid...I've been doing some reading online and it 
> > seems that graphite will adsorb aromatic acids, such as benzoic acid, 
> > I'm wondering if that has something to do with it. We're going to be 
> > trying a fresh crucible but if that doesn't work, well three times 
> > conspiracy and we'll know the benzoic is the culprit, just not why.
> >
> > Adam
> >
> > On Jul 30, 2005, at 10:18 AM, Dr W Meier-Augenstein wrote:
> >
> >> Hi Adam,
> >>
> >>
> >> Do both peaks broaden (H2 and CO) or only the H2 peak?
> >>
> >> If its only the H2 sample peak, I'd be tempted to say something's 
> >> messing up the He flow in the reactor (usually spilled silver 
> >> starting to build up at the bottom of the reactor tube); but for that 
> >> to happen after only ten runs is pushing it a bit.
> >>
> >> I take it all the usual suspects have been ruled out, such as air 
> >> breaking in due to problems with the AS isolation valve or a GC 
> >> column that hasn't been baked out for a while?
> >>
> >> If both sample peaks broaden and eventually vanish, my money would be 
> >> on the column. Perhaps the fact it happened when you run your 
> >> benzoate samples was merely a coincidence.
> >>
> >>
> >> Cheers,
> >>
> >> Wolfram
> >>
> >>
> >> On Jul 29 2005, Adam Zoltan Csank wrote:
> >>
> >>> Hello all,
> >>> I've been attempting to run some Benzoic acid on the TCEA, however, 
> >>> it seems that after a little over ten samples have been run my peaks 
> >>> begin to tail until they they disappear. The cause of this problem 
> >>> is that the graphite crucible seems to be softening and degrading 
> >>> and so is not allowing the samples to reach the hot spot. I am 
> >>> wondering if maybe the benzoic acid is reacting with the graphite 
> >>> somehow. Or if , since we have been cleaning and reusing our 
> >>> crucibles, the crucibles themselves weaken over time.
> >>> I would be curious to know if anyone else has had this problem, and 
> >>> if anyone has any ideas as to what could be the cause.
> >>> Thanks in advance,
> >>> Adam
> >>> Adam Zoltan Csank
> >>> Department of Geological Sciences,
> >>> University of Saskatchewan,
> >>> 114 Science Place,
> >>> Saskatoon, SK, Canada,
> >>> S7N 5E2
> >>> phone (o) (306) 966-5737
> >>>               (h) (306) 665-2797
> >>> email <[log in to unmask]>
> >>> http://homepage.usask.ca/azc461
> >>>
> > Adam Zoltan Csank
> > Department of Geological Sciences,
> > University of Saskatchewan,
> > 114 Science Place,
> > Saskatoon, SK, Canada,
> > S7N 5E2
> > phone (o) (306) 966-5737
> >              (h) (306) 665-2797
> > email <[log in to unmask]>
> > http://homepage.usask.ca/azc461
> >
>