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
>