Maybe BAKE OUT the GC Oven of the EA?
Raise the oven Temperature to 110 deg C for 4 hours, then return it to  
your regular operating T (55C, or 60C, or whatever it is). Our EA  
manufacturer recommends doing this every 3-4 months.
-- 
Charlotte Lehmann
Assistant in Instruction
Bates College
Department of Geology
206A Carnegie Science Building
44 Campus Avenue
Lewiston, ME 04240
Phone: 207-786-6485
FAX: 207-786-8334


Quoting "Stephen S. Howe" <[log in to unmask]>:

> Isogeochemists:
>
> I am having a new problem with unacceptable peak tailing during carbon
> isotopic analyses of samples using our Carlo Erba  NA 1500 Series 2 NC
> elemental analyzer interfaced with our Optima mass spectrometer.
>
> Last week, I finished the analysis of about 700 samples, about 600 of
> which were marine sediment in 5x9 mm tin capsules, followed by about
> 100 samples of marine mammal muscle tissue in 3.5x5 mm tin capsules.
> Chromatography was excellent during these runs, with just the faintest
> hint of tailing as the CO2 peaks came down to background only when the
> quartz inserts needed changing as they filled with ash and the sample
> drop position started moving slightly above the optimum hot zone,
> stretching the duration of the combustion very slightly.  The magnesium
> perchlorate water trap was changed after about 650 samples, and the
> remaining 50 or so analyses were fine.  I decided to change the
> oxidation and reduction chemicals in their respective furnace tubes
> after what appeared to be NO started to break through during the runs a
> second or two after the time N2 would be expected (even though the mass
> spec was tuned only to CO2 during these runs).  Sure enough, the copper
> turnings in the reduction tube were completely oxidized.
>
> I brought the reaction tubes back down to room temperature to change
> the chemicals, and replaced both the oxidizing and reducing chemicals
> with new chemicals of the same vintage as the chemicals used for the
> 700-sample run.  I loaded everything the same way I always do, taking
> pains to insure dead volume was at a minimum.  Leak tests were made and
> flow rates were set at room temperature and then both again at high
> temperature.  He, O2, and CO2 supplies are from the same cylinders as
> before, at the same delivery pressures as before both at the EA and the
> reference gas injector box.  Capillary positions remain unchanged and
> mass spec background when tuned to N2 and ion gauge readings are as
> good as ever.  Stability runs are excellent.
>
> Samples of the peach leaves standard (I usually run at least a dozen  
>  or so after changing chemicals) appear to be combusting  
> beautifully,  with a bright flash two seconds after the drop, as I  
> usually set.   However, the chromatography is not acceptable,  
> exhibiting pronounced  tailing as the CO2 peak attempts to return to  
> baseline, both for the  TCD and the mass spec traces.  Sometimes  
> this tailing extends for at  least 60 seconds beyond the time it  
> used to reach baseline.  The  shapes of the 2/1 and 3/1 ratio traces  
> are also noticeably affected.   Extending the run 60-120 seconds  
> past the usual length of 360  seconds allows the baseline to drop  
> some more between samples, but  I'm not able to get any consistency  
> among isotopic values, even with  adjusting the HS open timing to  
> optimize the acquisition window. I'm  getting values of -23 ± 2 per  
> mil, instead of my customary -25.9 ±  0.2 per mil.
>
> It seems logical that dead volume is to blame for causing the tailing
> on both the TCD and mass spec traces, since combustion is good,
> especially as the only thing I've changed are the chemicals in the two
> furnace tubes, but I really can't believe my packing was poor enough to
> cause this amount of tailing.  I really don't want to bring the EA back
> down to room temperature and repack the tubes again (let alone waste
> the new and costly chemicals I just put in).  Do any of you experts
> have any suggestions for something I might have missed or might try?
>
> Thanks for any help anyone can give.
>
> Steve
> -- 
> ---------------------------------------------------------------------------
> Stephen S. Howe                         Office:  (518) 442-5053
> Dept. of Earth & Atmos. Sciences        Stable Isotope Lab:  (518) 442-4471
> Earth Science 352B                      FAX:  (518) 442-5825
> University at Albany                    E-mail:  [log in to unmask]
> 1400 Washington Avenue        http://www.albany.edu/geosciences/sshowe.html
> Albany, NY  12222-0001	    http://www.albany.edu/geosciences/sirmslab.html
> ---------------------------------------------------------------------------