Hi Anindya--

You're doing good...

Depending on what gets into the system, you might need to clean the 
nafion tubing (see Isogeochem Wiki, under repair tips: 
http://isogeochem.wikispaces.com/repair+tips ).

Once you get above 30000 mV on mass 44, you can be hitting the limit, 
50000 mV, on mass 46--so funny numbers.  Also, how much sample and how 
much acid are you using, especially on the larger samples? 

It may also be time to re-tune, and it that doesn't hold, clean, your 
source.  You can check that with a standard ref 1 on-off sequence if you 
haven't already.  If the standard pulse isn't linear (<0.06 per mil per 
volt) in the 4 to 8 V range, then it's probably an issue in the mass 
spec, else it's a gasbench/prep issue. 

take care

gerry

Some generic tips, in case you missed something  (I'm using this as a 
test bed for postings to the isogeochem wiki):
New batch of phosphoric acid?
How are the peak shapes off the GC in the Gasbench?  If the retention 
times are delayed, there may be a clog in the system.  If the peaks 
aren't as large as they should be, you might have a clog or loose 
connection in the sampling line.
Are the open splits operating smoothly in the gasbench?
Is the He tank good?  Every now and then we'll be a bad He tank.
Did the mass spec suck air?  Sometimes the He tanks run out faster than 
I'd expect--not nice.  It can take a little while for the mass spec to 
settle down, and if we catch it soon enough, just a re-tuning.
Have you processed other sample types through the gasbench, e.g. 
equilibrated waters?  Occasionally you can get funky volatiles coming 
through with different samples, which gives you a wavey baseline much 
later on.
Have you baked out the GC column?  If so, how does the downstream nafion 
tubing look?
Changed the pump oil recently?  If so, has the system settled down yet?
How are the background gases? (see bad He tank above)

Anindya Sarkar wrote:
> Hi all
> We have been routinely analysing carbonates in a gas bench (Delta plus 
> XP) for last 1.5 years. we obtained good precision of ~0.01 to 0.03 
> per mil for oxygen (in a single measurement) over a range of  44 
> signal 3000 mv to 20000 mv (from 100 ug to several hundred ug level of 
> pure carbonate). Over last one month the precision level has been bad 
> (>0.1 per mil) particularly for low signal (like <7000 mv) or very 
> high signal (say >25000 mv). The precision is generally good between 
> 8000mv and 20000 mv even now. For water sample of ~ 12000 mv excellent 
> precision is obtained even now.  My question is
>
> 1) Why precision goes bad at >25000 mv signal? Is it that the nafion 
> tube cant cope up with so much of water produced n eventually 
> affecting the source?
> 2) Why precision has gone down for <7000 mv signal when we were 
> getting a good precision earlier for even 100 ug (~3000 mv) sample? In 
> fact we were getting ready to push the machine for analysing even 50 
> ug sample without cryofocussing n normal configuration of gas bench 
> (i.e. normal large vial).
> Does it indicate detereorating filament condition after 1.5 yrs of 
> usage? Trap/Box current though dont suggest that.
> ---------------------------------------------------------------------
> Anindya Sarkar
> Associate Professor & PI, National Stable Isotope Facility
> Department of Geology and Geophysics
> Indian Institute of Technology
> Kharagpur 721302
> West Bengal, INDIA
> Tel.: 0091-3222-283392 (O) 283393 (R) 220184 (R)
>        Cell: 09434043377
> Fax.: 0091-3222-282268
> http://anindya-sarkar.tripod.com
> ------------------------------------------------------------------