HI Greg--

That's normal, though not desired behavior.  At temp, the glassy C tube 
reacts with the alumina (or contaminants therein).  You can also get 
some diffusion through the alumina, and eventually small cracks opening 
up, at temperature too.  If you don't have a new alumina tube--get one 
now.  Note:
1.New tubes may take some time to "bake out"--and sometimes repacked 
reactors do too. (Depending on how you pack your tubes, if there's any 
traces of quartz wool in the hot zone--it will slowly react with the 
glassy C at temp.)
2.Check the label on the front of your furnace (take off front cover 
should be at top right) and see what the "offset" is.  The 
diffusion/leak through the alumina is temperature dependent.  If the 
offset is large, you might be running hot, i.e. above 1450, which will 
give you higher backgrounds over time.
3.Check your ramp rates on the jumo controller, going slowly, e.g. 
5oC/min, esp when >900, helps minimize thermal shock.
4.You could do an Ar leak check--mainly to see that the connections are 
not opening up a little at temp, but it's probably the alumina tube that 
needs to be replaced.

Sercon sells a moly tube that inserts between the glassy C and alumina 
to minimize background.  Don't have ref. handy, and I'm pretty sure it's 
sacrificial.  High backgrounds are annoying, but if stable, can be 
subtracted out.   If it's trending over time a lot and bounces around, 
then your numbers bounce around too. Also, if air is leaking in, you'll 
get N2 coming through, and get some NO forming in the source to mess 
with your mass 30--and you're standards will bounce around and 
drift--definitely time for new alumina tube.

take care,


On 7/18/2012 7:31 AM, Gregory A Wandless wrote:
> I have a mass 28 background from the TC/EA of around 900mV at 1450oC. 
> I changed the reactor and overnight the background dropped to about 
> 150mV but about an hour ago ( before I got in) the background jumped 
> back up to 900mV. Seems it has something to do with the reactor, is it 
> possible that a crack developed in the reactor? Any other ideas? 
> Before I the changed the reactor if I dropped the furnace temp the 
> background also dropped indicating to me that I didn't have a leak.
> Thanks,
> Greg