This is about the same background I have. 30 mV on mass 28. 

If you had a leak you will definitely know it as the background could
rise to 300-400 mV.

I believe that on the EA the O2 valve is open only on injection time
(opened for 10 sec). So if you have this background continuously it is
not necessarily contributed from the O2 cylinder. 

Good luck,

Ali Belbachir
Analytical Technologist
Canada Border Services Agency
79 Bentley Avenue, Ottawa, ON K1A 0L8
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-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Bruce Wegter
Sent: Friday, December 05, 2008 2:22 PM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Question on O2 purity and typical N2 background.

We have a Costech EA, a Conflo III, and a Thermo Delta V Advantage 
I.R.M.S.  Currently using continuous flow for C13 and N15 determination
on 
sediment core samples.  C13 analysis is very good.  N2 is the issue, our
N2 
background is typically in the 26mV - 28mV range (on N28, cup 2), we
have 
tried various grades of O2 combustion gas, from grade 4.5, 5.0 (< 5 ppm
N2, 
batch tested), and now grade 6.0 (supposedly < 0.2ppm N2, also batch 
tested), with no change at all in the N2 background.  Note:  this
background 
is present in blanks, (blank = no sample whatsoever, O2 injected, with
auto 
sampler inhibited) as well as "empty" tin capsules, and of course
samples and 
standards.  

We have (we believe) thoroughly leak checked the system with argon leak 
check techniques.  This identified a small leak at the water trap in the
EA, 
which we promptly corrected.  No other leaks were identified, typical
Ar40 on 
cup 3 reads < 70 mV (it has been lower than 60 mV).  All gas fittings, 
regulators, and lines from all gas cylinders were also checked, again no
leaks 
were identified.  We plumbed He into the O2 line to see if N2 background

would change with the He in the combustion line.  It did, N28 background
was 
2mV - 3mV on cup 2 with He in place of O2.

Question 1:  Is our N2 background (~28mV) typical?  If not, what is?

Question 2:  What grade O2 should we use (and is grade 6.0 the "best"?)?

Manual says grade 5.0 or better, we have seen no difference in N2 
background between 4.5, 5.0, and 6.0.  Should I request from my gas
supplier 
a specific O2 cylinder which has been tested and certified to < 0.2ppm
N2, 
not just batch tested?  Or, should I try a different supplier?  We have
had no 
other issues with our current supplier.  I realize that cert's can be
wrong, and 
our O2 cylinder grades might not really be what they are supposed to be,
but 
are we looking in the right direction, or barking up the wrong tree (as
Mom 
used to say!).
Question 3:  What else could give us our N2 background?

Any help or direction would be greatly appreciated.

Thank you in advance.

Bruce Wegter
Sciences Instrumentation Technician
Geosciences Department
Hamilton College - Clinton, New York  13323
e-mail:  [log in to unmask]