Hi Bruce,

Interested to hear someone else has spent a 
'heap-o-time' argon leak checking a Costech ...

We have a similar set up (Costech EA + Conflo III 
and Delta+XL) and observed similar leak problems 
immediately after installation. I then spent the 
better part of a week searching for a leak (and 
slowly losing what little sanity I have left ...) 
because of the same high N2  backgrounds that you describe.

Fortunately I have two peripherals (TCEA and EA) 
on the same mass spec which I can switch between 
using a Valco 4 port valve and which both use the 
same UHP helium supply.  With the TCEA in line 
mass (40) bkgrd was/is ~7mV.  Therefore, I 
discounted a bad tank of helium (as Ben pointed 
out can occur), and I started argon leak testing in earnest.

Initially the mass (40) background of the Costech 
was >150mV.  After bypassing everything ... the 
GC column, autosampler and reactors, h2o trap and 
TCD, the mass 40 was still >150mV.   I then argon 
leaked checked every fitting inside the EA and 
eventually found two leaks: 1) the TCD itself and 
2) a tee fitting between solenoids # 6 and 
#7.  Both of these were replaced/repaired by the 
Costech rep here in Canada under warrantee. This 
reduced the argon background down to ~45mV.  At 
this point I started putting components back in 
line.  With everything back in line mass 40 
remains ~45mV and mass (28) ~12mv,  mass (29) ~9mV and mass (30) ~50mV.

Regarding the O2 supply.  The oxygen portion of 
the Costech is isolated from the rest of the 
system except when the O2 pulse is requested,  so 
it can't be source of the elevated backgrounds. 
It can though affect your blanks if your O2 tanks 
is either contaminated or the pressure/flow isn't 
high enough.  Check that you've got at least 1 
bar of O2 pressure and a flow of 25 ml/min at the 
O2 vent.  If the O2 pressure is too low room air 
can back diffuse into your loops.  For-what-its-worth I use 4.3 purity O2.

So at the end of the day I am running with the 
backgrounds noted above, which are in agreement 
with what Ben, Gerard and Ali  report.  I know of 
one other very small leak inside the EA but the 
fitting is very hard to get to and I've decided to let it be.

At these background levels a N2 blank correction 
is the only way to achieve acceptable data and I 
make sure to run several empty tin cups before 
samples to get an accurate blank size.  FYI:  a 
3.2 x 4mm tin cup gives me a ~0.6 [Vs] peak area.

Cheers,
Steve





At 12:21 PM 12/5/2008, you wrote:
>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]

Stephen Taylor
Lab Manager, Isotope Science Lab,
University of Calgary, Physics & Astronomy,
2500 University Dr. N.W.
Calgary, Alberta,
Canada, T2N 1N4
office: 403 220-8268, Labs: 403 220-8273 or 220-6813
(f) 403 220 7773
[log in to unmask]
www.phas.ucalgary.ca/isl