HI Bruce and Ali--
We typically run with mass 28 background around 30 mV too.
However, I use cheap tanks of O2 (ok, I recently upgraded to extra dry,
which is also 4.8, from USP grade). The high purity O2 tanks have very
low levels of N2 contamination, but we've found them to be very very
heavy--like hundreds of per mil. Unfortunately, they're very hard to
measure directly--N2 levels to small to reliably pick up (yeah, we tried
6.0 before too). Therefore, the cheap tanks with N2 peaks in the 30 to
60 mV range (1 to 2 Vs areas, depends on injection size) and per mil
values close to 12 per mil work well. We can (ok Isodat does it for us)
measure the "blank" and use the blank corrected values.
The cheaper tanks may have more hydrocarbon contamination, but it's not
causing problems for our EA or our runs.
take care
gerry
Belbachir, Ali wrote:
> 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
> [log in to unmask]
>
>
> -----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]
>
>
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