Dear David,
> I have been trying to analyse nitrogen by EA-IRMS using a Carlo-Erba NCS
> 1500 and a Prism III. I've had a few problems with methodology which I
> think are basically solved. However, in the course of doing other work it
> has come to light that our 'ultra-pure' He is actually fairly dirty. We
> have just discarded a cylinder which had very high levels of water, CO2 and
> N2. The new cylinder is better, having negligable CO2 and water and only
> about half the concentration of N2. We have decided to test all new
> cylinders before use and put a water trap on the He line.
Your communication highlights two aspects of a salient point of
paramount importance.
(1) Labels like 'high purity', 'very high purity', 'extremely high
purity', 'super-purity' and 'spectroscopically pure' only exist to
justify the extortionate prices charged for products thus labelled;
what they mean is that the composition is unknown except for the
exaggerated claims of the supplier.
(2) Carrier gas purity is one of the major limiting factors for the
quality of IRMS measurements.
> Before I start yet another trial run I would appreciate comments on what
> concentration of N2 is acceptable? The present cylinder gives a N2 peak of
> about 5 E-11 amps but the previous cylinder gave > 1 E-10 amps. Samples
> would have peaks > 2 E-9 amps. Is it necessary or possible to trap out the
> N2 ?
A signal of of 1 e-11 to 5 e-11 on cup 1 (28) seems quite acceptable.
Trapping N2 is rather a tricky business (cf. previous discussions on
this subject on this list). On the other hand, 'ultra-pure He should
not contain that much. Over here, different purity standards must
meet certain criteria. If you buy He in Europa (including the UK) the
following quality criteria apply.
He 5.0 (= 99.999%): N2 - 5 vpm; O2 - 2 vpm; H2 - 1 vpm; CO2 - 0.5
vpm; THC - 0.5 vpm; H2O - 1 vpm.
He 5.5 (= 99.9995%): N2 - 1 vpm; O2 - 1 vpm; H2 - 1 vpm; CO2 - 0.5
vpm; THC - 0.5 vpm; H2O - 1 vpm.
He 6.0 (= 99.9999%): total impurities 1 vpm.
To trap nasties other than N2, a gas purifier might seem to be a good
idea. However, there is many a product on the market that promises
much and keeps little. Some of them actually add rubbish to the gas
and thus to one's misery. In my experience, the best system by far is
the furnace system distributed by Supelco (and no, I don't get any
royalties for this). I use the furnace in combination with their OMI
(indicating) purifier tube and I'm quite happy with the performance.
Hope this helps.
Cheers,
Wolfram
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Dr. W. Meier-Augenstein, CChem MRSC
Lecturer/Research Fellow
University of Dundee, Dept. of Anatomy & Physiology,
Small's Wynd, DUNDEE DD1 4HN, United Kingdom
Tel.: +44-(0)1382-34/5124, /4968
Fax: +44-(0)1382-34/5514
e-mail: [log in to unmask]
URL: http://www.dundee.ac.uk/anatphys/wma/wolfram.htm
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