Dear Myq,
There is no reason why an IRMS instrument could not measure isotope ratios
in any desired range with high precision for any applicable analysis gas.
The amplifications are matched because peak detection and quantification is
most precise for signals of comparable intensity.
The amplification for each detector are defined by the feedback resistor.
A simple swap of that resistor changes amplification accordingly. In
addition, it turns out that you can also attach multiple amps/counters to
the same faraday cup to amplify the signal to various ranges with no
measureable increase in noise (at least at continuous flow precisions).
For instance, for N2 you might use 1:100, 1:10, and 1:1 to give 3 different
ranges simultaneously. This way, everything from natural abundance to,
say, 70% 14N is on scale in at least 1 channel (use 1:100 for 99.5% 15N).
As Andrew Tait pointed out, you also need standards with isotope ratios
near those of your sample which means you need several throughout the
measurement range. Even in natural abundance work, HD measurements over a
range of, say 100 permil of more, require at least 2 standards (e.g. SMOW
and SLAP) for best calibration.
We were interested in the multiple amplification approach as we use
enriched isotopes, and >7 years ago did set up a system and showed that it
works (the data are unpublished). We did not pursue it, mostly because our
need to measure isotope ratios with high precision (4-6 significant
figures) over such a large range was minor, and in fact it is seldom the
factor which limits precision (or accuracy) in an enrichment calculation.
The effort to develop user-friendly software and standards to handle it was
too great. In most cases, quantitative dilutions of enriched materials or
analysis by GC/MS gives at least 2-3 sig figs and that is usually enough.
If anyone has such an application and would like more information on this
approach let me know.
Tom Brenna
At 10:15 PM 3/24/00 -0600, you wrote:
>> Can anybody tell me what 99.9 atm% carbon 13 would convert to if
>> expressed as the delta notation?
>
>While we are discussing this:
>
> Is there any reason why an IRMS could not calculate, with great
>accuracy, the enrichments close to 99.5 and above? The opposite is true for
>natural abundance of 15N - 99.5 % 14N. I realize that the collector for mass
>29 in this situation is much more sensitive, but couldn't you just swap the
>wires between the faraday cups to their respecitve amplifiers or perhaps turn
>the whole section upside-down so that the cups were in reverse order?
>
>myq
>
>
>--
> Myq Larson / Stable Isotope Analysis \ [log in to unmask]
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