Hi David, and everyone else,

> Andrew,

I take it this is addressed to me.  Oh yes, I said something about "your
average mass spec would be pushed to measure a ratio higher than 2" yonks
ago.  Aha!  Now I have to validate this...

> I'm curious how you arrived at a 45/44 ratio of 2 as the maxium
enrichment most instruments can handle.  Is it that the instrument can
only handle twice as much ion signal or did you take the standard ratio of
0.011237 and multiply it by about 200 because the resistors on the
collectors differ by 100?

With a Major resistor of 5E8Ohms and a Minor resistor of 5E10Ohms, the
Minor beam can read up to an FSD of 2E-10A.  With s ratio of 2, the Major
beam would be 1E-10A, which is pretty small.  I will qualify this twice:
Firstly, different mass specs behave differently at very small beam sizes,
so it's possible that some machines can cope with tiny Major beams better
than others.  This would be assuming that you were using a reference gas
close to the enrichment of your sample.  Using a natural abundance
reference gas could cause huge problems.  At natural abundance levels, the
pressure of 13CO2 is tiny compared to 12CO2, so the machine balances the
gases using the 44 beam.  BUT at a ratio of 2, there is only 33% 12CO2 in
the gas, so if you balance using the Major beam, the actual gas pressures
will be out by a huge amount, which could cause problems at the changeover
valve.  All Dual Inlet of course.  Secondly, it is fairly easy to change
the gain resistors, so with a 5E8 resistor on the minor, you would be able
to keep both beams on similar scales.

Oh, the Finnigan works in Volts, I think, so I'll quickly rephrase.  The
FSD on the Minor is 10V (1E-10A?), which would equate to a Major of 0.05V
(5E-11A?) on normal resistors.  With a Minor resistor identical to the
Major resistor, you would be able to use a Major of up to 5V, equating to
10V on the minor.  Figures are roughish, but IMO valid.

> I have a Finnigan Delta S and I wanted to find these limits of
measurement for both C13 and N15.  I would also like to determine the
maximum depletion that the instrument can measure.

Same as above, only backwards.  I would guess that a depletion of 1/100th
of natural abundance would be possible on standard resistors, and that
changing the Minor resistor to a higher value would increase precision
dramatically.

Hope this helps,

Andrew