Hi Charlotte,

The whole purpose to make a calibration curve is because not any system for
aqnalysis is perfect (that includes the IRMS). The combination of
preparation (i.e. producing the analyte gas) and subsequent measuring in
theory should result in a 1.0 curve fit if comparing measured with defined
values for your standard material, but in practise you get a curve deviating
from 1.0. Constructing the curve gives you the opportunaty to correct to a
'1.0-curve situation', as long as their is linearity in the system and the
deviation is not too large.
All laboratory manupilations and mass specs, even when using exactly the
same procedures and machines, still produce (slightly) different results -
that exactly is the reason why we have to calibrate!

The 'reference gas' (= monitoring gas; Georges you was just in time short
while ago!) does not matter for calibration at all, be it best it has an
isotopic composition not too much deviating from your analyte material(s).
Further, it only tells you about shifts or drastic changes that may occur if
problems in the IRMS happen. It certainly cannot be used for calibration
purpose or as reference gas as such.

Water can have different sources. Leaks are an option there, but also water
can be produced in an oxidation/combustion reactor (if you have such, you
did not give details there), for example - it depends on your system if this
is applicable. Hopefully it does not come form your monitoring gas bottle
(did you check if the gas is 'dry'?), since that is a matter you changed.

If you have 'water hanging around' as you state, how you can measure CO2
properly?

My two eurocents,
Pier.
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> From: Charlotte Lehmann <[log in to unmask]>
> Reply-To: Stable Isotope Geochemistry <[log in to unmask]>
> Date: Mon, 5 Feb 2007 10:52:48 -0500
> To: <[log in to unmask]>
> Subject: [ISOGEOCHEM] slope of standard curve for d13C
> 
> I recently ran NIST standards to develop a new standard curve for d13C
> bulk isotopes on our Delta V and am wondering to what to attribute the
> fact that the slope is less than 1.0. This time it was 0.93, resulting
> in enriched values that are much more enriched than they were six
> months ago. We have a new tank of Scientific CO2 for the reference
> gas, and it appears to have a d13C value of about -39.4 per mil
> (whereas our previous tank from the same supplier was -22 per mil).
> Any suggestions?
> 
> Also, and I don't know if this is contributing to the above, we have a
> fair amount of water hanging around as evidenced by Mass 18 values in
> the several thousand mV range, with no indication of leaks (Mass 40 is
> low). I just discovered that the gas purifier on the GC-GCC III side
> had a blown fuse. But the problem is evident on the EA-Conflo III side
> of things and we have never used a gas purifier there and not had this
> problem before to my recollection. Heaters are on at the valves and
> the source. We already replaced the needle valves with Nupro valves.
> 
> Spent some time running diagnostics last week: focus is good;
> linearity checks out. Amplifier test passes (although I can't pick the
> gas configuration to use because it only sees "CO" which we do not
> routinely use. Peak shape and flatness are good. System stability is
> excellent. Signal stability (on CO2) seems to be 2.5 times what the
> manual says it should be; not sure what that means or how to correct
> it...
> 
> I'll be interested to hear any ideas!
> -- 
> Charlotte Lehmann
> Research Technician
> Bates College
> Department of Geology
> 206A Carnegie Science Building
> 44 Campus Avenue
> Lewiston, ME 04240
> Phone: 207-786-6485
> FAX: 207-786-8334
>