Thank you for your response.

I have several reasons for believing that HCO2 is not the source of the 45
signal. First of all, the formation of HCO2 requires a source of CO2. This
is occurring at background levels, not when a peak goes through.

Without the water trap, I have 44, 45, and 46 signals of 9, 11, and 12 mV.
This is typical in my experience. The water level was 80 mV: higher than we
like to run with but no sign of mass 45 trouble in the background levels.

With the water trap, the 44, 45, 46 signals were 8, 16, and 21 mV (elevated
45 and 46). At the point that I took those particular readings, water was
74 mV. Usually the trap does a better job, but the point here is that with
nearly identical water levels, the problem is rectified by removing the
trap. This means that H2O + CO2 can't be the problem.

The software should be able to account for these higher backgrounds, except
the elevated 45 and 46 signals do not remain constant. So, the 44/45 signal
fluctuates.

Also, when you heat a water trap and monitor mass 18, the water comes off
in one large, smooth, tailing bump. Monitor mass 45 and I see sharp spikes
of up to 300 mV. If it was HCO2, it would have the same shape as water.

All this makes me suspect some decomposition product. Anyone else with ideas?

Paul Eby

At 05:25 PM 7/20/00 -0400, you wrote:
>Hi,
>
>The problem definitely lies in the nafion water trap as HCO2
>is formed in the ion source because water is reacting with
>the CO2. The reason that you still see an increase in mass 45 and 46 even
>after changing your Nafion trap seems to be that the capillary going from
>the water trap to the ion source is saturated with water as a result of
>the failure of your first nafion trap. Subsequently, the CO2 is still
>reacting with the remaining Water after the trap. I recommend using a
>heat gun to heat up the capillary going from the trap to the ion source
>thoroughly,including the open split, making sure to shut off the on/off
>valve that leads directly to the souce so that the water vapor won't
>end up there (you can also disconnect the capillary that feeds in to the
>source while evaporating the water). Hope this will help.
>
>Nabil Saad
>
>On Thu, 20 Jul 2000, Paul Eby wrote:
>
> > Hello all,
> >
> > I'm hoping someone can shed some light on a problem I am having (GC-IRMS,
> > MAT252). My background levels for CO2 are elevated, but on masses 45 and 46
> > only. The source seems to be my Nafion trap, as the 45 (and 46) signal
> > increases drastically (perhaps 200mV) when I heat the trap, and falls to
> > normal when the trap is removed. Mass 44 is unaffected throughout all of
> > this (~9mV).
> >
> > I replaced the Nafion tubing, but am still getting the same problem.
> > Another trap in the lab made from the same batch is working fine. Does
> > anyone have any suggestions? What compound can give masses 45 and 46 but
> > not 44? Nafion is a fluorinated polymer, but the only fluorine compound I
> > can dream up that has a mass of 45 is NCF. Doesn't seem very likely....
> >
> > Paul Eby
> >
> > [log in to unmask]
> > Biogeochemistry Lab
> > School of Earth and Ocean Science
> > University of Victoria
> >
> > phone: 250-721-6183
> > fax:   250-472-4620
> >