For anyone who was interested in this problem, I just wanted to say that
the problem has been resolved. The 45/46 signal was coming from the Nafion
water trap, but after replacing it the signal remained. Heating up the new
water trap increased the signal, but heating the connecting lines out of
the water trap did not.

A day later, the elevated signal was gone. I have no real explanation for
why it would take an extra day for the signal to disappear, if indeed it
was replacing the water trap that did the trick. I don't think it was due
to downstream contamination being purged out over time, as heating those
lines gave no change.

So, the problem is resolved, but not necessarily solved.

Paul Eby




At 04:34 PM 7/20/00 -0700, you wrote:
>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
>> >