Is the problem worse if the bellows have different pressures in
them? That could indicate a problem in linearity starting to affect
your D47 signals.
Also, have you checked for leaking across your valves? Perhaps even
a transient leak to atmosphere?
For the newbies reading this, to check for leaks fill up both
bellows with gas, shut off both valves to the bellows, then pump out
everything else, including capillary lines. Once everything else in
pumped out completely (rough, then turbo for a while), close all
valves. Compress bellows a bit. Then wait.....
You shouldn't see any loss of pressure in the bellows--though
pressure reading may fluctuate with temperature. But you may still
see some gas leaking.
Open up valve to roughing pump, with the readback from the pressure
gauge. Note any change and any fluctuation. Then open up the
different sections to the roughing pump and look for changes in
pressure readings. Do this one at a time, and close the previous
section if possible. If you find a section that causes a pressure
jump, say reading go from 10-3 to 10-2, note that. The cross
section between bellows and external gas connection may show
evidence of a leak, but you don't know which valve is leaking--it
could be atmosphere leaking in. Cap off the external connection,
pump it out, and check for leaks again.
Sometimes the leaks will go away if you just reseat the valve, i.e.
open and close it for a while. Certain leaks may not be too
bad--e.g. if the valve to an unused and capped off internal port is
leaking a little, but the cap is fine, just leave that valve open
all the time if that increase in dead volume is acceptable. Then
fix it next time instrument in down.
On 1/26/2016 2:52 AM, ZhangNaizhong
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Hi Andy and Albert,
Thank you so much for the
suggestions. Unfortunately, we have not solved this
d13C and d18O are quite well. To
the D47, we got the results of ca. 0.04‰ in last December; this month we
have changed the capillaries between sample side and
standard side without shut down the MAT253, now the results
become ca. 0.09‰ and
keep constant. At the same time, we measured D47 of left vs.
left and right vs. right, both of them can give the results
of ca. 0 (<0.01‰). If
the problem are the bellows, do you have any suggestion to
clean them? We will shut down and clean the ion
source/change the filament the day after tomorrow.
Yes, we are using the PBL
correction methods suggested by He et al. (2012). Actually Bo
is my friend and he helped us to apply this method in 2013.
And we have also compared the results between PBL correction
and no PBL correction. There is almost no difference to the
reference gas. By the way, our PBL of m/z 47 is around -15mV
when we want to get a m/z 44 of 16V.
Thanks a lot!
Geochemistry on behalf of "Albert S. Colman"
January 21, 2016 at 11:54 PM
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[ISOGEOCHEM] Problems of MAT253 on clumped carbonate
Do you monitor and correct for your "pressure baseline"
(He et al., 2012. Rapid Comm. Mass Spectr.
, 2837-2853.)? This shift in the m/z 47 baseline
upon introduction of sample CO2 into the source will scale
with the size of the m/z 44 ion beam (combination of
secondary electrons and main peak pressure broadening). A
mismatch in pressures between reference and sample side
will lead to different baselines for each.
When this pressure baseline is sizable (10s of mV on m/z
47 for a working ~16V signal on m/z 44), then a small
mismatch in reference v. sample pressure balancing can
lead to self-zero tests that give apparent non-zero D47
values in the range that you observe. That effect will
change with different source tunings. It will also drift
through time, just as many labs have noted that the slopes
of uncorrected HG and EG curves drift through time.
Even if you haven't been monitoring the pressure baseline
explicitly, the slope of your heated gas/equilibrated gas
lines is an indication of the magnitude of this baseline
effect. As we derive in the supplemental information
associated with He et al. (2012), the slope of the heated
and equilibrated gas lines is approximately equal to the
PBL_47/V_47,ref. You could apply this correction for post
processing of the data you've already collected to see if
this resolves your issue.
Albert S. Colman
Dept. of the Geophysical Sciences
University of Chicago
5734 S. Ellis Ave.
Chicago, IL 60637
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you for your suggestions. We have repaired the
problem of sensitivity decrease with the method
suggested by Hagit.
However, the first
problem became even worse. Recently we have
changed the capillaries from left to right, and
also from right to left, the values were not
reversed, but deviated farther from 0 (positive
as before), now the result was increasing
gradually from 0.04‰ to 0.10‰.
Does anyone ever faced this problem? I
am now worrying about the pollutions of
bellows. If so, how could we clean the
bellows? Or is there any other suggestion?
the way, does Nickel capillaries work better
than stainless steel ones? We are now
considering to change them to Nickel ones.
Isotope Geochemistry on behalf of Andrew Schauer
January 7, 2016 at 5:44 AM
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[ISOGEOCHEM] Problems of MAT253 on clumped
Are the bellows calibrated? Ours fell way off
calibration frequently. Make sure both sides are
actually 0% and 100% open when the software