We here experienced a problem with very low response
to N2, the peaks were around 5000mV and gradually (2
years) decreased to less than 800mV. The solutioni was
very simple, we simply performed the "autofocus"
operation. I don't know if it applies for your case.
Regards,
Matheus
--- Shapna Mazumder <[log in to unmask]> escreveu:
> Hi Charlotte,
> The trap and box current will indicate whether the
> filament has gone belly
> up, they are usually at ~0.7 mA on my Delta V. Also
> a slow magnet scan will
> determine whether your mass calibration has shifted.
> Shapna
>
>
> ----- Original Message -----
> From: "Charlotte Lehmann" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Tuesday, October 23, 2007 12:42 PM
> Subject: Re: [ISOGEOCHEM] Dramatic Sensitivity
> Decrease
>
>
> > We have just experienced a dramatic sensitivity
> decrease as well and I am
> > wondering, while I await ideas from our Thermo
> service engineer, what
> > ideas the list might have. Here's what transpired
> with our DeltaV
> > Advantage:
> > Everything had been running well with intensities
> for masses 44-45-46 in
> > the 4000-5000-6000 mV ranges and for masses 28 and
> 29 in the 2000 and
> > 1500 ranges, respectively. The filament was
> replaced and everything tuned
> > to good linearity on 9/10/07. Fresh column and
> water trap fillings were
> > installed in the EA. And the EA GC-oven was baked
> out at that time. The
> > helium tank was switched on 9/24/07. No problems
> seen. The source tripped
> > off to that enigmatic HV reading of 0.35 kV on
> 10/2/07. But everything
> > looked okay after I turned the source back on and
> ran okay until now. A
> > new Oxygen tank was put on-line on 10/11/07.
> Vacuum readings with valves
> > open and closed continue to look great. On
> 10/15/07 I noticed that the
> > source and inlet heaters were not turned on; so I
> turned them on. Not
> > sure when they got turned off.
> > Nearly 500 samples were run, with the ash trap
> being emptied as needed.
> > Last Weds. I emptied the ash trap in the Costech
> ECS4010. Shortly
> > thereafter I checked the intensities of masses
> 44-45-46 and they were
> > 4418, 5060, and 6160, respectively. Five days
> later, a sequence run was
> > initiated, and the intensities were 270, 310, and
> 380 mV respectively,
> > with a corresponding drop in BGD values for masses
> 44-45-46. Masses 28
> > and 29 show only 153 and 114 mV respectively;
> again with corresponding
> > drop in BGD values as well.
> > I have leak checked everything with a GOW-MAC CO2
> and He detector. I
> > can't get a peak center on mass 40 since the
> sensitivity is so low
> > (normally without leaks we are at 8 mV or so).
> > I have checked the ConfloIII to make sure
> everything is working properly
> > there.
> > I have "fiddled" with the EA valve, but it didn't
> improve anything to
> > open and close it. This was the service engineer's
> only suggestion so
> > far, and now I wait for other ideas...
> > I have checked from the GC Trace-GCC-III side to
> see if the sensitivity
> > loss is observed from there as well and IT IS. So,
> it appears to be a
> > problem with the source or source electronics.
> > I also tried shutting down the software and the
> computer and restarting
> > with no change in the situation.
> > Today I took the the opportunity to refresh the
> columns and water trap in
> > the EA while things were not going to be running
> (we often do this about
> > every 500-750 samples anyway). The copper wire was
> as normal about half
> > spent. There was NO brown film in the SS connector
> tube (from combustion
> > to reduction column) this time as there has been
> recently. These changes
> > in the EA have not affected the issue, because it
> is clearly is in the
> > IRMS source.
> > Any ideas would be greatly appreciated. Again, the
> only change between a
> > good run and the loss of sensitivity was that I
> emptied the ash trap in
> > the EA. I guess I will try turning on the analyzer
> heater while I wait
> > for ideas. I really hate the thought that the
> filament has gone belly up
> > in less than six weeks!
> > --
> > Charlotte Lehmann
> > Assistant in Instruction
> > Bates College
> > Department of Geology
> > 206A Carnegie Science Building
> > 44 Campus Avenue
> > Lewiston, ME 04240
> > Phone: 207-786-6485
> > FAX: 207-786-8334
> >
> >
> > Quoting Herbert Tobias <[log in to unmask]>:
> >
> >> Hello All,
> >>
> >> I have a question about a MAT 252 IRMS. I vented
> the instrument to
> >> replace the sampling capillary. I let it pump
> down for ~2hrs. When
> >> testing CO2 gas pulses from a dual inlet volume
> and injected CO2 from a
> >> GCC interface, the signal was 10x lower than
> normal. This means its the
> >> IRMS itself. I tuned the lenses, and now got 4x
> lower signal than
> >> normal,
> >> better but not good. The voltages and currents
> on the filament are
> >> normal.
> >> Has anyone seen this problem and know how to
> resolve it?
> >>
> >> Thanks in advance...
> >>
> >> cheers,
> >> -Herb
> >> Cornell University
> >>
> >>
>
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