Dear Charlotte,


When you change the filament, could you have a look at the filament wire with a magnifying lens to see if the wire is still straight or has sagged a bit? Similarly, could you look for uneven wear and tear (thinning of the wire)? In addition, before you remove the screw could you have a look to see if the filament assembly has some freedom of movement however slight?

Anything that moves the filament wire from the ideal centre plane (usually downwards) will lead to a dramatic loss in sensitivity and sometime even isotopic linearity. I guess what I am saying here is don't throw the filament away if the filament wire looks OK but if the assembly (ceramic holder) may have dropped just a teensy weensy bit

Also, if it's not too late already, before you go filament changing, give the focus suggestion one more try but this time do not use the default auto-tune settings. Either tune manually or increase the step width (and the number of iterations).

No offence to Bremen but the auto-focus feature is only half as intelligent as it would have us believe. I'm pretty sure I'm not the only one who has seen a signal improvement of -500mV (!) or worse after an auto-focus.


Best,

Wolfram


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"Facts do not cease to exist because they are ignored."

Aldous Huxley, "Proper Studies", 1927
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Dr W Meier-Augenstein
Senior Lecturer in Stable Isotope Forensics

Environmental Forensics & Human Health Research Group
EERC, S.P.A.C.E.
Queen's University Belfast
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-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Charlotte Lehmann
Sent: 24 October 2007 15:55
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Dramatic Sensitivity Decrease

Thanks for the suggestions everybody.
The box and trap values are as they should be 0.7-0.8 mA.
The slow magnet scan indicates that peaks are in the same positions
that they have been.
The peak shape looks "flat, symmetrical, and not noisy".
Autofocusing improves the intensities but does not give anywhere near
the values I was getting and the linearity check shows that it isn't
linear.
So, as per the Thermo service engineer, I am going to change the filament.
--
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 "Bolotin, Jakov" <[log in to unmask]>:

> Hi Charlotte,
>
> Your Box and Trap currents should both be around 0.7-0.8 mA. If the
> difference between those currents is high (for example: 1.1 and 0.4)
> then you should clean your source. If the currents are OK, I would too
> suggest trying "autofocus".
>
> Regards
>
> Jakov
>
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
> Behalf Of Charlotte Lehmann
> Sent: Dienstag, 23. Oktober 2007 21:43
> To: [log in to unmask]
> 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
>>
>>
>
>