Dear Imma,


When you say "the valve source" is closed, do you mean the needle valve at the source or the open-split valve of the GC/C-IRMS interface?

If you have indeed closed the needle valve and as a result no peaks are observed while the peak return the moment the needle valve is open I would strongly suspect you have air getting into the system either at the side-arm of the needle valve where the transfer capillaries are connected, or somewhere upstream in the interface.

Should that be the case I would be worried as to why you don't see any of the other peaks typical for air break-through or why you don't see peaks around 10,000 - 11,000 BDAC for CO2 when then ref gas valve is open.

However, that may be due to an unrelated yet coinciding problem such as a slightly misaligned filament (you mentioned having changed the filament) or a misaligned source flange resulting in a reduced gas flow into the ion source. As already mentioned by another list member, a problem with the magnet current controller cannot be ruled out either.


Time to call Thermo-Man to the rescue.


Best,

Wolfram


________________________________
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Imma Tolosa
Sent: 17 October 2007 17:22
To: [log in to unmask]
Subject: Re: Emission problem

Dear Wolfram and other colleagues,

1)   Doing a magnet scan from 4000 to 11000 BDAC with the source and HV OFF, there are no peaks showing up.
 2) Doing a magnet scan with the source and HV ON, gives the same peaks at 7300 BDAC with and without CO2 gas.
However, the final surprising observation is that the peaks at 7300 BDAC are showing up only when the valve source is open (and source and HV on; with or without CO2 gives the same peaks) . These peaks are absent (no peaks at all) if the valve source is closed (no helium, no CO2 in the source) and source (and HV) on.

I am really confused and I think I will wait for the Thermo engineer.
Thank you very much for all your suggestions.
Imma

________________________________
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Wolfram Meier-Augenstein
Sent: Wednesday, 17 October 2007 14:43
To: [log in to unmask]
Subject: Re: Emission problem
Dear Imma,


I have to admit what you are telling me has me stumped.  Based on what you are saying at one point the system reported peaks at a particular magnet setting (when performing a magnet scan from 4000 to 11000) but when you set the magnet to 7200 and increase the set value gradually to 7300, even 7400 in say increments of 10, the beams do not move at all.

Safe for gremlins in the electronics I cannot think of anything at the moment that would explain seeing peaks during a automated magnet scan but not during a manual magnet scan of the region where the auto-scan showed peaks.

Can you switch the source (and HV) off and do a magnet scan from 4000 BDAC to 11000 BDAC (instrument control) in steps of 10 and see if the peaks around 7300 BDAC still show up?

Also, with the source and HV on, can you do a magnet scan as above with the ref gas open (admitting CO2 into the source) and see how many peaks are present now?

Before all that, can you close the software and reboot your PC?

Other than that, I can only concur with Jakov's suggestion, call Thermo.


Best,

Wolfram



________________________________
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Imma Tolosa
Sent: 17 October 2007 11:10
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] Emission problem

Dear Wolfram,
    Thank you for your suggestions and time. I would like to add some more information to clarify the points you raised up:
1)  My instrument set-up is a GC-C-IRMS Delta C.
2) Now the magnet scan BDAC displayed one big peak about 7300 with and without the CO2 gas calibration. The display was voltage abundance and not normalized to the highest signal. Therefore all the scan is flat except at about 7300 BDAC.
3) Trying to adjust the magnet setting manually to 7300, the beam display does not change at all (no rising bars!).
4) I just did a HVDAC scan and I do not get any intensity (all flat signal). I also realized that the HVDAC displays 2030 and before it was 2040.

Do you think that the problem could be related to the High voltage although the led is on?
Any more suggestions would be much appreciated
Imma

________________________________
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Wolfram Meier-Augenstein
Sent: Wednesday, 17 October 2007 10:05
To: [log in to unmask]
Subject: Re: Emission problem
Dear Imma,


Since your post does not mention what you present instrument set-up is, what I'm going to suggest is based on a best fit for what you describe.

You say the LEDs for HV and Emission are on, the filament check's out fine (you measured across the feedthrough pins with all the cables connectors pulled off, right?), you see 3 peaks during a magnet scan and yet you think you have no emission? No emission, no peaks, even if the LEDs would be on for reasons best known to themselves.

In the Isodat software window, adjust the Magnet setting manually (instead of changing mass for cup 2 you change the magnet set value to 7200 and increase that until the beam display starts to show rising bars, eventually reaching a maximum (essentially showing you what you see during your mass calib scans). Now switch the Source off. If the beams drop down to zero (and come back once you switch on the source again) you know (a) you have emission and (b) ions hit the analyzer.

So, a mass calibration scan (essentially a magnet scan) gives you three peaks around 7300 BDAC and seemingly nothing else.

(1)     Magnet scan BDAC values as displayed on the screen are not hysteresis corrected (and neither are mass calibration BDAC values).
(2)     The magnet scan screen autoadjusts Y-scale to fit the highest signal (peak) in the window.

I am wondering if you either have a massive leak (not massive enough to prevent the source from coming on, or for the system to give a vacuum fail warning) but massive enough to admit plenty of air or if the ref gas valve for N2 (or CO) is (a) open and (b) admits N2 (or CO) at a rate to yield a signal for m/z 28 well in excess of what your analyzer can handle, pushing all the other peaks into the baseline.


I like the idea of the ref gas valve open (or stuck) admitting unreasonably high amounts of N2 (or CO) better since a leak should result in a 4th peak (O2) to the right of your 3 peaks.

However, as I said in the beginning, this is a best guess based on the information you provided.


Best,

Wolfram




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

Aldous Huxley, "Proper Studies", 1927
*****************************************************************

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
David Keir Building
39-123 Stranmillis Road
Belfast, BT9 5AG
United Kingdom

Tel.: +44 (0)28 9097 4015
Fax: +44 (0)28 9066 3754
Email: [log in to unmask]
URL (Isotope Lab): http://www.qub.ac.uk/eerc/facilities/siff.htm
URL (Research):
http://www.qub.ac.uk/eerc/people/academic_staff/wma/wm_a/efnhh.htm

________________________________
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Imma Tolosa
Sent: 16 October 2007 16:09
To: [log in to unmask]
Subject: [ISOGEOCHEM] Emission problem



Dear colleagues,

        I am not having emission signal on my Delta C instrument and I observed the following features:

1) The high voltage and emission LED are on, but no ions are being detected in any cup. The pin 1 and 10 of the filament indicates that the filament is fine (I already changed) and no shortcuts between the different pins of the source have been found.

2) If I do a mass calibration, there are 3 peaks between 7200 and 7400 BDAC. They do not look spikes because I repeated several times and they are always at the same place. The normal values for m/z 12, 28 and 44 should be 3216, 7440 and 10836 BDAC.

3) The baseline when the source and high voltage are off gives 10 counts and -95 mV, whereas normally should be 17 counts and 235 mV.

4) The UCF/Amplifier (with the source and high voltage off) gives an error because the "bounds exceeded". Intensity was only 5 mV without any oscillation whereas normal values were 119 mV.

I would appreciate any advice to solve the problem.
Thanks in advance

Imma TOLOSA

International Atomic Energy Agency - Marine Environment Laboratory

4 Quai Antoine 1er,  MC 98000 MONACO

Tel: (377) 97 97 72 41 Fax: (377) 97 97 72 76

Home Page: http://www.iaea.org/monaco

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