Molybdenum linearity issues using 7800 ICP-MS

Using ICP-MS 7800 with Helium mode for Mo assessment, as you can see from attached picture we are having some issues. Calibrating range goes from 0.1ppb (actually excluded so 1 ppb as the lower point but this changes nothing) up to 1000ppb, the higher standard seems to be overextimated (if you exclude it you would see its dot clearly above the new calibration line, also looking at CPS they seem ~10% higher than expected according to the lower points).
Also with that calibration the other standards calculated value is ~ 10% lower than the nominal value for the "closer" one and ~ 20% lower for the 1 & 10ppb std wich also points to a linearity problem.
This has been a constant issue in the last months so it's not a random effect that may be related to standard preparation.

I've noticed that the higher calibration point is determined in analog mode while the others are all in pulse mode, could there be a problem with P/A Factor? We run a separated P/A setting using Agilent PA tuning solutions 1+2 diluted 1:100 before calibration (with the "merge" option checked) and also use the p/a adjustment setting into acquisition so it should work properly. 

According to the latest tune report detector voltages seems good so shouldn't be a detector problem.

Any insight? Is there something I could try (besides narrowing the calibration range wich I don't want to do)?  

  • Hi,

    could you please have a look into the „LOG“ folder of masshunter.

    you find it under X:\Agilent\ICPMH\1\LOG

    there is a „pafactors.xml“. If this file (or the other log files) gets to big, it causes some problems. 
    close masshunter and rename the log file, when you then restart masshunter it will create a fresh one. 
    of course you then have to do another PA Calibration, but maybe this time it works…

    best regards chris 

  • Hello again, tried to cancel the log as suggested (the file was actually pretty big ~12MB with entries dating back to 2019) but it didn't solve the issue.
    I keep having this linearity problem that doesn't show for elements close in mass to the one of Mo (ie Se, Cd) or other elements that show the same behaviour in standard detection (higher std read in analog mode, others in pulse such has Nichel wich shows perfect linearity).

    Again any insight would be highly appreciated.


  • Have you tried enabling the "independent P/A factor" in settings?  Using this along with the P/A factor adjustment in the batch will allow for P/A factors to be collected for all tune modes.  Note that the independent P/A factor column header is added to the Acquisition Parameters section of the batch and also needs to be enabled in the batch.  I know this may be a long shot since you are not having issues with Ni which is in the same tune mode but perhaps the sensitivities are different enough.  Also, have you tried running Mo in a batch with only a single tune mode?

  • I've looked upon the settings but I can't find that "indipendent P/A factor" option, can you point me into the right menu of the software? Maybe I'm looking in the wrong section. Also I'm wondering if that could be a problem related to the reaction cell since Molybdenum 95 is interferred by Mn argide, is it possible that at higher concentration the helium flow is not sufficient to dump the MnAr interference leading to that overextimated count? I'm currently using 4.2 ml/min

  • In MassHunter 4.6, it is in Settings > Options> Selected Options> Independent P/A Factor

  • Brief update, found the option (htank you) and did a couple of runs with it, the only thing I could notice is that at 1000 ppb the Mo p/a factor is shown (and it gets determined in analog mode), for lower conc it just says "signal too low" and they are all determined in pulse mode. Is that normal?

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