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)?  

  • 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

Reply Children
  • 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?

  • You should only get a P/A factor when there is a standard high enough in concentration to result in an analog measurement.  Out of curiosity, did the independent P/A factor help your Mo linearity?

  • Nope, the issue still stands. However I tried to calibrate mass 95 96 97 in both modes (He and Nogas) and the linearity problem shows on all the isotopes in Helium mode while the linearity is good in Nogas mode regardless the mass. So the issue seems He mode related.
    About the P/A factor, using the "external" setting based on the instruction provided with the A+B solutions a 100ppb concentration is supposed to be the optimum so am I not supposed to get a good signal on the indipendent P/A factor during the batch when reading the 100ppb standard? Or is the 100ppb optimum intended in nogas mode (because He suppress signal intensity/CPS so it's normal to get a "signal too low" message on a 100ppb standard when read in He mode)?

Was this helpful?