Instability signal issue on ICP-MS 7900

Hello everyone,

We are facing stability problems on our ICP-MS 7900. We lose up to 10% of the signal when analysing twice the same sample at the beginning and at the end of our samples sequence, no matter which element is analyzed, with or without gas. RSD are low, signal is ok for blank.

There are days where it works perfectly fine and other not at all.

We tried to perform cleaning, change tubes, change cones but nothing is working to improve our results. We usually do not use any ISTD but we tried once with and it didn't help as the instability is not constant depending on element, time ....

Is there someone facing similar instability issue ?

Thank you very much for your help !

  • Hello, I suspect you are either experiencing drift due to matrix or instrument drift which is helped my internal standard correction.

    When you tried internal standard correction did you use a mix of several internal standards or just one? It's ideal to use a mixture of internal standards so you can select an I.S. close in mass to each analyte. The general rule is to choose one within 20 amu of the analyte mass. For example, a common mix contains Li6, Sc, Ge, Rh, In, Tb, Lu, and Bi since it covers a wide mass range. 

    You could also be experiencing drift due to excess buildup on the nebulizer/cones/lenses. Do you know the estimated %TDS of your samples? If you are running semi quant/quick scan you could also look at that data and determine if they are high in minerals/salt or TMS (total matrix solids). You could monitor the nebulizer gas back pressure during the analysis, an increase in BP would suggest buildup on the nebulizer causing a loss in sensitivity. If I suspect my nebulizer is getting excessive buildup, I will filter my samples with a 0.45 micron filter prior to analysis. 

    To combat higher matrix samples, you can switch to more robust plasma conditions. For example, if you are running low matrix plasma settings, you could try general purpose settings, or introduce HMI.

    Your software does have preset plasma conditions, but I'll list common settings below if you are not familiar with the presets. 

    Low Matrix: 1550W Forward Power, 8mm Sample Depth, ~1-1.1 L/min Nebulizer Gas Flow

    General Purpose: 1550W Forward Power, 10mm Sample Depth, ~1-1.1 L/min Nebulizer Gas Flow

    HMI: 1600W Forward Power, 10mm Sample Depth, Reduced Nebulizer Flow depending on level of HMI, could range anywhere from ~0.3-0.8 L/min, introduction of dilution gas ~0.6-0.15Lmin (total of nebulizer + dilution should be ~ 0.90-1 L/min). Preset HMI settings often include HMI-4, HMI-8, HMI-25, HMI-50, HMI-100. 

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