7900 ICP-MS Ca concentrations consistently higher in He mode when measuring 44Ca

I am trying to measure Ca in seawater samples and when using 44Ca in He mode, concentrations are consistently ~11% higher than what is expected from our standards. Additionally 44Ca in He mode is about 11% higher than 44Ca in no gas mode and 48Ca in He mode. This is consistent for every sample/standard such that if I multiply concentrations measured from 44Ca by 0.89 then they are about what they should be. He flow rate is 5 ml/min and energy discrimination is 5V. Samples are diluted 100x in 2% nitric and so plasma mode is in low matrix. Is this some sort of ArHe interference or something else? Just wondering how to fix this, thank you!

  • When I analyze acid digested oleochemicals I never use m/z 44 for Ca in He mode. There is a significant interference on this mass (maybe COO ??)

    Instead I use m/z 40 in H2 mode for calcium. Don't know if a possible COO interference is relevant for seawater samples though.

  • I did think about that but the curious part is why would 44Ca in no gas mode provide Ca concentrations expected of typical seawater (suggesting to me not much interference) but higher concentrations in 44Ca He mode, and to my understanding He mode should remove COO interference. I wonder if I should test these potential interferences, we have an artificial seawater recipe so maybe I can add in typical seawater elements one step at a time and see if there is an addition that invokes the response im seeing in 44Ca? If it is ArHe though it might be more reasonable to fiddle with measurement parameters first. Unfortunately we do not have H2 set up for our machine.

  • That is strange yes! But if the nogas mode give expected results why not use that Stuck out tongue winking eye?

    But seriously, I am quite certain that on m/z44 (He mode) there is a significant interference related to carbon which is not (totally) removed by He. I see this nearly everyday with my acid digested carbon-rich samples. On m/z 44 I find Ca a lot higher than on m/z40 (H2 mode) for samples and reference materials. I know it has to do something with  a carbon based interference because in watersamples I can measure calcium accurately on m/z 44 also. My He mode (7800 ICP) runs on 4.3 mL/min.

    Maybe you can add some (electronics grade) isopropanol to some samples to experiment and see what happens with the m/z44 signal. And/ or vary the He flow.

    Sr++ could also interfere on m/z88. Good luck Thumbsup

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