Optimization for Na and K on ICP-MS

So this is a broad question looking for resources.  I have been tasked with developing a ICP-MS method to test for low levels of Na and K (PQL of 1 ppm each).  I believe the easiest way to obtain these lower levels is to reduce my dilution factor (currently 100 fold) -samples (0.5g) are digested in 5 ml HNO3 then diluted to 50 ml with water.  This is done to decreased the acidity of the solution going through the ICP-MS (I was told a max of 10%).  Other than the prep/sample digestion any resources for element specific optimization? Our ICP-MS is 7800 series, Micro Mist Nebulizer, Model: x-lens, He gas cell (no H2).  Any other information please let me know.

  • I would recommend a cooler plasma (say 800 watts) to obtain lower levels of alkali metals on a 7800 ICP-MS.  This is kind of a semi-Cool plasma mode that can be run right along with hot plasma modes for other metals because the rf generator on our instrument is so robust that it can tolerate these power transitions and re-equilibrate rapidly.   Generally, Standards Additions is used for calibration for elements determined in this mode.  There are a number of plasma/lens parameters that must be set to optimize this semi-Cool mode and it can be combined with either H2 or He ORS to make it even more effective.

     

    HOWEVER, this approach depends on the sample matrix as to when this option can be exercised.  If it is a relatively clean matrix (high purity HNO3 or HCL, etc), this option will work just fine and allows DLs below 1 ppb for the referenced alkali metals.  If on the other hand, it is an environmental sample with  greater than ~0.5% TDS, this is not an option because there isn’t enough energy in the plasma to handle ionization of the matrix.   

     

    Since low level Na is fairly ubiquitous, lab cleanliness is paramount.  

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