Have you considered the possibility of spectral interferences - a good one
might be AlO (m/z = 27 + 18). Other possibilities but perhaps less likely
would include CaH (44, 1), MgOH (26,18,1), AlOH (27,17,1).
You could test this by preparing cocktails of the anticipated
interferences, some with and without analyte and you can then determine how
to implement correction equations for the interferences you find.



Regards



Michael B. Knowles
ICP-ES and ICP-MS Marketing
Varian Optical Spectroscopy Instruments

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