If I get Na result 100 ppm on ICP-MS and 95 ppm on ICP-OES, what is the reason? How can we explain this difference?
Perhaps you should first look at the precision of one of those values first. For example if the uncertainty of the 100ppm result on ICPMS is 5%, so 95-105ppm and the OES has similar uncertainty there is significant overlap of those 2 values if you include the error bars. In my world (environmental analysis) we would simply call this "analytical variability" and consider the values identical, statistically speaking.
In industries where 5% is a real big deal a lot of effort is required to reduce that uncertainty.
Perhaps you should first look at the precision of one of those values first. For example if the uncertainty of the 100ppm result on ICPMS is 5%, so 95-105ppm and the OES has similar uncertainty there is significant overlap of those 2 values if you include the error bars. In my world (environmental analysis) we would simply call this "analytical variability" and consider the values identical, statistically speaking.
In industries where 5% is a real big deal a lot of effort is required to reduce that uncertainty.