Ca & Na curve linearity Failing during calibration

hello Family. 

I recently run fish tissue samples for mercury determination on MP-AES 4210 . Unfortunately my calibration curves for both  Ca & Na have been failing ever since, i have tried selecting different wave lengths. the feature am using is a multi-calib i.e Ca, Mg, K and Na

I used the same standards used before and it use to be all smooth before mercury run. 

could this be contamination in the sample line? if so how do i rectify it.

the intensity is getting low immediately just after the 2nd standard

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