I am running P&T with a few internal standards (I.S.) and the naphthalene d8 I.S. is usually around 1/2 the response of the other I.S. of the same concentrations ... but on one instrument the naphthalene d8 response is only around 1/10 of the response of the other I.S. on that instrument... all purge flows / temps etc are identical. Autosampler is ATOMX XYZ. Any help much appreciated.