Although major advantages have been made in developing robust, easy-to-use ICP-AES instruments offering sub µg g⁻¹ detection limits and relative interference free operation, long-term drift of the analytical signal continuous to be problematic and necessitates regular re-calibration. The work presented here focuses on the effect of two instrumental parameters, i.e. the rf power and the nebuliser gas flow rate, on the robustness of the signals. The effects on the long-term stability when varying these two factors was systematically studied using an experimental design protocol. A "drift diagnosis" on thirty emission lines was performed at 12 different sets of operating conditions by repeated determination of a multi-element solution over several hours. The results were studied using standard parameters, i.e., Mg ratio, sensitivity, drift error, drift patterns and multi-way analysis. Parallel factor analysis (PARAFAC) was employed to analyse the 3-way data array generated: "emission lines × replicates × operating conditions". The physical interpretation of the new PARAFAC-factors is shown to enable a better understanding of the drift phenomenon by mathematically characterising the causes of long-term instability. Finally, the robustness of the technique using different operating conditions is evaluated and the appropriate use of internal standards to correct for drift is discussed.