This paper describes the theory of on-line additions of aqueous standards for calibration of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Establishment of a calibration curve enabled investigation of: fractionation, matrix effects, mass flow ratios, and the relative merits of wet and dry plasma conditions for laser ablation sampling. It was found that a wet plasma was much more tolerant of increased sample loading without reducing plasma robustness, leading to less severe and more constant mutual matrix effects. These findings indicate that the on-line addition of water is the preferred mode of operation for quantification by LA-ICP-MS. The analytical performance of the method was validated by the analysis of three certified reference materials: National Institute of Standards and Technology (NIST) 612 Trace Elements in Glass, European Reference Material (ERM) 681 Trace Elements in Polyethylene and British Chemical Standards (BCS) No. 387 Nimonic 901 Alloy. Analysis of NIST 612 was performed under both wet and dry plasma conditions, and the correlation with certified elemental concentrations was much better when a wet plasma was employed. Analyses of ERM 681 and BCS No. 387 were performed under wet plasma conditions, due to that method’s proven advantages. The differences between the found and certified elemental concentrations varied between 1–10% for the majority of elements, for all three certified reference materials.