Standard Addition Method for the Determination of 1 Pharmaceutical Residues in Drinking Water by Spe-

21 The study of the occurrence and fate of pharmaceutical compounds in drinking or waste 22 water processes has become very popular in recent years. LC-MS/MS is a powerful 23 analytical tool often used to determine pharmaceutical residues at trace level in water. 24 However, many steps may disrupt the analytical procedure and bias the results. A list of 27 25 environmentally relevant molecules, including various therapeutic classes and 26 (cardiovascular, veterinary and human antibiotics, neuroleptics, non-steroidal anti-27 inflammatory drugs, hormones and other miscellaneous pharmaceutical compounds) was 28 selected. In this work, a method was developed using Ultra Performance Liquid 29 Chromatography coupled to tandem Mass Spectrometry (UPLC-MS/MS) and solid phase 30 extraction (SPE) to determine the concentration of the 27 targeted pharmaceutical 31 compounds at the nanogram per liter level. The matrix effect was evaluated from water 32 sampled at different treatment stages. Conventional methods with external calibration and 33 internal standard correction were compared to the standard addition method. An accurate 34 determination of pharmaceutical compounds in drinking water was obtained by the 35