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Since 1940, poly- or perfluorinated alkyl substances (PFAS) have been largely used in many applications, including paints, fire foaming, household items, product packaging, and fabrics. Because of their extremely high persistency, they have been defined as "forever chemicals". Although the EU is taking action to reduce their use, their widespread occurrence in environmental matrices and their harmful effects on human health require the use of highly performing analytical methods for efficient monitoring. Furthermore, novel PFAS are constantly revealed by both EU and National environmental agencies. The objective of this work is to investigate the cause of the signal decrease during the analysis of a standard PFAS mixture in water-based matrices, by proposing an efficient technical procedure for laboratory specialists. The analyses were carried out on a mixture of 30 PFAS, including both regulated and unknown substances (which are expected to be introduced in the guidelines), characterized by different chemical features, using LC-vials of two different materials, namely, glass and polypropylene, and dissolved in two solvents, namely, water and water-methanol. The temperature of analysis and the concentration of PFAS were also considered through LC-MS analyses at different times, in the 0-15 h range. Depending on the chemical structure and length of the PFAS, sampling and treatment procedures may be adopted to tackle the decrease and the release from the containers, reducing the risk of underestimating PFAS also in real water matrices., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)