Schematic workflow for the studies of saliva compositional stability (a) immediately after collection, at 22 °C (0 to 48 h), 4 °C (0 to 48 h), and −20 °C (0 to 4 weeks), a donors: subjects S1–S3, b,c donors: subjects S1–S5; (b) after storage (−80 °C, 1–2 weeks), thawing and preparation for NMR analysis, with and without sodium azide, at 4 °C (d donors: subjects S1, S2, and S4, e donors: subjects S1, S3, and S4), and at 25 °C; (f donors: subjects S1, S4, and S5, g donors: subjects S1, S3, and S4). Please note that some of the subjects donated saliva more than once, in different days, and that no samples were pooled, in order for inter-individual and inter-day variability to be assessed.