## 2 Materials and Methods  
We collected 104 PET-bottled commercial mineral water samples, either sparkling, still and natural effervescent in Italy from 24 producers. Samples were shipped to the laboratory directly by producers, on a voluntary basis, after request by Mineracqua, the association of manufacturers of bottled-water products. Sampling was representative of the whole country, including the major islands since we collected the main mineral water brands form each Italian region (Figure 1). As soon as they arrived at the laboratory, the samples were placed in a cold room, stored at 4 \({}^{\circ}\)C, and analyzed within a few days. All samples were accompanied by a data sheet, compiled by the producer, giving information on the batch. The samples were organized into six groups, according to the sample type and size (0.5 or 1.5 L): natural still water 1.5 L (NW1.5), natural still water 0.5 L (NW0.5), natural effervescent 1.5 L (EW1.5), natural effervescent 0.5 L (EW0.5), sparkling carbonated 1.5 L (SW1.5) and sparkling carbonated 0.5 L (SW0.5). The analytical procedure was a fully automated method, previously described [11]. It is based on head-space solid-phase micro-extraction (SPME) and on isotopic dilution, with acetaldehyde-d4 as internal standard. Briefly, 3 mL water were prepared in 10 mL headspace vials. After addition of 6 \(\mu\)L of a 50 ng/\(\mu\)L solution of internal standard solution, AA was derivatized directly in water samples into the thermally stable oxime product [24; 25] by adding 20 \(\mu\)L of o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine hydrochloride (PFBHA) 10 ug/\(\mu\)L at 60 \({}^{\circ}\)C. The headspaces were extracted by solid-phase micro-extraction (SPME), with a 2-cm long tri-phasic fiber (Supelco, Bellefonte, PA) for 10 min. After extraction, the fiber was thermally desorbed in the GC injector, and the AA oxime derivative identified and quantified [26] against the isotopically labeled internal standard analog. Analysis was done by gas chromatography mass spectrometry (GC-MS) with an Agilent GC5890-MSD5975C system, using standard 70 eV electron ionization mode. The acquisition was Selected Ion Monitoring and full-SCAN mode (SIM/SCAN) in order to check for false positives due to contamination by unknown organic compounds. To quantify AA, ions with _m/z_ 209 and 213, for native and deuterated internal standard respectively, were used, with a Varian CP-Select 624 CB chromatography column (60 m,  
Figure 1: Map of Italy with the different sampling points. The black labels indicate the locations where samples were collected. The number inside the label indicates the number of different brand samples collected in each place. 0.32 mm I.D., 1.80 um film thickness). The GC oven temperature program was 80 \({}^{\circ}\)C for 1 min, 15 \({}^{\circ}\)C/min to 220 \({}^{\circ}\)C, held for 15 min. The initial pressure in the column was 29 kPa. Helium was used as a carrier gas at a constant flow rate of 0.8 mL/min. The injector operated in splitless mode and temperature was set at 250 \({}^{\circ}\)C. Transfer line temperature was maintained at 280 \({}^{\circ}\)C and ion source was set at 250\({}^{\circ}\)C. The linearity of the instrumental response was in the range of 1-600 \(\mu\)g/L with average regression coefficients of 0.9995. The limit of detection (LOD) and limit of quantitation (LOQ) was determined on the signal-to-noise ratio (S/N) of the lowest point in the calibration line. As an average LOD was 0.2 \(\mu\)g/L (with a S/N 3:1) and LOQ (with a S/N 10:1) was 0.7 \(\mu\)g/L. The coefficient of variation was 18 % (intra-day) and 13 % (inter-day). [PERSON]'s rank correlation analysis was used to investigate the correlation between AA concentration and the characteristics of the samples (size, CO\({}_{2}\) concentration, fixed residue, shelf life, time from sample collection to analysis, and distance from the production site).