2.2.2. Standards and Calibration Solutions

For each compound, internal standard included, two standard stock solutions were preliminarily prepared. The first set of standard solutions (S1) was prepared diluting reference standards in methanol at a concentration of about 9 <sup>×</sup> 10<sup>7</sup> <sup>μ</sup>g/L. The second set of standard solutions (S2) was prepared diluting S1 solutions in methanol (1:100 dilution) to obtain a concentration of about 9 <sup>×</sup> 105 <sup>μ</sup>g/L.


**Table1.**E-liquidscompositionandinformation:manufacturingcountry,%ofthemaincomponents,characteristicflavorandnicotinecontent(expressedasmg/mL



*Atmosphere* **2020** , *11*, 374

\* mg/g; (-) means that information was not provided on product label.

 as

Starting from S2 and with subsequent dilution with methanol, five solutions for each compound were prepared in the concentration range, approximately 20.0–450.0 μg/L (S3–S7). In order to simulate e-liquid basic composition, five matrix-matched calibration solutions for each compound were prepared by adding 100 μl of the corresponding S3–S7 solutions and 100 μl of benzene-d6 solution (S2 set) in a headspace (HS)-vial containing 1 ml of laboratory-made liquid (90% propylene glycol, 10% water). Similarly, a blank solution was also prepared by adding 100 μl of benzene-d6 solution (S2 set) and 100 μl of methanol in a HS-vial containing 1 ml of laboratory-made liquid (90% propylene glycol, 10% water). Both blank and matrix-matched calibration solutions were used for calibration, resulting in five concentration levels for each compound in the dynamic range between limit of quantification (LOQ) value and 45.0 μg/L.
