Fragmentation Patterns of Volatile Organic Compounds via PTR-MS

For the determination of the fragmentation patterns, we adapted a procedure described in the literature.13 Individual VOC solutions were prepared in de‐ionized water at concentrations ranging from 0.03 to 1% (v/v) for the aldehydes and fatty acids and at 100 mg/L for the phenols. A 500‐mL glass bottle, equipped with a Drechsel head, was filled with 100‐mL de‐ionized water and kept in a water bath at 37°C. A flow of clean air was generated by means of a pump, connected to a hydrocarbon trap (Supelco Supelpure HC, Sigma Aldrich). An airflow of approximately 500 sccm was applied to the bottle. Measurements were conducted employing a commercial PTR‐MS instrument (PTR‐TOF 1000, Ionicon Analytik GmbH, Innsbruck, Austria). At the beginning of the measurement, drift tube conditions were: temperature 110°C, pressure 2.30 mbar, and voltage 600 V, resulting in an E/N of 144 Td (1 Townsend = 10⁻¹⁷ V cm²). The PTR‐MS inlet was connected to the bottle head with a flow of 40 sccm, letting the excess flow out by means of a t‐piece. The inlet consisted of a PEEK tubing heated at 110°C. Once temperature and signal were stable, 1 mL of standard solution was added to the bottle by means of a syringe: several mass peak traces rapidly increased following addition and subsequently decreased, showing a time evolution pattern that was characteristic for each parent compound. This allowed to attribute unambiguously, for each compound and set of conditions, parent ion, and relative fragments or adducts. After standard solution addition and the initial equilibration phase, E/N was decreased in a stepwise fashion, with 1‐min steps. The voltage applied were as follows: 600, 550, 500, 450, 400, 350, 300, and 200 V, resulting in E/N = 144, 132, 120, 108, 96, 84, 72, and 48 Td, respectively. At the end of each experiment, the reagent ion was switched, and a new measurement was started using a fresh bottle, employing the same protocol and standard solution.
The effect of the reduction in air water content on fragmentation patterns was studied by employing the same experimental setup. The measurement was started using water‐saturated air and setting the instrument drift voltage at 350 V. Once steady conditions were reached, the switch to dry conditions were achieved by connecting a CaCl₂ cartridge in between the Drechsel bottle and the PTR‐MS inlet. The cartridge consisted of a 10‐mL glass vial equipped with a PTFE septum, filled with anhydrous calcium chloride (CaCl₂, Sigma‐Aldrich). After cartridge connection, approximately 30 seconds were required to re‐establish steady conditions. Since the volume of the cartridge was relatively low compared with the inlet flow, no measurable retention of analytes onto the cartridge could be observed. Measurement was then continued for approximately 1 minute, allowing to establish fragmentation patterns under dry conditions.