Sensors 2026 , 26 , 2049
7of 16
interface temperature 250 ◦ C. Full scan mode was used over the range of 30–400 amu (or Da), at a rate of 10 scans s − 1 ; the extraction frequency was 30 Hz. As described previously, an autosampler was used. Between analyses, a blank was run to prevent carryover. The analyses were performed in triplicate.
2.6. Electronic-Nose 2.6.1. Coating of the Piezoelectric Quartz Crystals
Piezoelectric quartz crystals were 9 MHz AT-cut, HC-45 with gold electrodes (Euro- quartz Limited, Crewkerne, UK). The piezoelectric quartz crystals were coated on both sides with solutions of the coating compounds, namely Nafion, TBA-PMnW11, Mn(TPFPP)Cl, THEED, Mn-Pht, and PVP, using drop deposition followed by spreading by a spin-coater Süss Delta 10 BM (SÜSS MicroTec Lithography GmbH, München, Germany) or by spray deposition (BADGER model 200, Badger Air-Brush, Co., Bellwood, IL, USA). Solvents were then allowed to evaporate until the quartz crystal oscillation frequency became stable. The amount of coating was estimated from the difference between the bare crystal’s frequency and the coated crystal’s frequency after solvent evaporation. 2.6.2. Set-Up of the Electronic Nose and Analytical Methodology Figure 2 schematically shows the experimental setup. Compounds from the SPME fiber were thermally desorbed in a homemade oven set to 250 ◦ C. A nitrogen flow carried the desorbed VOCs to the 6 sensors, operating simultaneously. The experimental layout for the e-nose, as well and the PVC piezoelectric quartz crystals cell design, were elsewhere [9].
Figure 2. A simplified schematic experimental set-up of the array of six sensors: N 2 —nitrogen flow, F—SPME fiber, O—oven, V—distribution valve, OX—homemade oscillator, PS—power supply, PC—output of frequency of oscillation to a counter and storing data in a personal computer, T— polymeric cell housing the piezoelectric quartz crystal, C—piezoelectric quartz crystal inside the cell. The first step, the extraction step, consisted of sampling the volatile compounds from papers for 30 min, as described in detail in Section 2.5. After this period, the SPME fiber
https://doi.org/10.3390/s26072049
Made with FlippingBook interactive PDF creator