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was retracted and introduced into the pre-heated oven (O). Next, the volatile compounds were thermally desorbed from the SPME fiber and a nitrogen carrier gas (N2) transported them to the distribution valve (V). This valve splits the sample stream among the six (6) piezoelectric crystals, each one with a different chemical coating. Each piezoelectric quartz crystal (C) was housed in a cell (T) and connected to electronic homemade oscillators (OX). The oscillation frequencies of the sensors were simultaneously monitored and stored on a PC at intervals of 1 s, using a Counter/Timer device PXI 1033 from National Instru- ments, Austin, TX, USA, and software written in LabView 7.0. Figure S1 in the Supplemen- tary Materials shows the simultaneous frequency recordings from the six sensors. A long baseline shows the frequency stability of all sensors. Their different oscillation frequencies are dictated by the amount and acoustic properties of the coatings. When compounds are released from the SPME fiber, carried by the nitrogen flow, and reach each coated crystal, the frequencies of the six sensors start decreasing simultaneously, as the main flow is equally divided into six streams by the distribution valve (V) and the sensors are equidistant from it. Due to the continuous nitrogen flow and the reversibility of interactions between coatings and volatile compounds, frequency increases were soon observed, as compounds were swept away and initial frequencies were promptly restored. The difference between the baseline frequency, obtained under pure nitrogen, and the minimum frequency recorded was computed for each sensor. The SPME fiber (F) was removed from the oven (O) only when the oscillation frequency of all the crystals had returned to baseline values, ensuring the complete recovery of all sensors and complete fiber desorption. 2.6.3. Sampling/Desorption Optimization Sorption of paper volatiles by SPME and desorption/carrier gas flow for the analysis were optimized using the simplex method. The objective was to maximize the sensors’ responses. Three variables were considered: the number of paper circles used in sampling, the exposure time of the SMPE fiber to the paper sample, and the nitrogen flow rate.
3. Results and Discussion 3.1. Paper pH
Paper typically contains mainly cellulose, a natural polymer forming long chains (fibers), and can be classified into three main types—rag, acidic, and contemporary (neutral/alkaline)—as a function of their different acidity (pH) and degree of polymer- ization (DP) [18]. Paper pH depends on pulping and bleaching processes, compound degradation, sizing reagents, and deliberate uses of alkaline stock. Contemporary paper is made from highly processed wood pulp with alkaline reserves. The pH of the analyzed paper samples is presented in Table 2.
Table 2. pH values measured for each paper according to TAPPI standard 509 [hydrogen ion concen- tration (pH) of paper extracts—cold extraction method]. pH values are an average of 3 replicates.
Sample
pH*
Sample
pH*
Sample
pH*
P1 P2 P3 P4 P5
9.04 ± 0.02 9.05 ± 0.01 9.42 ± 0.01 9.62 ± 0.01 10.08 ± 0.00
P8 P9
9.92 ± 0.01 9.92 ± 0.01 9.44 ± 0.00 9.45 ± 0.00 9.18 ± 0.01
P15 P16 P17 P18 P19
9.54 ± 0.01 10.00 ± 0.02 9.39 ± 0.00 10.15 ± 0.01 9.60 ± 0.02
P10 P11 P12
https://doi.org/10.3390/s26072049
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