## 4 Discussion  
The sensor duplex has been developed to provide a competitive system for the early detection of the acetic acid released during the degradation of cinematographic films produced using cellulose acetate. As mentioned in the introduction, this material's degradation process consists of two phases: an initial one, which is water-dependent, the kinetic of which is quite slow. Once the concentration of acetic acid reaches a certain concentration in air, an autocatalytic process triggers, which quickly leads to irreversible damage to the film. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Film Name**} & \multicolumn{3}{c}{**Acetic Acid Concentrations (ppb) at Elapsed Time**} \\ \cline{2-4}  & **Day 0** & **Day 15** & **Day 30** \\ \hline Film 90 & 37,540 \(\pm\) 703 & 37,150 \(\pm\) 751 & 37,990 \(\pm\) 720 \\ \hline Film 88 & 1100 \(\pm\) 29 & 1063 \(\pm\) 32 & 1107 \(\pm\) 32 \\ \hline Film 94 & 428 \(\pm\) 8 & 414 \(\pm\) 6 & 418 \(\pm\) 5 \\ \hline Film 96 & 287 \(\pm\) 2 & 290 \(\pm\) 1 & 290 \(\pm\) 1 \\ \hline Carsta 64 & 380 \(\pm\) 9 & 369 \(\pm\) 11 & 382 \(\pm\) 7 \\ \hline Carsta 65 & 875 \(\pm\) 19 & 864 \(\pm\) 21 & 886 \(\pm\) 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Acetic acid values calculated at test start, after 15 days and 30 days of continuous exposure of BIOS1 + BIOS2 sensor duplex to vapors released by real cinematographic film samples. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Code**} & **Result with A–D Strips (ppm)** & **Result with BIOS1-BIOS2 Sensor Array (ppm)** \\ \hline Film 99 & \(>\)20 & \(>\)20 \\ \hline SCS 00060-1 & 1–2 & 1.687 \\ \hline SCS 00060-4 & \(>\)20 & \(>\)20 \\ \hline SCS 00060-5 & 3–5 & 5.215 \\ \hline SCS 00061-3 & \(>\)20 & \(>\)20 \\ \hline SCS\_00057A & \(>\)20 & \(>\)20 \\ \hline SCS\_00058A & 6–8 & 6.277 \\ \hline SCS\_00059 & \(>\)20 & \(>\)20 \\ \hline SCS\_00060-3 & 10–15 & 13.085 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._It is thus fundamental to detect the release of acetic acid from the cinematographic films as soon as possible in order to apply safekeeping techniques. The developed sensor duplex proved reliable for the accurate quantification of acetic acid among all the volatiles released during cinematographic films degradation. This was possible thanks to the selection of the two MOS sensors, one based on tungsten oxide, the other based on tin oxide. It is known from literature [34] that MOS sensors employing SnO\({}_{2}\) as thin layer are responsive towards oxidable vapors, particularly alcohols. This is due to the formation of an oxygen layer in proximity with the SnO\({}_{2}\) particles. This interaction causes a shift in the electron distribution in the SnO\({}_{2}\) particles, with consequent variation of the conductivity of the thin layer. When an oxidable gas is introduced in the system, oxygen leaves the proximity of the tin oxide particles to oxidize the gas, causing a reorganization of the electrons in the tin oxide particles, with consequent resistance variation. On the other hand, tungsten oxide thin films do not show these phenomena [35], proving responsive to both oxidizing and reducing gasses. In the examined case, the two compounds that influenced the response of the employed sensors were 1-butanol (an oxidable vapor) and acetic acid (a vapor without marked oxidizing or reducing properties). The chosen sensors responded to both gases but with markedly different profiles. When exposed to butanol, the tin oxide sensor recorded a reduction of 90% of resistance when exposed to 1 ppm, while when exposed to acetic acid it showed a similar reduction only when exposed to concentrations exceeding 20 ppm. On the other hand, the tungsten oxide sensor shows an overall lower decrease of resistance, reaching a 60% decrease when exposed to 1 ppm of butanol or when exposed to 6 ppm of acetic acid.