# Identification by GC-MS Analysis of Organics in Manufactured Articles through a D-Optimal Design
## 3 Results  
### Chemometric Model  
The results of [PERSON]'s algorithm are reported in Figure 1.  
Figure 1: Log(M) as a function of the number of experiments.  
The plot describes the behavior of the logarithm of M as a function of the number of experiments. From the plot, it can be observed that the log(M) value increases up to 11 experiments; then, after a brief decrease, it continues increasing to a value of 25 experiments. Since the performance of 25 experiments would have been too expensive and time-consuming, a good trade-off involved the preparation of 11 experiments.  
Since 7 experiments had already been performed, only 4 new experiments were carried out by choosing among the 32 calculated by the D-optimal design. Table 2 reports the experiments performed.  
The results from the D-optimal design were then evaluated, as a screening approach, to estimate the significance of the experimental parameters under exam. The dependent response (Y) under exam was the number of identified compounds in the analyzed samples (A, B and C). As reported in Figure 2, only the dichloromethane and heptane parameters were significant.  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & **\# Experiment** & **Acetone** & **Dichloromethane** & **Heptane** & **Extraction** & **Time GC Analysis** \\ \hline \multirow{6}{*}{Before DOE} & 1 & \(+1\) & \(-1\) & \(-1\) & \(-1\) & \(-1\) \\  & 2 & \(-1\) & \(+1\) & \(-1\) & \(-1\) & \(-1\) \\  & 3 & \(-1\) & \(+1\) & \(-1\) & \(-1\) & \(+1\) \\  & 4 & \(+1\) & \(-1\) & \(-1\) & \(-1\) & \(+1\) \\  & 5 & \(-1\) & \(-1\) & \(+1\) & \(+1\) & \(+1\) \\  & 6 & \(-1\) & \(+1\) & \(-1\) & \(+1\) & \(+1\) \\  & 7 & \(+1\) & \(-1\) & \(-1\) & \(+1\) & \(+1\) \\ \hline \multirow{6}{*}{After DOE} & 8 & \(-1\) & \(+1\) & \(-1\) & \(+1\) & \(-1\) \\  & 9 & \(-1\) & \(-1\) & \(+1\) & \(+1\) & \(-1\) \\ \cline{1-1}  & 10 & \(+1\) & \(+1\) & \(-1\) & \(+1\) & \(-1\) \\ \cline{1-1}  & 11 & \(+1\) & \(+1\) & \(+1\) & \(+1\) & \(+1\) \\ \hline \end{tabular}
\end{table}
Table 2: Experimental parameters: where for acetone, dichloromethane, heptane \(+1\) is yes, \(-1\) is no; for extraction \(+1\) is sonication process, \(-1\) is Soxhlet process; for time of GC analysis \(+1\) is 30 min, \(-1\) is 24 min.  
Figure 2: Significance of the experimental parameters in terms of MLR coefficients from the D-optimal analysis. The * and ** symbols indicate the significant \(p\)-values lower than 0.05 and 0.01, respectively.  
In particular, the use of dichloromethane as a solvent favored the extraction and identification of a higher number of compounds, and the absence of heptane within the analytical procedure was preferred. The use of Soxhlet or sonication extraction was not significant, but Soxhlet extraction, when employed, was preferred to sonication since it provided a slightly higher number of identified compounds.  
The use of acetone and the time of GC-MS analysis were both non-significant parameters. Therefore, the faster GC-MS analytical conditions were used to diminish the time of analysis. Moreover, since the use of acetone provided a higher (but not significant) number of identified compounds, its combination with Soxhlet extraction was evaluated too, in parallel with the one involving dichloromethane.