## 3 Results and Discussion
### Feature Selection  
The LIBS spectra of penicillin contain a large amount of information, much of which may not be relevant for the identification of different penicillin manufacturers. In previous literature, it was confirmed that it is necessary to perform feature selection on LIBS spectral data to filter interference or redundant information, to facilitate the analysis of LIBS spectral data and improve the accuracy of spectral classification [17; 43]. In this study, we evaluated the importance of each PPT, AC and ACPT spectral line for the manufacturer identification based on the decrease in the Gini index of RF. A significant decrease in the Gini index indicates that the spectral line is more important for classification. The RF algorithm was  
\begin{table}
\begin{tabular}{l l r} \hline \hline
**Type** & **Element** & **Line/Band Wavelength (nm)** \\ \hline \multirow{6}{*}{PPT} & CN & 358.0, 384.8, 385.7, 386.5, 388.0 \\  & Ca & 393.1, 396.8, 420.7, 422.6, 429.9 443.1, 732.6 \\  & Fe & 404.1, 445.1, 453.2, 577.7, 579.6, 580.7, 582.8, 648.9, 714.8, 849.8, 853.8 \\  & & 497.7, 498.5, 499.6, 500.1, 501.0, 503.2, 505.9, 516.8, 518.7, 520.6, 522.0, 550.9, 553.9, 558.9 \\  & Na & 558.9 \\  & O & 611.3, 615.8, 645.8, 671.8, 691.0, 693.6, 720.2, 777.4, 844.7 \\  & H & 656.2 \\  & N & 742.3, 744.2, 746.8, 818.8, 821.6, 822.3, 824.2, 862.9, 865.6, 868.0 \\  & K & 766.4, 769.9 \\ \hline \multirow{6}{*}{AC} & C & 247.8 \\  & Mg & 279.1, 279.7, 284.7 \\  & CN & 358.0, 384.8, 385.7, 386.5, 388.0, 414.5, 416.2, 417.6, 419.1 \\  & Ca & 393.1, 396.8, 420.7, 422.6 \\  & C\({}_{2}\) & 497.7, 498.5, 499.6, 500.1, 501.0, 503.2, 505.9, 516.8, 518.7, 520.6, 522.0 \\  & Na & 588.9 \\  & O & 777.4, 844.7 \\  & H & 656.2 \\  & N & 742.3, 744.2, 746.8, 818.8, 821.6, 822.3, 824.2, 862.9, 868.0 \\ \hline \multirow{6}{*}{ACPT} & CN & 358.0, 384.8, 385.7, 386.5, 388.0 \\  & Ca & 393.1, 395.7, 422.6 \\  & Fe & 445.1, 453.2 \\  & C\({}_{2}\) & 497.7, 498.5, 499.6, 500.1, 503.2, 505.9, 516.8, 518.7, 520.6, 522.0, 550.9, 553.9, \\  & Na & 588.9 \\  & O & 691.0, 693.6, 777.4, 844.7 \\  & H & 656.2 \\  & N & 742.3, 744.2, 746.8, 818.8, 821.6, 822.3, 824.2, 862.9, 868.0 \\  & K & 766.4, 769.9 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Corresponding wavelengths of the elements or molecular bands in the penicillin LIBS spectra.  
introduced in a previous study [16]. As shown in Figure 2, there were 44 spectral peaks in the LIBS spectra of PPT. We calculated the integrated intensities of the 44 spectral peaks and normalized the 44 data using the maximum normalization method. The 44 normalized data were used as the inputs of RF to obtain 44 Gini indices.  
For the same type of penicillin, the spectral lines of different elements had different effects on the identification of different penicillin manufacturers. The importance of each spectral line for the three types of penicillin is shown in Figure 3. In the LIBS spectra of PPT, the C\({}_{2}\) (497.7-505.9 nm) molecular band had the highest importance weight, and the spectral line of Na (588.9 nm) ranked second. The spectral lines of Na (588.9 nm) and Mg (279.7 nm and 279.1 nm) have relatively high importance in the LIBS spectra of AC. In the LIBS spectra of ACPT, the spectral lines of Na (588.9 nm) and K (766.4 nm) have great influence for the identification of different ACPT manufacturers. In the LIBS spectra of the three types of penicillin, the spectral line of Na (588.9 nm) has a relatively significant impact on the manufacturer identification.