## 3 Results  
### Optimization of SPME Arrow Fibers  
The polarity and volatility of the material to be analyzed determined the extraction efficiency of different SPME Arrow fibers [36]. Therefore, an appropriate SPME Arrow fiber was selected for analyzing the volatile compounds of distilled spirits using five commercial sorbents (120 \(\upmu\)m CAR/PDMS, 120 \(\upmu\)m DVB/CAR/PDMS, 120 \(\upmu\)m PDMS, 120 \(\upmu\)m DVB/PDMS, and 100 \(\upmu\)m PA). Figure 1 shows the total number of peaks detected for each class of volatile compounds in distilled spirits. The Arrow fibers are classified according to their relative polarity. PDMS is relatively nonpolar; CAR/PDMS and DVB/PDMS are semi-polar; and PA and DVB/CAR/PDMS are polar [37]. Fifty-four volatile compounds, the highest number, were identified in the Arrow fiber with the CAR/PDMS sorbent and can be divided in the following nine classes: acetals (2), acids (1), alcohols (11), aldehydes (2), esters (20), furans (5), hydrocarbons (9), lactones (3), and phenols (1). Thirty-seven volatile compounds, the second highest number, were identified in the fiber with the DVB/CAR/PDMS sorbent: acetals (1), acids (1), alcohols (9), aldehydes (2), esters (13), furans (4), hydrocarbons (5), lactones (2), and phenols (0). SPME Arrow fibers with PDMS, DVB/PDMS, and PA could detect 32, 35, and 29 volatile compounds, respectively.  
Figure 1: Total number of peaks detected for each class of volatile compounds in Soju (GUL-5) using five types of HS-SPME Arrow fibers (CAR/PDMS, DVB/CAR/PDMS, PDMS, DVB/PDMS, and PA).  
The normalized peak areas for each class of the volatile compounds obtained using SPME Arrow fibers with different sorbents were compared (Figure 2). The CAR/PDMS, with a total peak area of 77,745 \(\times\) 10\({}^{6}\), showed a high normalized peak area for all the classes of volatile compounds, except for acids. The Arrow fiber with PA showed the highest normalized peak area for the acid class, with a total peak area of 51,206 \(\times\) 10\({}^{6}\). The total peak areas observed with DVB/CAR/PDMS, PDMS, and DVB/PDMS were 57,485 \(\times\) 10\({}^{6}\), 25,751 \(\times\) 10\({}^{6}\), and 46,151 \(\times\) 10\({}^{6}\), respectively. The results obtained with the CAR/PDMS fiber showed completely different trends from the other four fibers.  
The reproducibility of SPME Arrow and SPME fibers was evaluated to select a suitable fiber type for analyzing the volatile compounds of Soju. SPME Arrow and SPME extraction methods were tested under the same conditions: type of fiber, CAR/PDMS; extraction temperature, 40 \({}^{\circ}\)C; extraction time, 30 min; number of runs, triplicate. Supplementary Figure S1 shows the average of relative standard deviation (RSD) and standard deviation of RSD. The SPME Arrow fiber showed the following data: peak area, 7.7414 \(\times\) 10\({}^{10}\); the average of RSD, 3.61%; and standard deviation of RSD, 4.71%. The SPME fiber had the following features: peak area, 1.0687 \(\times\) 10\({}^{10}\), which is approximately seven times less than that of the SPME Arrow fiber; the average of RSD, 4.41%; and standard deviation of RSD, 9.92%. These results showed that the SPME Arrow fiber can be used for the analysis of volatile compounds in distilled spirits as it had better reproducibility than the SPME fiber.