## 2 Results
### Non-VOCs Detected by GC-TOFMS  
A total of 268 non-VOCs (160 identified non-VOCs) including organic acids, alcohols, sugars and benzene derivatives were detected by GC-TOFMS (Table S3). Most of these non-VOCs were detected in distilled liquors for the first time. Organic acids (46 compounds) were the most frequently detected non-VOCs, accounting for 17% of the total non-VOCs, followed by sugars (43 compounds), benzene derivative (30 compounds), and alcohols (27 compounds) (Figure 2A). These four classes of compounds constituted about 55% of the total non-VOCs detected. Esters, the dominating VOCs detected by HS-SPME-GC-TOFMS, were barely detected. The contents of non-volatile acids in Chinese baijius (283-727 mg/L) were generaly higher than in Western liquors (16-378 mg/L) (Figure 2B). Lactic acid was the major non-volatile acid in Chinese baijius (125-484 mg/L) accounting for 7-53% of the total non-VOCs. However, lactic acid was detected in only small amounts in Western liquors (1-44 mg/L). Sugars only comprised about less than 2% of the total non-volatile profile in Chinese baijius, but were the major non-VOCs in Western liquors, comprising from 30% to 90% of total non-VOCs except for some vodkas (V2 and V3) in which no sugar was detected (Figure 2C). Six-carbon (C6)-derived sugars, such as fructose and mannose, dominated the non-VOCs in Western liquors. Fructose accounted for more than 9% of the total non-VOCs in Western liquors, with the highest abundance (56%) found in B1 (Table S3 and Table S4). The non-volatile alcohols detected were mainly polyols. Glycerol was the main non-volatile alcohol with content ranging from 9 to 553.2 mg/L in the Western liquors assessed. In contrast, the distribution of alcohols was more balanced in Chinese baijius. For example, in addition to glycerol, 2-methyl-1,3-propanediol and 2,3-butanoli were the main non-volatile alcohols, with 2,3-butanoli  
Figure 2: Non-volatile organic compounds (non-VOCs) in distilled liquors detected by GC-TOFMS. (**A**) Number of compounds in major classes of non-VOCs in 24 representative distilled liquors. (**B**) Contents of major classes of non-VOCs. (**C**) Relative concentrations of major classes of non-VOCs. accounting for more than 50% of the total non-volatile alcohols in Chinese soy sauce aroma type liquor (Table S3 and Table S4). Although the contents of benzene derivatives were stable at 123-468 mg/L in the distilled liquors assessed, their relative concentration varied considerably, constituting 2-78% of total non-VOCs (Figure 2B,C). Benzene, ethyl, and benzene,1,4-dimethyl- were the most common benzene derivatives (Tables S3 and S4). Characterization of Differences and Similarities of Chinese Baijius and Western Liquors by PCA and Correlation Network  
In addition to the direct comparisons of VOCs and non-VOCs in Chinese baijius and Western liquors, we used more holistic analytical methods such as principal component analysis (PCA) and correlation network analysis to evaluate systemic differences amongst the samples. PCA, an unsupervised model that considers all variables, clusters or separates samples by reducing the dimensionality of variables. The first 10 principal components (PCs) of the VOCs-based PCA model described more than 80% of the data, with the first two PCs representing more than 42% of the variance in the data. The Chinese baijius were on the left of PC1 and all the Western liquors were located on the right of PC1. The first eight PCs of the non-VOCs-based PCA model described more than 80% of the data, with the first two representing more than 44% of the variance in the data. The Chinese baijius and vodkas were located on the lower left of PC1 and the rest of the Western liquors were distributed on the right of PC1 or above PC2. In both scores plots, Chinese baijius could be seperated from the Western liquors, suggesting their compositional differences (Figure 3A,B). Similar result was also obtained through heatmap analysis by combining VOCs and non-VOCs (Figure S1).