Statistical Analysis
The Xcalibur software (Thermofisher Co., Milford, MA, USA) were used to for mass spectrum analyse (Kang et al., 2020).
Since it is a trace quantity and there is also the tissue embedding agent and PET membrane strongly interference, we used an extraction characteristics ion method to quantify the trace compounds in LCM samples. We firstly identified 56 compounds from the three origins by directing extraction. Then, the quantitative analysis was executed for each compound of the LCM samples by extraction a single characteristics ion. Relative quantification of identified compounds in samples to be compared were based on the mean ionic intensity of the characteristic ions (Lovestead and Urness, 2020)
The hierarchical cluster analysis (HCA) results were demonstrated as heatmaps with dendrograms and the pearson correlation coefficients (PCC) were calculated using the function clustering and heatmaps were obtained by the pheatmap package in R.
Principal component analysis (PCA) and orthogonal partial least squared discriminant analysis (OPLS-DA) were conducted to evaluate the differences in chemical profiles of groups using the EZinfo software 3.0 (Version 3.0; Waters Co., Milford, MA), all the data is taken logarithmically (Lyu et al., 2020).
One-way analysis of variance (ANOVA) of quantitative analysis results of compounds from three geographic origins were performed using SPSS Statistics 20.0. ANOVA, with letters indicating significant differences (p < 0.05).