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).