Fig. 2
Si and O isotopic fingerprints of SiO2 NPs of different origins. a, b Si isotopic composition of SiO2 NPs grouped according to sources. c, d O isotopic composition of SiO2 NPs grouped according to sources. The dots in a and c represent the samples tested in this study, and those in b and d include available data in the literature20,21,32,33,34,35,36,37,38,39,40,41. In b and d, *P < 10-4, **P < 10-4, and ***P < 10-4. Unpaired Student’s two-tailed t-test was used. e, f, Si-O 2D isotopic fingerprints of SiO2 NPs with source differentiation by two straight lines (δ18O = 13‰ and δ30Si = 0.1‰). The f is a partial enlarged view of e. The different colored zones represent different sources (NQ or ND or ES+EF+EP). The error bars represent 2s.d. (n = 2-5). g, h Si-O 2D isotopic fingerprints of SiO2 NPs with linear discriminant analysis (LDA) into three (g) or five classes (h). The zones defined by colored contour lines representing virtual distribution ranges of different sources given by the LDA-based classifiers. The color and thickness of the contour lines correspond to the respective sources and the probabilities of a sample being predicted to be the related class (0.5, 0.4, 0.3 for the thick, normal, and the thin one, respectively). Note that the ND and NQ samples in g, h include both the real samples used in this study and pseudo-samples constructed using the δ30Si and δ18O data reported in the literature (see Supplementary Methods 1.4)
