Distribution of significance (from the t-test) obtained in the selected K_p – V_rest interval by shuffling 250 times the time order of the observed spectra, for each of the four selected chemical species. The vertical dashed (dash-dotted) lines represent the borders of the intervals enclosing the 95% (99.73%) of the significance values, corresponding to the 95% (99.73%) quantile of the distributions. These intervals are reported in Table 4. Since the 50% of the significance values (~20 000 values) are <0.2 σ for all the 4 chemical species, the plots of the distributions are limited on the y-axis in the interval [0; 650] for clarity.