Calibration curves of the H/C atomic ratio as a function of the H⁺/C⁺ (top left panel) and H⁻/C (bottom left panel) ion ratios. The black dots refer to the 14 calibration samples with their corresponding number (see Table 1). Right panels: enlargements of their respective left panel. The black line is the linear regression and the grey zone represents the predicted confidence band at 68%.