Fig. S2.

(A–E) Isotopic fine structure for the [C₁₆H₃₃MgO₄]⁻ molecule ion. [C₁₆H₃₃MgO₄]⁻ was identified via its isotopologues in the Novato methanolic extract, as seen in ESI-FT-ICR mass spectra. The black spectrum is the experimental mass spectrum from Novato extract, and the red profile represents the theoretically computed isotopic pattern at natural abundance of C, H, O and Mg. The observed isotopic distribution is primarily caused by the isotopes ^24/25/26Mg, ^12/13C, ^16/18O and ^1/2H. To separate each isotopologue at FWHM, a minimum mass resolving power of R ∼125,000 at m/z = 313 is required. Beyond CHOMg, the total combinatorial molecular complexity within the sample set includes N and S compositions. The mass resolving power of R ∼500,000 helps to exclude more complex combinatorial formula solutions, which contain, for example, nitrogen and sulfur and discriminates unambiguously the CHOMg from the CHNOS chemical compositions.