High-field FTICR-MS data evaluation of natural organic matter: are CHON5S2 molecular class formulas assigned to (13)C isotopic m/z and in reality CHO components?

The analysis of dissolved organic matter (DOM) using high-field Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) poses challenges in molecular formula assignment. The consideration of (13)C isotopes provides new insights into the consistent elemental formula solutions. Modern software helps to overcome misinterpretation, but false assignments of molecular classes to mass peaks have rarely been elucidated until now. It will be demonstrated that this can be important with formula assignments comprising exactly five nitrogen and two sulfur atoms in DOM data sets: the molecular class CHON5S2. The existence of such components in DOM under tripeptide Met-His-Cys formed with the formula C14H23O4N5S2 cannot be excluded; however, components containing 5 N and 2 S should be suspected to not be highly abundant. The true elemental compositions of such unusual "N5S2 moieties" were calculated using Suwannee River fulvic acid (SRFA) data from the literature and one data set from acidic pit lake pore water. The replacement of a H3N5S2 moiety with a (13)C1(12)C5O4 moiety explained more than 95% of the questionable "N5S2 moieties". This finding was proved by calculation of δ(13)C‰ values from relative peak intensities.