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 ¹³C 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 ¹³C₁¹²C₅O₄ moiety explained more than 95% of the questionable “N₅S₂ moieties”. This finding was proved by calculation of δ¹³C‰ values from relative peak intensities.