Negative Ion Laser Desorption/Ionization Time-of-Flight Mass Spectrometric Analysis of Small Molecules Using Graphitic Carbon Nitride Nanosheet Matrix

Ultrathin graphitic carbon nitride (g-C₃N₄) nanosheets served as a novel matrix for the detection of small molecules by negative ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) was described for the first time. In comparison with conventional organic matrices and graphene matrix, the use of g-C₃N₄ nanosheet matrix showed free matrix background interference and increased signal intensity in the analysis of amino acids, nucleobases, peptides, bisphenols (BPs), and nitropolycyclic aromatic hydrocarbons (nitro-PAHs). A systematic comparison of g-C₃N₄ nanosheets with positive and negative ion modes revealed that mass spectra produced by g-C₃N₄ nanosheets in negative ion mode were featured by singly deprotonated ion without matrix interference, which was rather different from the complicated alkali metal complexes in positive ion mode. Good salt tolerance and reproducibility allowed the determination of 1-nitropyrene (1-NP) in sewage, and its corresponding detection limit was lowered to 1 pmol. In addition, the ionization mechanism of the g-C₃N₄ nanosheets as matrix was also discussed. The work expands its application scope of g-C₃N₄ nanosheets and provides an alternative approach for small molecules.