Mass spectrometrically detected directly coupled high performance liquid chromatography/nuclear magnetic resonance spectroscopy/mass spectrometry for the identification of xenobiotic metabolites in maize plants

Reconstructed ion chromatograms have been used to identify relevant high performance liquid chromatography (HPLC) peaks in a directly coupled high performance liquid chromatography/nuclear magnetic resonance spectroscopy/mass spectrometry (HPLC/NMR/MS) experiment. This has been applied to a study of the metabolism of a model compound, 5-nitropyridone (2-hydroxy-5-nitropyridine), in maize plants grown hydroponically. By monitoring the on-flow reconstructed ion chromatogram corresponding to the 5-nitropyridone fragment at m/z 143, and additional molecular ions corresponding to metabolites identified as products from similar compounds, relevant peaks were identified rapidly for subsequent stopped-flow 1H NMR spectroscopic analysis. The combination of coupled HPLC/NMR/MS enabled the direct identification of three metabolites, namely the N-glucoside, N-malonylglucoside, and O-malonylglucoside. This work demonstrates the power of HPLC/NMR/MS for the structural elucidation of xenobiotic metabolites in complex biological matrices (such as plant material) with minimal sample preparation. In particular, using mass spectrometry for the initial identification of relevant HPLC peaks allows the analysis of complex samples without the necessity for other spectroscopic markers, such as 19F NMR signal for fluorinated compounds or UV spectroscopy for molecules with strong UV chromophores.