Highly Efficient Ionization of Nitro-aromatic Compounds using Photoelectron Induced Atmospheric Pressure Ionization (PAPI)

摘要

All three ionization methods were capable of detecting nitro-aromatic compounds efficiently. Signal pattern (amount of fragmentation/ion transformation processes) is different: PAPI: (1) Generally only one signal for each compound is observed. (2) [M-H]~(-) for compounds with acidic proton, otherwise [M]~(-). (3) O_(2)~(-) adduct/dimer formation in rare cases. PTR: (1) [M+H]~(+) formation. (2) Fragmentation observed, particularly for hydroxy-nitro-species. DA-APPI: (1) Main signals are M~(+)/[M+H]~(+) in positive mode; M~(-)/[M-H]~(-) in negative mode. (2) Very complex mass spectra due to the use of a dopant. PTR-TOF-MS shows a very good linearity over a wide concentration range. Linearity ranges of FT-IR (high conc.), PTR-TOF-MS (large conc. range), and PAPI (low conc. range) is of benefit for atmospheric chemistry studies. The estimated detection limits of PAPI are in the pptV-range. The low background noise level of PAPI mass spectra allows for detection of minor product signals. Optimum performance is obtained through parallel PTR and PAPI MS measurements, resulting in virtually orthogonal structural product information (proton affinity/gas phase basicity vs. electron affinity/gas phase acidity.