Influence of needle voltage on the formation of negative core ions using atmospheric pressure corona discharge in air

The dependence of the formation of negative core ions Y− and their hydrated cluster ions Y−(H2O)n on needle voltage has been examined by using atmospheric pressure corona discharge ionization mass spectrometry. An insect pin (200 μm in diameter, 20 mm in length, and ca. 1 μm in the tip radius) was used as the corona needle. At the lowest effective needle voltage (−1.9 kV), water cluster ions OH−(H2O)n with a core ion OH− were observed. Furthermore there was a discontinuity between the abundances of OH−(H2O)3 at m/z 71 and OH−(H2O)4 at m/z 89, suggesting that the cluster OH−(H2O)3 is more stable than OH−(H2O)4. The ion OH−(H2O)3 which may be referred to as “the first hydrated shell” is formed via hydration of the core ion OH− in air. Various different core ions Y− appeared as the hydrated cluster ions Y−(H2O)n with increasing needle voltage. At relatively low voltage (−2.3 kV), the dominant hydrated clusters observed were OH−(H2O)n and O2H−(H2O)n. At the highest corona voltage (−3.5 kV), the well-known long-lived core ion NO3− and its hydrated clusters NO3−(H2O)n were mainly observed. The resulting negative core ions OH−, NO2−, NO3−, HNO3−, CO3−, and CO4−, which are atmospherically important ions or stable terminal ion species in tropospheric atmosphere, are discussed from the standpoint of ion evolution in air.