What is the advantage of alternative current (AC) corona discharge for APCI mass spectrometry?

A comparative study on direct current (DC) and alternating current (AC) corona discharge ionization for positive and negative mode of operation was performed by mass spectrometry. The needle, positioned 3 mm in front of the inlet of a mass spectrometer, was moved upward and intensities of ions were measured as a function of vertical distance d. For DC corona discharge, the unipolar plasma sizes for positive and negative ions were found to be nearly the same with d ≈ ∼5 mm. In contrast, for AC corona with 15 kHz, while positive ions were found to be much more confined in a narrow region with d ≈ ∼2 mm, negative ions were detected with much larger d ≈ ∼10 mm. This marked difference of unipolar plasma sizes of positive and negative ions for AC corona was ascribed to the much larger diffusion lengths of electrons than those of positive ions. In the negative mode of DC and AC corona discharge ionization, the intensity of CH₃COO⁻ (m/z 59) originating from an acetic acid impurity was found to increase with d. This peculiar behavior was attributed to the proton transfer reaction of O₂⁻ with acetic acid vapor taking place distant from the tip of the electrode needle. The ionization efficiency of AC corona is higher than that of DC corona for both positive and negative modes of operation. The higher ionization efficiencies for AC corona are attributed to the generation of remnant electrons and positive ions for positive and negative phase AC corona, respectively. These remnant charges play crucial roles as the triggers for plasma breakdown in every cycle of RF frequency. The ion intensities measured by proximity corona discharge ionization are almost proportional to the analyte concentrations when they are less than 1 ppm.