Probe oscillation control in tapping-mode scanning probe electrospray ionization for stabilization of mass spectrometry imaging

Mass spectrometry imaging (MSI) is used for visualizing the distribution of components in solid samples, such as biological tissues, and requires a technique to ionize the components from local areas of the sample. Tapping-mode scanning probe electrospray ionization (t-SPESI) uses an oscillating capillary probe to extract components from a local area of a sample with a small volume of solvent and to perform electrospray ionization of those components at high speed. MSI can be conducted by scanning the sample surface with a capillary probe. To ensure stable extraction and ionization for MSI, the probe oscillation during measurements must be understood. In this study, we examined the changes in oscillation amplitude and phase due to the interaction between the oscillating probe and the brain tissue section when the probe tip was dynamically brought close to the sample surface. The changes in the probe oscillation depended on the oscillation frequency and polarity of the bias voltage applied to the solvent because an electrostatic force shifted the frequency of the probe oscillation. These findings suggest that controlling the probe oscillation frequency is important for stabilizing MSI by t-SPESI. This study highlights the importance of controlling probe oscillation frequency to stabilize MSI by t-SPESI. The change of the probe oscillation amplitude/phase are influenced by oscillation frequency during approach.