Abstract
A time-dependent postextraction differential acceleration (PEDA) potential was used to temporally focus increasingly heavy ions in a stigmatic imaging mass spectrometer, allowing them to be imaged with high mass and spatial resolutions over a broad mass-to-charge (m/z) range. By applying a linearly rising potential to the ion extraction electrode, sequential m/z ratios were subjected to a changing electric field, allowing their foci to coincide at the detector. Using this approach, at least 75% of the maximum mass resolution was obtained over a 300–600 Da range when the ion microscope was focused around 450 Da, representing more than a 10-fold increase over the conventional single-field PEDA method.
Highlights
High-throughput imaging of spatially localized chemical distributions plays a central role in many disciplines
Pathology and drug discovery benefit from the rapid identification of biomarkers or proteins in assays and tissues. These and similar experiments often employ mass spectrometry imaging (MSI) to record and assign mass spectra to defined spatial coordinates on a sample.[1−4] MSI provides a wealth of information: results can be integrated over mass to extract images of distinct compounds or over position to compare mass spectra from different regions of a sample
We showed that the mass range over which the mass resolution of our ion microscope reached at least half that of the focused m/z (410 Da) was just ∼8% of the 300 Da range that could simultaneously be spatially resolved.[27,28]
Summary
High-throughput imaging of spatially localized chemical distributions plays a central role in many disciplines. The PEDA technique immediately extracts surface ions following ionization to retain their spatial information and corrects for their initial velocity dispersion by raising the potential of the extraction electrode after the ions have passed through it In this way, ions with slower initial velocities gain more kinetic energy from the voltage pulse than faster ions, creating a temporal focus point that can be superimposed on the detector to improve the mass resolution. In this way, the spatial information of the ion packet is retained due to the electric field in the extraction region, while the mass resolution is increased by the differential acceleration and energy focusing imparted by the voltage pulse. This is a consequence of positioning the focal points of the linear pulse near the center of the simulated mass range in Figure 2c, whereas the optimized quadratic pulse begins to focus ions at lower m/z
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