In mass spectrometry imaging, spatial resolution is pushed to its limits with the use of ion microscope mass spectrometric imaging systems. An ion microscope magnifies and then projects the original spatial distribution of ions from a sample surface onto a position-sensitive detector, while retaining time-of-flight mass separation capabilities. Here, a new type of position-sensitive detector based on a chevron microchannel plate stack in combination with a 512 ϫ 512 complementary metal-oxide-semiconductor based pixel detector is coupled to an ion microscope. Spatial resolving power better than 6 m is demonstrated by secondary ion mass spectrometry and 8 -10 m spatial resolving power is achieved with laser desorption ionization. A detailed evaluation of key performance criteria such as spatial resolution, acquisition speed, and data handling is presented. (J Am Soc Mass Spectrom 2010, 21, 2023-2030 © 2010 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry M ass spectrometry imaging (MSI) [1-3] measurements allow the visualization of the spatial structure and identification of the molecular masses from complex surfaces. High spatial resolution is accomplished with ion-microscope mass spectrometers, where ions are extracted from the sample surface and projected onto a position-sensitive detector. A spatial resolution better than 4 m has been reported using UV/IR laser surface probes in matrix assisted laser desorption ionization (MALDI) [4 -8]. A pulsed primary ion beam as a surface probe can achieve higher spatial resolving powers (1 m) [9,10]. The spatial resolution can be further improved by using a more focused primary ion/laser desorption ionization surface probe. However, fragmentation of the surface molecules and long measurement times are undesired side effects of decreasing the surface probe area. For instance, at a 2 ϫ 2 m pixel size (4 m lateral resolution) and a sample size of 1 ϫ 1 mm, a typical measurement comprises 250,000 measurement points and can last several hours. An alternate approach to increase the spatial resolution is the use of microscope mode MSI. In the microscope mode, surface molecules are desorbed and ionized over a large sample area, typically 200 -300 m in diameter. An ion microscope employs ion optics to project the ionized surface compounds onto a position-sensitive detector while magnifying the image and retaining the spatial information defined by the sample surface. With a field of view of 200 ϫ 200 m and a sample size of 1 ϫ 1 mm, a microscope mode imaging experiment involves 25 measurement points and retains the 4 m lateral resolution given the corresponding ion optical magnification factor. The ion optical magnification allows high-resolution images to be obtained independent of the ionization source. Microscope mode MSI enables fast, highresolution large area imaging provided that an adequate, i.e., fast and position-sensitive, detector is used to record high quality molecular images [4].Position-sensitive detectors most commonly used for microsc...

Native mass spectrometry (native MS) has emerged as a powerful technique to study the structure and stoichiometry of large protein complexes. Traditionally, native MS has been performed on modified time-of-flight (TOF) systems combined with detectors that do not provide information on the arrival coordinates of each ion at the detector. In this study, we describe the implementation of a Timepix (TPX) pixelated detector on a modified orthogonal TOF (O-TOF) mass spectrometer for the analysis and imaging of native protein complexes. In this unique experimental setup, we have used the impact positions of the ions at the detector to visualize the effects of various ion optical parameters on the flight path of ions. We also demonstrate the ability to unambiguously detect and image individual ion events, providing the first report of single-ion imaging of protein complexes in native MS. Furthermore, the simultaneous space-and time-sensitive nature of the TPX detector was critical in the identification of the origin of an unexpected TOF signal. A signal that could easily be mistaken as a fragment of the protein complex was explicitly identified as a secondary electron signal arising from ion-surface collisions inside the TOF housing. This work significantly extends the mass range previously detected with the TPX and exemplifies the value of simultaneous space-and timeresolved detection in the study of ion optical processes and ion trajectories in TOF mass spectrometers.

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