Combining MALDI-2 and transmission geometry laser optics to achieve high sensitivity for ultra-high spatial resolution surface analysis

A transmission geometry optical configuration allows for smaller laser spot size to facilitate high-resolution matrix-assisted laser/desorption ionization (MALDI) mass spectrometry. This increase in spatial resolution (ie, smaller laser spot size) is often associated with a decrease in analyte signal. MALDI-2 is a post-ionization technique, which irradiates ions and neutrals generated in the initial MALDI plume with a second orthogonal laser pulse, and has been shown to improve sensitivity. Herein, we have modified a commercial Orbitrap mass spectrometer to incorporate a transmission geometry MALDI source with MALDI-2 capabilities to improve sensitivity at higher spatial resolutions.

A compact high-resolution X-ray powder diffractometer

A new powder diffractometer operating in transmission mode is described. It can work as a rapid very compact instrument or as a high-resolution instrument, and the sample preparation is simplified.

A new powder diffractometer operating in transmission mode is described. It can work as a rapid very compact instrument or as a high-resolution instrument, and the sample preparation is simplified. The incident beam optics create pure Cu Kα₁ radiation, giving rise to peak widths of ∼0.1° in 2θ in compact form with a sample-to-detector minimum radius of 55 mm, reducing to peak widths of <0.05° in high-resolution mode by increasing the detector radius to 240 mm. The resolution of the diffractometer is shown to be governed by a complex mixture of angular divergence, sample size, diffraction effects and the dimensions of the detector pixels. The data can be collected instantaneously, which combined with trivial sample preparation and no sample alignment, makes it a suitable method for very rapid phase identification. As the detector is moved further from the sample, the angular step from the pixel dimension is reduced and the resolution improves significantly for very detailed studies, including structure determination and analysis of the microstructure. The advantage of this geometry is that the resolution of the diffractometer can be calculated precisely and the instrumental artefacts can be analysed easily without a sample present. The performance is demonstrated with LaB₆ and paracetamol, and a critical appraisal of the uncertainties in the measurements is presented. The instantaneous data collection offers possibilities in dynamic experiments.

Targeted multiplex imaging mass spectrometry in transmission geometry for subcellular spatial resolution

Targeted multiplex imaging mass spectrometry utilizes several different antigen-specific primary antibodies, each directly labeled with a unique photocleavable mass tag, to detect multiple antigens in a single tissue section. Each photocleavable mass tag bound to an antibody has a unique molecular weight and can be readily ionized by laser desorption ionization mass spectrometry. This article describes a mass spectrometry method that allows imaging of targeted single cells within tissue using transmission geometry laser desorption ionization mass spectrometry. Transmission geometry focuses the laser beam on the back side of the tissue placed on a glass slide, providing a 2 μm diameter laser spot irradiating the biological specimen. This matrix-free method enables simultaneous localization at the sub-cellular level of multiple antigens using specific tagged antibodies. We have used this technology to visualize the co-expression of synaptophysin and two major hormones peptides, insulin and somatostatin, in duplex assays in beta and delta cells contained in a human pancreatic islet.

Direct imaging of single cells and tissue at sub-cellular spatial resolution using transmission geometry MALDI MS

Discussions about MALDI imaging frequently turn to the topic of spatial resolution and the eff orts of some researchers in the field to push towards routine imaging of tissue sections at a cellular scale. Some factors that limit resolution are, the size of the focused desorption laser beam and analyte delocalization from the solution-based sample preparation. With solvent-free matrix application techniques analyte delocalization is less of a concern and the size of the focused laser is the major limiter of spatial resolution. In the Special Feature, Professor Caprioli and co-workers at Vanderbilt University demonstrate a new instrumental approach for improving spatial resolution. They have modifi ed a MALDI-TOF system to use transmission-mode geometry, in which the desorption laser is focused onto the matrix crystals from behind and through the target and sample rather than conventional front-side illumination where the laser is focused onto the crystals directly. They show that by moving the laser source behind the sample target, they can optimize the laser focus to achieve cellular resolution for MALDI imaging.