High-Spatial-Resolution Multimodal Imaging by Tapping-Mode Scanning Probe Electrospray Ionization with Feedback Control

Direct extraction and ionization techniques using minute amounts of solvent can be employed for the rapid analysis of chemical components in a sample without any sample preparation steps. This type of approach is important for mass spectrometry imaging of samples with multiple chemical components that have different spatial distributions (i.e., biological tissues). To improve the spatial resolution of such imaging, it is necessary to reduce the solvent volume for extraction and deliver it to the sample surface. This report describes a feedback control system applied to tapping-mode scanning probe electrospray ionization. By combining the measurement technique of capillary probe vibration with the dynamic distance control system between the probe and the sample, the vibration amplitude of the probe is maintained while the probe scans over uneven samples. This method allows simultaneous high-resolution imaging of molecular distribution, surface topography, and amplitude/phase changes in the probe vibration. Such multimodal imaging is demonstrated on rhodamine B thin films in microwells and on a mouse brain tissue section. This technique can generally be applied to examine the multidimensional molecular distribution and the surface profiles of various objects.

Recent advances of ambient ionization mass spectrometry imaging in clinical research

The structural information and spatial distribution of molecules in biological tissues are closely related to the potential molecular mechanisms of disease origin, transfer, and classification. Ambient ionization mass spectrometry imaging is an effective tool that provides molecular images while describing in situ information of biomolecules in complex samples, in which ionization occurs at atmospheric pressure with the samples being analyzed in the native state. Ambient ionization mass spectrometry imaging can directly analyze tissue samples at a fairly high resolution to obtain molecules in situ information on the tissue surface to identify pathological features associated with a disease, resulting in the wide applications in pharmacy, food science, botanical research, and especially clinical research. Herein, novel ambient ionization techniques, such as techniques based on spray and solid-liquid extraction, techniques based on plasma desorption, techniques based on laser desorption ablation, and techniques based on acoustic desorption were introduced, and the data processing of ambient ionization mass spectrometry imaging was briefly reviewed. Besides, we also highlight recent applications of this imaging technology in clinical researches and discuss the challenges in this imaging technology and the perspectives on the future of the clinical research.

Oversampling To Improve Spatial Resolution for Liquid Extraction Mass Spectrometry Imaging

Liquid extraction mass spectrometry imaging (MSI) experiments provide users with direct analysis of biological surfaces with minimal sample preparation. Until now, much of the effort to increase spatial resolution for MSI with liquid extraction techniques has focused on reducing the size of the sampling area. However, this can be experimentally challenging. Here, we present oversampling as a simple alternative to increase the spatial resolution using nanospray desorption electrospray ionization (nano-DESI) MSI. By imaging partial rat spinal cord tissue sections, two major concerns with oversampling are addressed: whether endogenous molecules are significantly depleted from repeated sampling events and whether analytes are redistributed as a result of oversampling. In depth examination of ion images for representative analytes show that depletion and redistribution do not affect analyte localization in the tissue sample. Nano-DESI MSI experiments using three times oversampling provided higher spatial resolution, allowing the observation of features not visible with undersampling. Although proper care must be taken to ensure that oversampling will work in specific applications, we envision oversampling as a simple approach to increase image quality for liquid extraction MSI techniques.

Ion Source with Laser Triangulation for Ambient Mass Spectrometry of Nonplanar Samples

The analysis of nonplanar samples in ambient mass spectrometry poses a formidable challenge. Here, an ion source equipped with laser triangulation for analyzing nonplanar surfaces was constructed. It was designed as a two-position device, where the sample height was measured using laser triangulation and the target compounds were then analyzed. Thanks to a stage movable in xyz, the ion source maintained an optimal vertical distance between the sample and the sampling capillary for each measured spot during the surface analysis. The xyz-coordinates for the movement of the sample stage were computed using the laser sensor data in such a way as to avoid direct contact of the sampling capillary and the measured surface. The ion source performance and its ability to analyze various morphologies were tested using desorption electrospray ionization with plastic objects coated by 2,5-dimethoxybenzoic acid. The experiments showed excellent performance for nonplanar samples but also revealed some limitations especially on object edges and steep slopes. The applicability of the ion source operated in desorption electrospray ionization and desorption atmospheric pressure photoionization was examined for food and pharmaceutical samples. Chemicals on the surface of nonplanar samples were probed along a line extending across the surface of the measured objects. The device provided high-quality spectra, regardless of the sample height at the measured spot. The automatic adjustments of the sample stage in xyz proved to be beneficial for analyzing nonplanar samples and for simultaneous measurement of samples with various dimensions and shapes.

Structural Analysis of Natural Products

Current mass spectrometry, nuclear magnetic resonance spectroscopy, and X-ray diffraction are presented as structure elucidation tools for analytical chemistry of natural products. Discovering new molecular entities combined with dereplication of known organic compounds represent prerequisites for biological assays and for respective applications as pharmaceuticals or molecular markers. Liquid chromatography is briefly addressed with respect to its use in mass spectrometry- and nuclear magnetic resonance-based metabolomics studies.