Dynamic Range Expansion by Gas-Phase Ion Fractionation and Enrichment for Imaging Mass Spectrometry.

In the analysis of biological tissue by imaging mass spectrometry (IMS), the limit of detection and dynamic range are of paramount importance in obtaining experimental results that provide insight into underlying biological processes. Many important biomolecules are present in the tissue milieu in low concentrations and in complex mixtures with other compounds of widely ranging abundances, challenging the limits of analytical technologies. In many IMS experiments, the ion signal can be dominated by a few highly abundant ion species. On trap-based instrument platforms that accumulate ions prior to mass analysis, these high abundance ions can diminish the detection and dynamic range of lower abundance ions. Herein, we describe two strategies for combating these challenges during IMS experiments on a hybrid QhFT-ICR MS. In one iteration, the mass resolving capabilities of a quadrupole mass filter are used to selectively enrich ions of interest via a technique previously termed continuous accumulation of selected ions. Second, we have introduced a supplemental dipolar AC waveform to the quadrupole mass filter of a commercial QhFT-ICR mass spectrometer to perform selected ion ejection prior to the ion accumulation region. This setup allows the selective ejection of the most abundant ion species prior to ion accumulation, thereby greatly improving the molecular depth with which IMS can probe tissue samples. The gain in sensitivity of both of these approaches roughly scales with the number of accumulated laser shots up to the charge capacity of the ion accumulation cell. The efficiencies of these two strategies are described here by performing lipid imaging mass spectrometry analyses of a rat brain.

Enabling drug discovery and development through single-cell imaging.

INTRODUCTION: Single-cell imaging-based assays are an area of active and growing investment in drug discovery and development. This approach offers researchers the capability to interrogate rare subpopulations of cells with minimal sample consumption and multiplexed readouts. Recent technological advances in the optical interrogation and manipulation of single cells have substantially increased the throughput and sensitivity of these assays. Areas covered: In this review, the authors focus on three classes of single-cell imaging-based analyses: single-cell microscopy combined with microfluidics, mass spectrometric imaging for subcellular compound localization, and imaging mass cytometry (IMC). They provide an overview of each technology and recent examples of their utility in advancing drug discovery, based on the potential for scalability, multiplexing, and capability to generate definitive data on cellular heterogeneity and target engagement. Expert opinion: Understanding target engagement and heterogeneity at the single-cell level will enable the development of safer and more effective therapies, particularly for new modalities like CAR-T cell therapies and gene editing approaches (AAV, CRISPR). Successful adoption of new single-cell imaging-based approaches in drug discovery will require tandem investment in advanced computational analysis and bioinformatic approaches, due to the complexity and multivariate nature of single-cell imaging data.

Matrix pre-coated MALDI MS targets for small molecule imaging in tissues.

A new sample preparation method for MALDI tissue imaging has been developed for the analysis of low molecular weight compounds that employs matrix pre-coated MALDI targets. Tissue sections need only to be transferred onto the pre-coated target before analysis for fast and easy sample preparation. Pre-coated targets have a homogenous matrix coating with uniform crystals of approximately 1-2 µm and do not require solvents that may lead to analyte delocalization within a tissue section. We report here the use of matrix pre-coated targets for imaging of lipids, peptides, and pharmaceuticals in tissues.