Constant-Distance Mode Nanospray Desorption Electrospray Ionization Mass Spectrometry Imaging of Biological Samples with Complex Topography

Abstract

A new approach for constant-distance mode mass spectrometry imaging (MSI) of biological samples using nanospray desorption electrospray ionization (nano-DESI) was developed by integrating a shear-force probe with the nano-DESI probe. The technical concept and basic instrumental setup, as well as the general operation of the system are described. Mechanical dampening of resonant oscillations due to the presence of shear forces between the probe and the sample surface enabled the constant-distance imaging mode via a computer-controlled closed-feedback loop. The capability of simultaneous chemical and topographic imaging of complex biological samples is demonstrated using living Bacillus subtilis ATCC 49760 colonies on agar plates. The constant-distance mode nano-DESI MSI enabled imaging of many metabolites, including nonribosomal peptides (surfactin, plipastatin, and iturin) on the surface of living bacterial colonies, ranging in diameter from 10 to 13 mm, with height variations up to 0.8 mm above the agar plate. Co-registration of ion images to topographic images provided higher-contrast images. Based on this effort, constant-mode nano-DESI MSI proved to be ideally suited for imaging biological samples of complex topography in their native states.

High Spatial Resolution Imaging of Mouse Pancreatic Islets Using Nanospray Desorption Electrospray Ionization Mass Spectrometry

“…20–22 We recently introduced an approach for robust nano-DESI MSI with high spatial resolution by adding a shear force probe, which enables precise control of the distance between the sample and nano-DESI probe. 19,23 Herein, we used this approach for imaging of pancreatic islets with high spatial resolution.…”

“…In this study, we employ the recently developed constant-distance nano-DESI MSI approach 19,23 to obtain first insights into the localization of lipids and metabolites in both the pancreatic islets and surrounding exocrine tissue. This approach enables molecular imaging of individual pancreatic islets with a lateral resolution of ~11 μm.…”

Towards High-Resolution Tissue Imaging Using Nanospray Desorption Electrospray Ionization Mass Spectrometry Coupled to Shear Force Microscopy

“…[26–28] One notable exemption is DESI that can tolerate surface irregularities of the order of several microns. Several approaches have been developed to facilitate constant distance ambient imaging using liquid extraction techniques.…”

“…[26] Shear force microscopy enables imaging of sample topography by measuring a shear force between an oscillating finely-pulled nanopipette and a sample surface. [29] The distance between the sample and the shear force probe is continuously adjusted throughout the experiment to maintain the oscillation amplitude of a resonant mode most sensitive to the sample surface at a constant value.…”

“…[30] By incorporating the shear force nanopipette into the nano-DESI probe, we were able to conduct constant-distance nano-DESI MSI of living microbial communities directly from agar plates. [26] In these experiments, the shear force probe allowed us to follow variations in sample height of up to 0.8 mm while acquiring mass spectral data with a moderate spatial resolution of ~200 μm. Herein, we demonstrate that this approach enables imaging of thin tissue sections with high spatial resolution.…”