[Recent progress in mass spectrometry imaging using nanospray desorption electrospray ionization]

Ambient mass spectrometry imaging (MSI) enables hundreds of analytes in tissue sections to be directly mapped at atmospheric pressure with minimal sample preparation. This field is currently experiencing rapid growth, with numerous reported ambient ionization techniques resulting in a "hundred flowers bloom" situation. Nanospray desorption electrospray ionization (nano-DESI), developed by the Laskin group in 2010, is a widely used liquid-extraction-based ambient ionization technique that was first used for mass spectrometry imaging of tissue in 2012. The nano-DESI probe comprises a primary capillary and a nanospray capillary, with the latter efficiently transferring analyte-containing droplets via a tiny liquid bridge formed between the probe and sample surface, thereby enabling nanoelectrospray ionization (nano-ESI) in front of the inlet of a mass spectrometer. The advantages of nano-DESI MSI include minimal sample preparation, high spatial resolution, and high sensitivity. These features are well-suited for imaging various sample types, including frozen tissue sections, microbial communities, and environmental samples. A PubMed-database search using the "nano-DESI" keyword revealed 72 related articles in the 2010-2024 period, with 34 of them published between 2021 and 2024, which indicates that nano-DESI has rapidly developed as an ambient ionization technique over recent years. Herein, we briefly introduce key nano-DESI-MSI research progress reported in the past three years with the aim of better understanding and facilitating the use of this technology. We first discuss advances in ion-source development. Since no commercial nano-DESI source exists, designing and constructing ion sources remain technical challenges that limit its development. Nano-DESI has been successfully coupled with various types of mass spectrometer, including LTQ Orbitrap, quadrupole-Orbitrap (Q Exactive), 6560 IM QTOF, timsTOF Pro2, triple quadrupole, and FTICR. These couplings have significantly expanded the applications range of the nano-DESI technique. Secondly, lipid analysis is a major nano-DESI-MSI applications area. While the complexities of lipid structures present great challenges for nano-DESI MSI, new nano-DESI coupling techniques have enabled the identification and imaging of fine lipid structures. Several novel imaging methods have recently been introduced to address difficulties associated with identifying lipid structures, such as distinguishing carbon-carbon double bonds (C=C) and sn-positional isomers. We finally highlight recent research progress in the nano-DESI MSI of intact protein assembles and proteoforms, which is a growing hotspot in the field. Unlike small lipid molecules, large protein molecules are very challenging to image and consequently demand higher instrumental performance (e.g., ionization efficiency, mass range, and sensitivity). In a similar manner to the ESI technique, nano-DESI tends to generate multiply charged molecular ions, which endows it with a significant advantage when imaging large protein molecules. Recent years have witnessed important nano-DESI-MSI progress for studying protein-ligand interactions and identifying and imaging endogenous proteoforms. In summary, this article focuses on nano-DESI research progress in terms of ion-source development, lipid-structure analysis, and spatial proteomics over the past three years and discusses key challenges that need to be addressed in the field.