High-Speed Tandem Mass Spectrometric in Situ Imaging by Nanospray Desorption Electrospray Ionization Mass Spectrometry

Abstract

Nanospray desorption electrospray ionization (nano-DESI) combined with tandem mass spectrometry (MS/MS), high-resolution mass analysis of the fragment ions (m/Δ=17,500 at m/z 200), and rapid spectral acquisition enabled simultaneous imaging and identification of a large number of metabolites and lipids from 92 selected m/z windows (± 1 Da) with a spatial resolution of better than 150 μm. Mouse uterine sections of implantation sites on day 6 of pregnancy were analyzed in the ambient environment without any sample pre-treatment. MS/MS imaging was performed by scanning the sample under the nano-DESI probe at 10 μm/s while acquiring higher-energy collision-induced dissociation (HCD) spectra for a targeted inclusion list of 92 m/z values at a rate of ~6.3 spectra/s. Molecular ions and their corresponding fragments, separated using high-resolution mass analysis, were assigned based on accurate mass measurement. Using this approach, we were able to identify and image both abundant and low-abundance isobaric and isomeric species within each m/z window. MS/MS analysis enabled efficient separation and identification of isomeric and isobaric phospholipids that are difficult to separate in a full-scan mode. Furthermore, we identified several metabolites associated with early pregnancy and obtained the first 2D images of these molecules.

Mass Spectrometry Imaging with Isomeric Resolution Enabled by Ozone‐Induced Dissociation

“…While such representations have their own value (for example, tissue differentiation and classification), their correlation with biochemical composition and metabolic activity is tenuous, as these signals represent the composite distributions of a range of isomers. Tandem mass spectrometry (MS/MS) can aid in certain cases3 but fails for structural isomers that yield identical product ions under conventional ion activation conditions. Methods enabling isomeric resolution are required to reach the goal of performing MSI on structurally defined chemical species.…”

Ambient Molecular Analysis of Biological Tissue Using Low-Energy, Femtosecond Laser Vaporization and Nanospray Postionization Mass Spectrometry

“…High spectral resolution and accuracy-based mass measurements play an important role in unambiguous identification of drugs, lipids, and metabolites species from tissue samples in mass spectrometry [10,16]. We note that the high mass resolving power and accuracy of our QTOF analyzer enabled the identification of PC(6:0/ 6:0).…”

“…Molecular profiling and imaging of animal tissue samples with minimal sample preparation have been demonstrated by several ambient ionization MS techniques, including DESI [9,25,61], nano-DESI [10,22], IR-MALDESI [11,62], and LAESI [12,63]. To investigate the possibility of using LEMS for the analysis of animal samples, mouse brain was chosen in our analysis.…”

“…The ion at m/z 184.1 corresponds to either free phosphocholine molecules or phosphocholine fragments of PC lipids, which supports the detection of phosphatidylcholine species from the brain by LEMS. Further analysis by MS/ MS or an ion mobility analyzer are required to confirm and elucidate the exact chemical structure of possible isobaric and/or isomeric ions because of the vast structural complexity and diversity of lipids [10,63].…”

“…The goal of understanding complex biological systems, ranging from intact body [7] to tissue sections [8][9][10][11][12], to body fluids [13,14], to single cells [15,16], has been the driving force for the advent of various ambient ionization MS techniques in the last decade. Plasma-based techniques like direct analysis in real time (DART) or low temperature plasma (LTP) are ideal for the analysis of volatile analytes [17,18], and have been used to explore the hydrocarbon profile of live flies [7] as well as the metabolite distribution of a chili fruit [19].…”