Imaging Mass Spectrometry with Gas-Phase Charge Inversion Ion/Ion Reaction Separates Isobaric Lipids

Isobaric compounds, molecules sharing the same nominal mass but with their atoms arranged differently, pose a significant challenge in imaging mass spectrometry experiments. These compounds, found within a diverse array of chemical compounds in tissue samples, often lead to composite images comprising multiple substances. A recent study in the Journal of the American Society for Mass Spectrometry, authored by Jonathan T. Specker and Boone M. Prentice in the Department of Chemistry at the University of Florida in Gainesville, Florida, USA, introduces a novel approach that addresses this issue using gas-phase charge inversion ion/ion reactions, providing a promising method for separating isobaric lipids (1).

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The complexity of tissue samples often results in the presence of multiple isobaric compounds, making it essential to separate and identify them accurately. High-resolution accurate mass (HRAM) measurements can sometimes resolve such compounds, but the feasibility of this method depends on the instrument platform and the desired experimental time scale. Alternatively, tandem mass spectrometry (MS/MS) offers an approach to improve molecular specificity. Traditional MS/MS methods rely on collision-induced dissociation (CID) to fragment compounds of interest and subsequently analyze the product masses. However, gas-phase ion/ion reactions offer an alternative approach to selectively react with specific classes of analytes.

In this study, researchers employed gas-phase charge inversion ion/ion reactions to selectively resolve phosphatidylcholines (PCs) in isobaric lipid mixtures. By utilizing a 1,4-phenylenedipropionic acid (PDPA) reagent dianion, the team was able to react with [M + H]⁺, [M + Na]⁺, and [M + K]⁺ ion types, generating demethylated product anions for each PC, denoted as [PC – CH₃]⁻. These product anions are no longer isobaric and exhibit mass differences of 22 Da for protonated ions and 16 Da for sodiated ions, both versus potassiated ions respectively.

The application of this innovative reaction method was exemplified through the differentiation of isobaric lipids in the imaging mass spectrometry analysis of rat brain tissue. By employing gas-phase charge inversion ion/ion reactions, the researchers were able to achieve a level of molecular specificity that was otherwise challenging to attain. The mass differences induced by the reaction provided a clear distinction between compounds that would have appeared identical in traditional mass spectrometry approaches.

The significance of this method lies in its potential to greatly enhance the accuracy of analyte identification in imaging mass spectrometry experiments. Rather than relying solely on HRAM measurements, which may not always be feasible due to instrumentation limitations, researchers can use gas-phase charge inversion ion/ion reactions to selectively differentiate compounds, even when they share the same nominal mass.

This method offers a valuable tool for researchers and scientists engaged in a wide range of applications, from pharmaceuticals to clinical diagnostics. As the methodology continues to develop, it may open new avenues for the study of complex chemical mixtures, further expanding our understanding of the molecular composition of diverse samples.

This article was written with the help of artificial intelligence and has been edited to ensure accuracy and clarity. You can read more about our policy for using AI here.

Reference

(1) Specker, J. T.; Prentice, B. M. Separation of Isobaric Lipids in Imaging Mass Spectrometry Using Gas-Phase Charge Inversion Ion/Ion Reactions. J. Am. Soc. Mass Spectrom. 2023, 34 (9), 1868–1878. DOI: 10.1021/jasms.3c00081