Solvent-Free Analysis Facilitates Ion Mobility Spectrometry-Mass Spectrometry Research

Solvent-free analysis can make it possible to perform chemical analysis of molecules that are difficult or impossible to solubilize and avoid chemical reactions that change the structure of certain molecules in solution. Spectroscopy recently spoke with Sarah Trimpin, an assistant professor in the Department of Chemistry at Wayne State University, about her work with solvent-free analysis and imaging mass spectrometry (MS).

Spectroscopy: What are the latest developments in your research in solvent-free analysis? Have you had any breakthroughs?

Trimpin: My work in developing a solvent-free MS analysis method that could be applied to nonvolatile compounds began with developing a solvent-free matrix-assisted laser desorption–ionization (MALDI) method. Even though the early work was with polymers, my goal was to analyze complex insoluble biological materials such as membrane proteins. The complexity of the samples required separation in addition to that achieved by the mass analyzer, which led me to study ion mobility spectrometry–mass spectrometry (IMS-MS) in Professor David Clemmer’s laboratory (Indiana University, Bloomington). IMS-MS provides a solvent-free, nearly instantaneous two-dimensional separation by size and shape (IMS dimension) and mass-to-charge (MS) dimension. Our latest breakthroughs relate to new ionization methods in MS, some of which are applicable with the solvent-free preparation methods developed for MALDI.

Spectroscopy: What are the biggest challenges you are facing in this work?

Trimpin: Analyzing complex biological systems presents many challenges, including large numbers of compounds, a wide range of concentrations (over six orders of magnitude), solubility-restricted materials, and the need to analyze compounds in a spatial and temporal context. Structural determination of unknown compounds provides additional challenges, often requiring advanced instrumental capabilities such as high mass resolution and mass measurement accuracy and highly sensitive ionization methods. Advances in MS capabilities such as electron transfer and capture dissociation, IMS-MS, and high resolution are making important inroads into providing the instrumental tools that we need, but advances in ionization will be crucial for success.

Spectroscopy: What effect do you expect solvent-free analysis to have on the mass spectrometry and “omics” communities?

Trimpin: The most near-term utility of solvent-free analysis is in imaging where the use of solvents can be detrimental in preserving spatial resolution. The difficulty here is not that we expect to analyze insoluble compounds, but that the soluble compounds need to be associated with the matrix to be ionized. We have had success by applying the dry matrix to tissue before it dries out. A longer-term effect of solvent-free methodology may be its impact on analyzing solubility-restricted compounds.

Spectroscopy: Your work toward achieving solvent-free analysis also applies to imaging MS, particularly for brain signaling. What are your goals in this area?

Trimpin: Solvent-free analysis is not a goal of our research, but a tool that we are developing in parallel with other tools to move us closer to the goal of analyzing complex systems with both high spatial and temporal resolution regardless of solubility. For tissue imaging, we are currently not limited by the use of solvents with regard to spatial resolution, but by issues such as ion abundance, dynamic range, ion suppression, and the ability to identify compounds directly from tissue. We have developed laserspray ionization that operates from atmospheric pressure or vacuum to produce multiply charged ions, which allows the use of high performance mass spectrometers with advanced fragmentation capabilities for direct identification and the use of laser ablation in transmission geometry for high spatial resolution and faster analysis by use of a single laser shot for each image pixel. We set high goals for ourselves and imaging signaling lipids is one of the goals we are striving to achieve.

Spectroscopy: Can you give us a preview of the talk you are giving at this year’s EAS conference, “New Ionization Approaches for Mass Spectrometry: From Laserspray Ionization to Matrix Assisted Vacuum Ionization”?

Trimpin: About three years ago, in an atmospheric-pressure MALDI imaging experiment using laser ablation in transmission geometry and using close proximity of the matrix to the ion entrance aperture of the mass spectrometer rather than voltage to transmit ions, we discovered that multiply charged ions were produced from peptides and small proteins very similar to electrospray ionization (ESI). MALDI produces mostly singly charged ions from these compounds. The observed multiply charged ions were not being produced by photoionization from the laser, but within the heated inlet of the mass spectrometer. Simply passing a matrix and analyte mixture, prepared identically to MALDI sample preparation, including solvent-free sample preparation, through a heated inlet tube to the mass analyzer produced abundant analyte ions. We proposed that the process produced charged matrix and analyte droplets or particles and that evaporation of the matrix produced the multiply charged ions similar to ESI. We were able to extend the process of producing multiply charged ions to vacuum MALDI ion sources by using matrix compounds that are sufficiently volatile so that if charged droplets are produced by laser ablation, they could evaporate the matrix under vacuum conditions and release multiply charged ions. To determine what factors are important for a good matrix compound to produce multiply charged ions from atmospheric pressure or vacuum, we learned that a wide range of compounds produced these ions if sufficient heat was applied for matrix evaporation. Astonishingly, we found a compound that produces abundant ions when placed in the vacuum of a mass spectrometer requiring no laser, voltage, or heat. This extremely simple ionization method is applicable to small molecules, peptides, and proteins. We believe that this method will have a major impact in clinical laboratories as well as with field-portable instruments.