"From Fundamentals to Applications: Converting Novel Ionization Processes into Sensitive Ionization Technologies for Use in Mass Spectrometry"
Professor Sarah Trimpin, Wayne State University
Matrix-assisted ionization (MAI) mass spectrometry (MS) spontaneously transfers compounds, at least as large as the 66 kDa bovine serum albumin protein, into gas-phase ions when the matrix:analyte sample is simply exposed to sub-atmospheric pressure conditions of a commercially available mass spectrometer. This new ionization process can be assessed from atmospheric pressure at the inlet of a mass spectrometer or by introduction of the sample directly into vacuum using, for example, an intermediate pressure MALDI source. Gas-phase ions are observed multiply protonated even using aprotic matrices such as 3-nitrobenzonitrile and 1,2-dicyanobenzene. Over 40 additional MAI matrices have been discovered and all have in common that charge separation occurs when ejected into a tube connecting a higher to a lower pressure region. While there is no specific structural motif, functionalities leading to less volatility and charge competition, e.g., carboxylic acids, typical for MALDI matrices, are not present. Instead, all MAI matrices have in common that they sublime in vacuum, and thus are no more likely to contaminate the mass spectrometer ion optics than solvents used with ESI or APCI. The method has excellent sensitivity having a limit of detection of <50 attomoles, depending on the mass spectrometer used, and producing clean full-scan mass spectra consuming only a few femtomoles of, for example, drugs and peptides. Complex mixtures can be acquired in seconds directly from buffered solutions, biological fluids and tissue. By placing matrix only on the feature of interest on a surface and exposing the surface to the vacuum of the mass spectrometer, ions are observed only from compounds exposed to the matrix solution allowing rapid interrogation of ‘features of interest’. We more recently engaged in designing and building dedicated sources that best suit the ionization processes enabling highest analyte ion abundance with little chemical background. In this presentation, our current thoughts relative to the mechanism of ionization along with unique applications utilizing MAI-MS with small portable mass spectrometers, automation, (ultra)high resolution MS, ion mobility spectrometry, and/or advanced fragmentation technology such as electron transfer dissociation will be presented enabling novel ways to achieve e.g., proteomics and lipidomics answers easier and more promptly with special emphasize directly from biological surfaces.
"Ambient Ionization of Aerosol Particles"
Koty Swanson, Laboratory of Professor Gary Glish, University of North Carolina Chapel Hill, Department of Chemistry
Analysis of aerosol particles primarily involves filter collection followed by extraction and analysis by GC/MS or LC/MS. This off-line method can take hours for a given aerosol sample and introduces additional complexities such as chemical oxidation/degradation and extraction bias. Online methods for aerosol analysis have been developed including Extractive Electrospray Ionization (EESI) which allows for the modification of the solvent composition for optimized extraction of compounds from aerosols. One such modification is the addition of metals to the electrospray solvent, which enables metal cationization of compounds from aerosols. An increase in sensitivity is observed for some molecules that are lithium, sodium, or silver cationized compared to the protonated molecule formed in EESI with an acid additive. Tandem mass spectrometry of metal cationized molecules can also significantly improve the ability to identify a compound. One disadvantage to EESI is that the source is very sensitive to spatial positioning of components, often causing large variation in signal from run-to-run and day-to-day. Two novel sources designs have been developed to overcome the variations in signal for EESI. Coaxial EESI is designed such that three concentric capillaries carry solvent, aerosol and nebulizing gas to the electrospray tip prior to interaction. This design has been shown to outperform standard EESI in reproducibility and signal-to-noise despite a reduction in sensitivity for oleic acid. A second device, Liquid Overflow Capture Electrospray Ionization, has also been developed that utilized a solvent interface for aerosol introduction followed by aspiration via the Venturi effect. This allows particles to be dissolved and electrosprayed on a timescale on the order of seconds. This device also increases reproducibility and signal-to-noise and does not suffer the same loss in sensitivity as coaxial EESI. Both devices have been used to sample from complex aerosols such as the aerosol generated from the pyrolysis of cellulose.