Mass spectrometry instrumentation and ion/ion reaction method development for the fundamental analysis of gas phase biomolecules

Boone M Prentice, Purdue University

Abstract

Mass spectrometry (MS) and tandem mass spectrometry (MSn) approaches play vital roles in the molecular analysis of many different types of analytes (e.g., proteins, peptides, etc.). The typical molecular mass spectrometry experiment involves making/sampling ions, probing ions, and transmitting/measuring ions. While the work described herein covers a broad range of technical mass spectrometry instrumentation and experimentation topics on each of these three main segments, the fundamental study of physical and chemical ionic behaviors is a common theme throughout. A technique which has been previously developed to manipulate droplets generated via electrospray ionization (ESI) during the ionization process (viz. the making/sampling phase) was quantified using a mathematical model. This technique involves the controlled introduction of volatile acidic and basic reagents into the interface region of the mass spectrometer where electrospray droplets are undergoing desolvation on the sub-millisecond time scale. Quantitative pH results from this model show good correlation with experimental results. Several projects were also completed involving ion transmission phases. One such projected was the construction and implementation of a miniature ion funnel interface, based off of the design reported by Julian et al. This interface is designed to increase the transmission efficiency (~100x) of ions generated via atmospheric ionization techniques into the high vacuum region of the mass spectrometer. Once the ions are inside the mass spectrometer, it was discovered that introducing a monopolar DC field during a number steps in a typical 3-D ion trap MS experiments could result in significant enhancements in instrument performance (e.g., ion accumulation efficiency, ion/ion reaction control, mass analysis). The final phase of a mass spectrometry experiment involves measuring and detecting the ions. A novel method for performing mass analysis in a 3-D ion trap was developed and characterized which involves scanning the quadrupolar DC potential applied to the ion trap while holding the RF potential. This `downscan' can offer some advantages over the traditional RF scan for ions of high m/z values, including a larger scanable mass range as well as the opportunity for improved resolution at high mass. Perhaps more chemically interesting is the work herein devoted to the development and characterization of several different methods to structurally interrogate ions (viz. the "probing" phase). The design and development of two ion activation approaches using a 3-D ion trap mass spectrometer, dipolar direct current collision induced dissociation (DDC CID) and photodissociation (PD), have been other major areas of research. Interesting characteristics of DDC CID, relative to the conventional single-frequency resonance excitation approach commonly employed, are its non-resonant or broad-band nature and the fact that ions of all m/z values are activated simultaneously. Key to the information derived from activation techniques such as the one mentioned above and from reactions performed inside the mass spectrometer is the ion type. Our research group is actively pursuing targeted chemical derivatization techniques that can be performed via ion/ion reactions ( viz. reactions involving a cation and anion) in the gas-phase as part of MSn workflows for biopolymer characterization. Here, this targeted gas-phase chemistry is extended to analytes containing carboxylic acid groups and carboxylate groups using carbodiimide and fixed charge ammonium reagents, respectively. In the spirit of exploring a wider range of chemistries, studies have been conducted involving non-covalently-bound cluster ions as reagent ion types. Relatively stable non-covalent cluster-type ion complexes have been used as multi-functional reagents in ion/ion reactions. This type of methodology allows for multiple covalent modifications to be achieved in a single ion/ion encounter and at the `cost' of only a single analyte charge. These experiments spurred further investigation into the surprisingly gas phase reactive behavior of arginine residues cationized with sodium towards sulfo-NHS ester reagents. Additionally, another area of research aims to implement a means to cool a 3-D ion trap so as to extend the lifetimes of solvated ions, which are typically short lived under ambient MS conditions. This instrumentation opens up numerous avenues for both fundamental and applicative studies of a wide variety of different chemical reactions, allowing the study of solution-phase-limited reactions using the inherent benefits of mass spectrometric analysis. This instrumentation design currently involves the use of copper blocks connected to the end-cap electrodes that are be cooled via their attachment to a custom built, in-vacuum liquid nitrogen dewar.

Degree

Ph.D.

Advisors

McLuckey, Purdue University.

Subject Area

Chemistry|Analytical chemistry|Physical chemistry

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