Novel Pyroelectric and Switched Ferroelectric Ion Sources in Mass Spectrometry: Implementation and Applications

Citation

Neidholdt, Evan Lyle (2010) Novel Pyroelectric and Switched Ferroelectric Ion Sources in Mass Spectrometry: Implementation and Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/M86B-8T98. https://resolver.caltech.edu/CaltechTHESIS:03292010-222225209

Abstract

Instrumentation development for ambient mass spectrometry is an ever-changing and ever-growing field, often emphasizing the development of field-portable, ruggedized instrumentation. This thesis presents work in the area of ion source development. Specifically, two novel ionizers for mass spectrometry are constructed, implemented, and tested on a variety of systems. The first, the ambient pressure pyroelectric ion source (APPIS) comprises a z-cut lithium niobate or lithium tantalate pyroelectric crystal which is subjected to thermal cycling. Thermal cycling and the pyroelectric effect result in the buildup of excess charge on the z-faces of the material. For a temperature change of only 30 K, an electrical potential as high as 70,000 V could be built up if no discharging occurs. At ambient pressure, electrical discharges occur between the oppositely charged crystal faces and regions of different potentials on the same crystal face. Monitoring of the discharges using an inductive pickup reveals that the time frame of ion production corresponds to that of electrical activity on the crystal face. Additionally, the ions observed in the mass spectra differ when different ambient gas compositions are present. This, and comparison to APCI experiments reveals that ionization with APPIS is a gas phase process and observed product ion species are highly dependent on ambient gas composition. An application of APPIS, whereby APPIS is used as an ion source for the detection of chemical warfare agents, is presented. Agents in the V and G classes possess amine functionality, and as a result have relatively high proton affinities and are well suited to ionization and detection using chemical ionization with APPIS. Simulants for the nerve agents VX and Tabun were detected as singly protonated species using APPIS.

A second ion source for ambient mass spectrometry is also presented, the switched ferroelectric plasma ionizer (SwiFerr). The source comprises a 1 mm thick plate of barium titanate with an electrode on one face and a grounded metallic grid on the opposite face. If an audio frequency high voltage waveform is applied to the electrode, domain structure is formed and the high electric fields across domain walls give rise to electron emission and plasma formation. At ambient pressure, the plasma produces chemical ionization reagent ions which participate in proton transfer reactions with trace species in ambient air. The source is useful for examining organic vapors and solid samples. Solid sampling is achieved through use of a pneumatic aspirator, which can aspirate powders into the SwiFerr source for analysis. Powders of the drugs loperamide and ibuprofen were ionized and detected using SwiFerr. A second-generation SwiFerr source has been designed and implemented, and is described. The second-generation source is constructed in a unibody fashion, such that the rear electrode, grid, and electrical contact wires are attached as a single unit using conductive adhesives. This allows for a source which is further miniaturized and capable of insertion into a standard Swagelok fitting, for ease of integration into existing instrumentation. The source is particularly sensitive, and an application involving detection of trace explosives is presented. Nanogram quantity samples of TNT were ionized and detected with SwiFerr after volatilization using a rudimentary thermal desorption apparatus. Sensitive detection of TNT suggests use of SwiFerr in applications where sensitive detection in field portable instrumentation is desired. To this end, plans for a stand-alone power supply for this ionizer as well as a supply designed for operating the source in a constant discharge mode at low power are presented in an appendix.

Additional work not related to ion source development is also presented in this thesis. Chapter 6 presents advances in free radical initiated peptide sequencing (FRIPS). The 20 amino acid basis set has been analyzed for reactivity with acetyl radical, and distinct reactivity classes are observed. Chapter 7 presents a study of regioselective cleavage at aspartic acid residues by various cations. Selective cleavage at aspartic acid is observed for any cation which is not mobile along the peptide backbone, and fragmentation proceeds through a salt bridge mechanism.

Thesis Files