Hadamard transform time-of-flight mass spectrometry [electronic resource] : instrumentation and application to desorption electrospray ionization

This thesis describes the construction and use of a Hadamard transform time-of-flight mass spectrometer (HTTOFMS). HTTOFMS is a multiplexed TOF technique that employs a modulation/demodulation strategy to decouple both mass range and resolution from duty cycle. By multiplexing, HTTOFMS has a 100% duty cycle allowing it to efficiently couple to continuous ionization sources such as electrospray ionization. The multiplexing is accomplished by spatially modulating a continuous ion beam using a Bradbury-Nielsen Gate. The theory of HTTOFMS and the development and implementation of the HTTOFMS are the subjects of Chapters 2 and 3 respectively with Chapter 1 providing a brief introduction to mass spectrometry. A desorption electrospray ionization (DESI) source with a circular, rotating sample platform was built and interfaced with the HTTOFMS. By chemically patterning a surface and rotating it at programmed rates, the maximum sampling rates of DESI were analyzed to test its viability for high throughput screening. The platform showed DESI is capable of analyzing in excess of 100 samples/s in the absence of MS sensitivity concerns. This experiment is detailed in Chapter 4. The DESI rotating disc platform was also used to couple capillary-based electrokinetic separations to MS. Traditional electrokinetic separation buffers have high salt and for some separation modes detergent concentration. These buffers can cause substantial ion suppression and source contamination in mass spectrometers so they are generally avoided. Because DESI exhibits high salt tolerance, it allowed the bridging of electrokinetic separations and MS without buffer concerns. The coupling occurs by depositing the effluent from a capillary column onto the rotating paper-covered disc. The temporal separation of the eluting analytes (i.e., the electropherogram) is spatially encoded on the surface as the disc rotates. Then, using DESI, surface-bound analytes are preferentially ionized, reducing the effects of ion suppression and inlet contamination. This work is detailed in Chapter 5. Two other experiments are discussed in Chapter 6: a kinetics project studying polypeptide hydrolysis and an attempt to observe spatial stratification of different species in an ion guide quadrupole. The rapid spectral acquisition rates of HTTOFMS make it an ideal mass analyzer for kinetics experiments. To purse these experiments, a stopped-flow (SF) apparatus was also constructed for the HTTOFMS instrument. Using the SF-HTTOFMS, the hydrolysis of polypeptides by trypsin was studied. The SF-HTTOFMS was validated using a standard assay and an optical SF instrument. After validation, the hydrolysis of unlabeled polypeptides with either one and two cut sites was studied and fit with Michaelis-Menten curves. The imaging detector of the HTTOFMS instrument enabled the study of ion guide stratification of different mass-to-charge ratio species, a theoretically suggested phenomenon. Theory suggest that as the charge density inside an ion guide quadrupole increases that lower mass-to-charge ratio species focus more tightly, but it has not been seen experimentally. Data is presented that suggests the observation of this behavior, and future work is suggested. The thesis closes with an appendix of useful MATLAB scripts for processing the data used in various experiments.

Keywords: Chemistry
Advisor: Zare Richard N. primary advisor. Thesis advisor
Fayer Michael D. advisor. Thesis advisor
Wandless Thomas advisor. Thesis advisor