Doctoral Preliminary Oral Exam – Elizabeth Groetsema

About the event

Speaker:  Elizabeth Groetsema

Group:  Professor Clowers

Title:  Utilizing the Higher Order Mathieu Space Stability Zones in Digitally Operated Quadrupole Time of Flight Mass Spectrometry

Abstract:

Quadrupole guides and mass filters, which are made of four conducting rods, are vital components of many mass spectrometry instruments. A quadrupolar potential field is created within the center of the rods by applying a potential to each rod pair resulting in the potentials being 180 degrees out of phase with each other. Typically, quads are driven with sinusoidal RF waveforms, and a mass filter is created by keeping the frequency constant and ramping the AC and DC voltages. Digital quads, however, are driven with rectangular RF waveforms that are defined by the duty cycle. The duty cycle states what percent of the AC period of the waveform is in the high state. Additionally, digital quads hold the AC voltage constant and ramp the frequency to create a mass filter, no DC voltage is required. The Mathieu space stability diagrams are graphs of the solutions of radial ion stability and can be manipulated by controlling the duty cycle of a digital quad. This increases accessibility to these higher order Mathieu space stability zones which yield higher theoretical resolving power than the primary zone. This work will explore the adaptation of a quadrupole time of flight (QTOF) mass spectrometer for digital operation and its ability to access and utilize these higher order zones. In the low mass range (<1500 m/z), the isolation windows of both sine quads and digital quads will be compared to demonstrate the improved isolation of low m/z analytes to ameliorate tandem experiments, leading to less ambiguity in the identification of molecular structures. In the high mass range (>4,000 m/z), digital operation of the quad will expand the working range from 4,000 m/z, the current limit, to 10,000 m/z. Proteins within this range will be isolated and then fragmented to probe structural information from native charge states. This simple switch to digital operation will enable refined analysis of low mass analytes and it will double the working range of the QTOF.