PRECISION DIGITAL WAVEFORM TECHNOLOGY IN NEXT-GENERATION QUADRUPOLE MASS SPECTROMETERS

Gregory Forrest Brabeck

Mass spectrometry is among the most widely used analytical techniques, employed to answer myriad chemical and biological questions. It provides information about the quantity, mass, and structure of the chemical components of complex mixtures and interfaces readily with many orthogonal separation techniques. Despite their utility, prevailing mass spectrometer designs face fundamental limitations on their ability to handle, resolve, and accurately determine the mass of large biomolecules with low charge and native-like structure. Digital quadrupole mass spectrometers exhibit improved resolving power at high mass and enable a suite of analytical ion handling operations. Chapter one presents an overview of the operation and experimental applications of digital quadrupole mass spectrometers. Duty cycle modulation controls axial ion motion in a linear quadrupole for storage, gas phase chemistry and ejection with precise kinetic energy. Duty cycle modulation also controls radial ion stability to enable high-accuracy mass filters and novel methods of efficient tandem MS. Chapter two discusses duty cycle modulation’s effects on ion stability in 2D and 3D ion traps. Matrix-based calculations analytically solve the Hill differential equation to generate unique stability diagrams for rectangular driving waveforms. These exact solutions are compared to the most common approximations and are found to present experimentally relevant differences. Chapter three extends matrix methods to arbitrary periodic driving potentials to calculate the pseudopotential well depth anywhere in the stability diagram. This proxy for ion trapping and transmission efficiency permits prediction of the performance of novel quadrupole mass filters. The differing performance characteristics of digital and sinusoidal mass filters are discussed. Chapter four experimentally demonstrates several novel methods for achieving collision-induced dissociation (CID) is a single digital linear ion guide. The duty cycle and frequency of digital waveforms may be switched on demand, permitting ions to be toggled between stable and unstable trajectories. Two methods of excitation resulting unique fragmentation patterns are achieved by varying ions’ proximity to the high mass stability boundary. The final chapter returns to simulation studies to systematically examine the effects of frequency- and amplitude-asymmetric driving waveforms on ion stability. Asymmetric waveforms provide supplemental quadrupolar excitation and create unstable bands within that stability diagram.

PRECISION DIGITAL WAVEFORM TECHNOLOGY IN NEXT-GENERATION QUADRUPOLE MASS SPECTROMETERS

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