Development of The Fourier Transform Electrostatic Linear Ion Trap for The Analysis of Gas Phase Ions
The electrostatic linear ion trap (ELIT) is a relatively new type of mass analyzer wherein ions are axially confined between two opposing reflectrons. The image charge induced on a centrally located pickup electrode can be digitized, Fourier transformed, and calibrated to generate a mass spectrum. The ELIT itself is relatively compact, requires no RF-supply or magnets, consumes little power, and is compatible with both pulsed and continuous ionization sources. Additionally, its linear geometry allows for in-situ tandem mass spectrometry (MSn), and two different modes of mass analysis: Fourier transformation (FT) and multiple-reflection time-of-flight (MR-TOF). In this dissertation, limitations to the Fourier transform mode of mass analysis are explored and mitigated such that the home-built device is shown to be capable of high-performance mass analysis. The current procedure by which ions are accumulated, bunched, and injected into the ELIT was computationally modelled by ion-optical simulations in SIMION. It was demonstrated that the process concentrates the ion cloud within the injection quadrupole and minimizes the time required for the bunched packet to exit once gated out. These effects were shown to increase the charge density of the packet while maintaining the injected kinetic energy distribution, thereby increasing the signal-to-noise of the image charge measurement. The quantitative and qualitative results of the simulations were found to be in good agreement with experimental results on the ELIT. Using lower analysis pressures and longer transient lengths, the ion frequencies were found to drift throughout data acquisition when captured in the ELIT via mirror-switching. This phenomenon was determined to be a result of the transient voltage recovery of the power supplies used for mirror-switching in response to a pulsed capacitive load. Two methods were explored to correct the frequency drift and thereby generate high resolution mass spectra. The first method used a central electrode to capture ions via in-trap potential lift and dual image charge detection to compensate for losses in mass resolution associated with increasing the length of the electrostatic trap. Although successful in their objectives, these changes resulted in a narrower m/z range for a single ion injection event relative to mirror switching. Furthermore, the dual detector approach resulted in a higher noise floor and a more complicated frequency spectrum due to asymmetries in the electric fields of the ion trap. The second approach utilized a circuit, based on the AD210AN isolation amplifier, to compensate for the voltage perturbation by superimposing the measured and inverted perturbation onto the electric field of the opposing reflectron. In doing so, the turning point of the ion packet in both reflectrons moved concurrently and therefore variations in the path length were minimized. With this circuit enabled, frequency shifts due to mirror-switching were minimized, and thus pressure-limited theoretical resolutions were achieved. The use of mirror-switching led to a much greater m/z range than in-trap potential lift for a single ion injection which was demonstrated via simulation and experimental results. In a pressure-limited circumstance, where the transient decay of the signal is dominated by collisions with the background gas, there are three methods by which to increase the mass resolution: increase the signal-to-noise (S/N) of the detector, lower the background pressure, or analyze the signal harmonics. The S/N of the A250 image charge detector was increased by a factor of 1.9 by replacing the former input junction field effect transistor (JFET), NTE452, with a better JFET, the BF862. A differentially pumped region was installed between the injection quadrupole and the ELIT which effectively decoupled the source and cooling gas from the ELIT chamber pressure. Ultimately, both modifications increased the amount of time for which the signal remained detectable and therefore a longer transient could be recorded. Alternatively, for a given transient length, the mass resolution was observed to linearly increase with the harmonic order. Signal harmonics on the ELIT are shown to be a direct result of the non-cosine signal generated by the charge sensitive preamplifier in response to a pulsed image charge. The harmonics were used to achieve a high nominal mass resolution, determine a protein ion’s charge state, and resolve four isobaric drug molecules. Adding to the list of experiments that can be performed in an FT-ELIT, ion-neutral collision cross sections (CCS) were derived from the pressure-broadened, frequency-domain linewidths. The energetic hard-sphere ion-neutral collision model, described by Guo et. al., was used to relate the recorded image charge to the CCS of the molecule. In lieu of mono-isotopically isolating the mass of interest, the known relative isotopic abundances were programmed into a Lorentizan fitting algorithm such that the linewidth was extracted from a sum of Lorentzians. Using a series of tetraalkylammonium cations, an excellent correlation was observed between the CCSs derived on the ELIT and those derived on a dedicated drift tube ion mobility spectrometer. With sufficient signal-to-noise, the determined cross sections were reproducible to within a fraction of a percent and were comparable to those reported on dedicated ion mobility spectrometry instruments.
McLuckey, Purdue University.
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