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Broadband Phase Correction of Fourier Transform Ion Cyclotron Resanonce Mass Spectra

Title: Broadband Phase Correction of Fourier Transform Ion Cyclotron Resanonce Mass Spectra.
Name(s): Xian, Feng, author
Marshall, Alan G., professor directing dissertation
Hendrickson, Christopher L., professor co-directing dissertation
Hill, Stephen, university representative
Dalal, Naresh S., committee member
Roper, Michael, committee member
Department of Chemistry and Biochemistry, degree granting department
Florida State University, degree granting institution
Type of Resource: text
Genre: Text
Issuance: monographic
Date Issued: 2012
Publisher: Florida State University
Florida State University
Place of Publication: Tallahassee, Florida
Physical Form: computer
online resource
Extent: 1 online resource
Language(s): English
Abstract/Description: It has been known for 35 years that phase correction of Fourier transform ion cyclotron resonance (FT-ICR) mass spectral data can in principle produce an absorption-mode spectrum with mass resolving power as much as a factor of 2 higher than conventional magnitude-mode display, an improvement otherwise requiring a (much more expensive) increase in magnetic field strength. However, temporally dispersed excitation followed by time-delayed detection results in steep quadratic variation of signal phase with frequency. We developed a robust, rapid, automated method to enable accurate broadband phase correction for all peaks in the mass spectrum. Low-pass digital filtering effectively eliminates the accompanying baseline roll. Experimental FT-ICR absorption-mode mass spectra exhibit at least 40% higher resolving power (and thus an increased number of resolved peaks) as well as higher mass accuracy relative to magnitude mode spectra, for more complete and more reliable elemental composition assignments for mixtures as complex as petroleum. Absorption-mode FT-ICR mass spectrum, which is produced by automatic broadband phase correction algorithm, demonstrates baseline distortion even with low-pass filter baseline correction. Significant baseline roll affects peaks picking algorithm and results in incorrect peak height measurement. Isotopic distribution in spectra presenting large baseline roll couldn't display correct information. Thus, identification and characterization of biomolecule become much more difficult. In Chapter 2, we designed a fast, robust and automated baseline correction process. Each minimum data point of reversed peak in absorption-mode spectrum has been collected as bases of baseline model, and then further linear interpolation and boxcar smoothing technique help to complete the baseline model. Finally, the baseline model is subtracted from original spectrum to produce a flat baseline. This algorithm has been experimentally proven to automatically flatten baseline of crude oil, environmental sample and biomolecule FT-ICR mass spectra. More peaks have been identified from absorption-mode spectrum with flat baseline without loss of mass accuracy. Isotopic distribution also demonstrates very accurate profile. Apodization function and zero-filling are two basic steps in data processing of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and their effect on the conventional Fourier transform ion cyclotron resonance (FT-ICR) experimental and simulated magnitude-mode mass spectra and single-peak absorption-mode spectra are well known. In Chapter 4, we examine the effects of each of twelve apodization (window) functions and 0, 1, and 2 zero-fills for absorption-mode Fourier transform mass spectra peak height-to-noise ratio, mass measurement accuracy, and mass resolving power for dense FT-ICR mass spectra of petroleum. Half function windowing is best for resolving close absorption-mode doublets, whereas full function windowing is best for resolving magnitude-mode doublets. Absorption-mode offers significantly higher mass resolving power than magnitude-mode for any given windowing function. Half apodization increases absorption-mode mass accuracy, irrespective of the choice of window function. One (but not more than one) zero-fill improves mass accuracy for absorption-mode mass accuracy but not for magnitude-mode. Peak height-to-noise ratio for both absorption and magnitude spectra is improved by zero-filling. Although we have successfully demonstrated the automated phase correction method for complex Fourier transform ion cyclotron resonance (FT-ICR) mass spectrum, we can't express the exact quadratic phase function of frequency from calculated phase for discrete data point. In Chapter 5, we applied stationary phase method to excitation and detection signal and derived the accurate phase spectra for both the linear chirp excitation and detected FT-ICR signals analytically. Because phase spectrum of detected signal represents correct variation of accumulated phase with frequency, it could be directly used to recover the absorption-mode FT-ICR mass spectra. Also, the phase correction of FT-ICR mass spectra from stored waveform inverse Fourier transform (SWIFT) by phase spectrum has been experimentally described. The analytically phase correction results are compared to the previous results produced by automated phase correction method in terms of resolving power and mass measurement accuracy Except for phase correction based on mathematical calculation of accurate phase for different frequencies. Scientists have demonstrated that simultaneous excitation and detection (SED) enable Fourier deconvolution to provide broadband phase correction with no user interaction. However, the capacitive nulling technique which is applied in SED method for removing the saturated excitation signal in front of detected signal is not practical due to unstable capacitors. In Chapter 6, we describe a new data processing procedure to enable broadband phase correction of FT-ICR mass spectra by SED without any hardware modification. The resulting absorption-mode spectra yield improvement in resolving power as well as reduction in assignment errors relative to conventional magnitude-mode spectra. The Fourier deconvolution procedure has the additional benefit of correcting for spectral variation resulting from nonuniform power distribution over the excitation bandwidth and phasing spectra from different excitation waveforms (e.g., SWIFT with different magnitude modulations). Fourier transform mass spectrometry (FTMS) of the isolated isotopic distribution for a highly charged biomolecule produces time-domain signal containing large amplitude signal "beats" separated by extended periods of much lower signal magnitude. Signal-to-noise ratio for data sampled between beats is low, due to destructive interference of the signals induced by members of the isotopic distribution. Selective blanking of the data between beats has been used to increase spectral signal-to-noise ratio. However, blanking also eliminates signal components, and thus can potentially distort the resulting FT spectrum. In Chapter 7, we simulate the time-domain signal from a truncated isotopic distribution for a single charge state of an antibody. Comparison of the FT spectra produced with or without blanking and with or without added noise clearly show that blanking does not improve mass accuracy and introduces spurious peaks at both ends of the isotopic distribution (thereby making it more difficult to identify posttranslational modifications and/or adducts). Ergo, blanking should never be employed: it has no advantages and major disadvantages.
Identifier: FSU_migr_etd-6074 (IID)
Submitted Note: A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Degree Awarded: Spring Semester, 2012.
Date of Defense: March 30, 2012.
Keywords: Absorption, Baseline Correction, FT-ICR, Mass Accuracy, Resolving Power, SED
Bibliography Note: Includes bibliographical references.
Advisory Committee: Alan G. Marshall, Professor Directing Dissertation; Christopher L. Hendrickson, Professor Co-Directing Dissertation; Stephen Hill, University Representative; Naresh S. Dalal, Committee Member; Michael Roper, Committee Member.
Subject(s): Chemistry
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Host Institution: FSU

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Xian, F. (2012). Broadband Phase Correction of Fourier Transform Ion Cyclotron Resanonce Mass Spectra. Retrieved from