Apodization Specific Fitting for Improved Resolution, Charge Measurement, and Data Analysis Speed in Charge Detection Mass Spectrometry

Abstract

Short-time Fourier transforms with short segment lengths are typically used to analyze single ion charge detection mass spectrometry (CDMS) data either to overcome effects of frequency shifts that may occur during the trapping period or to more precisely determine the time at which an ion changes mass or charge, or enters an unstable orbit. The short segment lengths can lead to scalloping loss unless a large number of zerofills are used, making computational time a significant factor in real-time analysis of data. Apodization specific fitting leads to a 9-fold reduction in computation time compared to zero-filling to a similar extent of accuracy. This makes possible real-time data analysis using a standard desktop computer. Rectangular apodization leads to higher resolution than the more commonly used Gaussian or Hann apodization and makes it possible to separate ions with similar frequencies, a significant advantage for experiments in which the masses of many individual ions are measured simultaneously. Equally important is a >20% increase in S/N obtained with rectangular apodization compared to Gaussian or Hann, which directly translates to a corresponding improvement in accuracy of both charge measurements and ion energy measurements that rely on the amplitudes of the fundamental and harmonic frequencies. Combined with computing the fast Fourier transform in a lower-level language, this fitting procedure eliminates computational barriers and should enable real-time processing of CDMS data on a laptop computer.

Accurate Sizing of Nanoparticles Using a High-Throughput Charge Detection Mass Spectrometer without Energy Selection

“…Peaks corresponding to individual ion signals within the STFT are traced and fit with their characteristic functions to avoid error due to scalloping and sampling artifacts in the FT and ensure amplitudes (and therefore charge) and frequency are measured with high accuracy. This method, which also uses custom-built Python software, has been described in detail elsewhere . The individually traced ion frequencies and amplitudes are then converted to measurements of m / z , charge, and ion energy. ,,,, Calibration values used to convert frequency to m / z and convert harmonic amplitude ratios (HARs) to ion energies were determined from SIMION simulations of the instrument trap geometry .…”

“…It is also possible the larger, much more massive ∼100 nm particles are not as efficiently transmitted and confined in the early stages of the CDMS instrument due to their large masses and aerodynamic acceleration effects that have been explored in other work. 18 However, regardless of the number of particles, recent advances in CDMS data analysis methods 31 have made it possible to measure and plot the masses of individual ions in real time using a desktop computer (the same computer used for the automated TEM image analysis method) such that no significant time cost outside of the experiment duration was incurred for data processing.…”

“…Recent developments in CDMS methodology, including methods to simultaneously analyze multiple ions, determine individual ion energies during the measurement process in situ , and determine signal amplitudes with greater fidelity, have prompted the design of CDMS instrumentation aimed at maximizing acquisition speed while retaining high-accuracy mass measurement capabilities. For state-of-the-art CDMS instruments that incorporate electrostatic ion traps to make accurate charge and m / z measurements, ion energies must be known. , Previous CDMS instruments have incorporated energy filtering ion optics to define a narrow band of ion energies to be analyzed, with the centerline of the energy passband used as the assumed energy for calculating the mass of each ion from its oscillation frequency. , However, the use of an energy filter decreases sensitivity because fewer ions are allowed to reach the detection stage.…”

“…22,25−28 CDMS has also been used to analyze polymer-based nanoparticles, but these studies had significant instrumentation-based limitations in either mass accuracy or the speed of data acquisition. 18,23 Recent developments in CDMS methodology, including methods to simultaneously analyze multiple ions, 29 determine individual ion energies during the measurement process in situ, 30 and determine signal amplitudes with greater fidelity, 31 have prompted the design of CDMS instrumentation aimed at maximizing acquisition speed while retaining high-accuracy mass measurement capabilities. For state-of-the-art CDMS instruments that incorporate electrostatic ion traps to make accurate charge and m/z measurements, ion energies must be known.…”

Characterizing Heterogeneous Mixtures of Assembled States of the Tobacco Mosaic Virus Using Charge Detection Mass Spectrometry

“…Synthesis of rTMV and cpTMV mutants was performed as described elsewhere. ,,,, Briefly, for rTMV and cpTMV mutants not containing non-canonical amino acids, BL21-Codonplus (DE3)-RIL cells were transformed with the appropriate pET vector, and colonies were selected for inoculation of Terrific broth cultures. Cultures were induced with 30 μM isopropyl-β- d -thiogalactopyranoside (IPTG) at an OD 600 of 0.6–0.8 and allowed to grow 14–18 h at 20 °C before harvesting cell pellets and storing them at −20 °C.…”