Fine Structure in Isotopic Peak Distributions Measured Using a Dynamically Harmonized Fourier Transform Ion Cyclotron Resonance Cell at 7 T
The Institute for Energy Problems of Chemical Physics, Russian Academy of Sciences, Leninskij pr. 38, k.2 Moscow, Russia 119334. Analytical Chemistry
(Impact Factor: 5.64).
03/2012; 84(5):2275-83. DOI: 10.1021/ac202804f
The fine structure of isotopic peak clusters in mass spectra of reserpine and substance P are measured using Fourier transform ion cyclotron resonance mass spectrometry at a 7 T magnetic field. The resolved peaks in the fine structure consist of (13)C, (15)N, (17)O, (18)O, (2)H, (33)S, (34)S, and combinations of them. A recently introduced high-resolution ion cyclotron resonance cell (Nikolaev, E. N.; Boldin, I. A.; Jertz, R.; Baykut, G. J. Am. Soc. Mass Spectrom. 2011, 22, 1125-1133) is used in these experiments. The positions of the experimentally obtained fine structure peaks on the mass scale agree with the isotopic distribution simulations with ≤200 ppb error. Some deviation from the theoretical isotopic distribution is observed, less abundant peaks in the fine structure patterns are a little suppressed compared to the larger ones. We present a method for atomic composition determination using accurate mass data and fine isotopic structure of the mass spectrum. Our method combines the traditional atomic composition determination from accurate mass data by enumeration of all possible formulas within constraints defined a priori with the estimation of element coefficients from resolved isotopic structures. These estimated values allow one to narrow the search space for the composition and therefore to reduce the number of candidate formulas.
Available from: Jean Futrell
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ABSTRACT: The recently designed dynamically harmonized Fourier transform ion cyclotron resonance (FT-ICR) cell creates a more harmonized electric field for the detection of the cyclotron motion of ions and prolongs the ion transient from seconds to minutes. In order to achieve its best performance, phase correction was applied in the spectra, and new advantages of the absorption-mode were revealed.
Spectra were acquired from both simple standard and complex mixtures using either narrowband or broadband mode, and the data were processed to compare the performance of the spectra in magnitude and absorption-mode.
The research shows that phase correction works well with data from Nikolaev's new cell, which produces the maximum improvement in resolving power (2×), and improves the match with the theoretical intensities of the isotopic peaks. In addition, the harmonic peaks can be diagnosed immediately in the absorption-mode.
The manuscript demonstrates absorption-mode spectra from Nikolaev's ICR cell, which will be of interest to the community. The improved relative peak intensities and immediate identification of harmonic peaks will facilitate data interpretation.
Available from: Gleb Vladimirov
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ABSTRACT: The recently introduced ion trap for FT-ICR mass spectrometers with dynamic harmonization showed the highest resolving power ever achieved both for ions with moderate masses 500-1000 Da (peptides) as well as ions with very high masses of up to 200 kDa (proteins). Such results were obtained for superconducting magnets of very high homogeneity of the magnetic field. For magnets with lower homogeneity, the time of transient duration would be smaller. In superconducting magnets used in FT-ICR mass spectrometry the inhomogeneity of the magnetic field in its axial direction prevails over the inhomogeneity in other directions and should be considered as the main factor influencing the synchronic motion of the ion cloud. The inhomogeneity leads to a dependence of the cyclotron frequency from the amplitude of axial oscillation in the potential well of the ion trap. As a consequence, ions in an ion cloud become dephased, which leads to signal attenuation and decrease in the resolving power. Ion cyclotron frequency is also affected by the radial component of the electric field. Hence, by appropriately adjusting the electric field one can compensate the inhomogeneity of the magnetic field and align the cyclotron frequency in the whole range of amplitudes of z-oscillations. A method of magnetic field inhomogeneity compensation in a dynamically harmonized FT-ICR cell is presented, based on adding of extra electrodes into the cell shaped in such a way that the averaged electric field created by these electrodes produces a counter force to the forces caused by the inhomogeneous magnetic field.
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