Isotopic beat patterns in Fourier transform ion cyclotron resonance mass spectrometry: implications for high resolution mass measurements of large biopolymers

Dynamics of ions of intact proteins in the Orbitrap mass analyzer

“…As previously demonstrated in [6], time domain signals from multiply-charged biopolymer ions exhibit characteristic and predictable beat patterns due to the phase relationship and constructive/destructive nature of the interference between the signals arising from various isotope and charge state populations. Useful frequency information in these time domain signals is extracted only from equally spaced "beats" while time span between these beats is essentially lost for signal acquisition.…”

“…Following [6], let us consider isotopic beat patterns for the particular case of the Orbitrap mass analyzer and start from the general equation for the frequency of axial oscillations in this analyzer:…”

Spectral accuracy of molecular ions in an LTQ/Orbitrap mass spectrometer and implications for elemental composition determination

“…However, this is not likely to be occurring in the Orbitrap as this phenomena requires longer time to manifest (Ͼ5 s) than the scan duration (1.52 s) required for 100 K resolving power measurements. Rather, the phenomena of isotopic beat patterns in which closely spaced yet unresolved frequencies cause constructive (or destructive) interference [27] and has been shown to effect measured relative abundances [28] may explain the abundance shortfalls for the smaller isotope peaks observed in our work (Makarov, personal communication). Indeed, our observation that the fluorophosphazine ion [C 30 H 19 O 6 N 3 P 3 F 48 ] ϩ (m/z 1521.9715) from the Agilent tune mix, which has limited fine structure due to its low oxygen and high fluorine content, does not display a large difference in spectral accuracy between low and high resolving powers as does thiostrepton and the other natural products with significant fine structure that we studied with similarly high mass (see Table 2), is consistent with these considerations although additional effort may be needed to more rigorously explain these phenomena.…”

“…From the perspective of MS instrument design and operations, spectral accuracy can provide an objective criterion to evaluate how the instrument measurement reflects the true isotope response of a given ion, and can help with optimization of MS operating conditions and/or signal processing. The extensive studies on the behavior of ions in ion cyclotron resonance cells [27,31] as well as work on signal processing strategies [32] may illustrate the types of studies that may lead to a better understanding of Orbitrap fundamentals and how to further improve its already impressive spectral accuracy characteristics.…”

“…What is also clear from this study is that attaining better spectral accuracy for a given ion can improve the chance of arriving at a unique elemental composition. Achieving this goal may require a better understanding of how isotopes of different abundances behave inside the Orbitrap and how best to process the resulting analog and digital signals [27]. Without high enough spectral accuracy, additional selection criteria may need to be employed, such as some empirical chemical rules suggested by Kind and Fiehn to further eliminate possible formulas [1].…”