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

In addition to mass accuracy, the ability of a mass spectrometer to faithfully measure the isotopic distribution of an ion, defined as spectral accuracy, is also important. Although time-of-flight mass spectrometers are reported to possess high spectral accuracy capability compared with other mass spectrometers, the Orbitrap has not yet been investigated. Ten natural products (moxidectin, erythromycin, digoxin, rifampicin, amphotericin B, rapamycin, gramicidin S, cyclosporin A, vancomycin, and thiostrepton) ranging in molecular weight from 639 to 1663 Da were measured on an LTQ/Orbitrap mass spectrometer with resolving power settings of 7.5, 15, 30, 60, and 100 K. The difference in the observed profile isotope pattern compared with the theoretical calculation after peak shape calibration, denoted spectral error, was calculated using the program MassWorks (Cerno Bioscience, Danbury, CT, USA). Spectral errors were least at 7.5 K resolving power (≤3%) but exceeded 10% for some compounds at 100 K. The increasing spectral error observed at higher resolving power for compounds with complex fine structure might be explained by the phenomena of isotopic beat patterns as observed in FTICR. Several compounds with prominent doubly charged ions allowed comparison of spectral accuracies of singly-versus doubly-charged ions. When using spectral error to rank elemental compositions with formula constraints (C0–100H0–200N0–50O0–50Cl0–5S0–5) and a mass tolerance ≤2 parts-per-million, the correct formula was ranked first 35% of the time. However, spectral error considerations eliminated >99% of possible elemental formulas for compounds with molecular weight >900 Da.