Coalescence and self-bunching observed in commercial high-resolution mass spectrometry instrumentation.

RATIONALE: Self-bunching and coalescence are well-known effects in Fourier transform ion cyclotron resonance (FTICR) and multi-reflection time-of-flight (TOF) mass spectrometry. These detrimental effects can also be observed in currently more frequently used high-resolution mass spectrometry (HRMS) instruments, such as the Orbitrap and single-reflection TOF.
METHODS: A modern single-reflection TOF and a Q-Orbitrap were used to produce conditions in which self-bunching and coalescence were observed. This was done by infusion experiments of several isobaric compounds. The peak widths of some low mass isobaric ions as well as the mass resolution of such mixtures were investigated. Attention was paid to possible self-bunching and coalescence effects.
RESULTS: For the utilized TOF mass spectrometer, the measured peak widths of the ions become significantly narrower (self-bunching) when increasing the ion abundance. On the other hand, isobaric ion pairs (delta < 30 milli m/z units) became unresolvable above a certain ion abundance (coalescence). The tested Orbitrap shows similar behavior, although coalescence appeared only at delta <15 milli m/z units. Coalescence was shown to affect the quantitative data, while self-bunching can lead to biased relative isotopic ratios.
CONCLUSIONS: The conventional measurement of a peak width does not truly reflect the mass resolving power of modern HRMS instrumentation. The mass resolving power is better demonstrated by resolving a mixture of isobaric compounds. Measurements obtained at low and high ion abundances should be investigated. Coalescence and self-bunching can reduce the truly available mass resolving power and therefore negatively affect quantitative and qualitative measurements. This article is protected by copyright. All rights reserved.