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

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.

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.

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.

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.