If I remember all my analytical chemistry classes right, Mikes explanation is already excellent!
A perfect correspondence between mass-over-charge (m/z) and time-of-flight (TOF) assumes that all ions are EXACTLY accelerated to the same kinetic energy. However, the acceleration energies will in practice not be perfect, which results in a kinetic energy distribution. A reflectron will further narrow this energy distribution, but still cannot make it perfect. So the kinetic energy and thus also to the TOF of an isotope is a distribution, however a VERY narrow one. That means there is some chance for each ion to have an energy that is so different from the mean, that the TOF will fall in a bin that is assigned to a neighboring isotope mass. While this chance is very small, if you have a LOT of ions a small, but predictable, fraction of those ions will by chance end up in a neighboring +/- 1 mass TOF bin. Also if the mass calibration (TOF ~ m/z relationship) is not well done, this effect can be appreciably stronger.
I think in the mass cytometry field, this is usually called an 'abundance sensistivity' spillover (as opposed to +/- 1 spillover which is solely due to isotopic impurities).
In our empirical assessment of 'spillover' in mass cytometry, we actually found that +/- 1 spillover was virtually absent with the machine used (<0.05%):
https://www.ncbi.nlm.nih.gov/pmc/articl ... gure/fig2/However, one reason why we initially started looking into spillover in mass cytomerty, was because we had a machine which must have not been setup correctly. This lead to a appreciable and consistent amount of +/- 1 spillover in a number of channels of a dataset I analysed.
Also, I can confirm seing it e.g. in Ir193→ Pt194 when having really high Ir193 levels.
Thus, while rare on a well calibrated machine, I would definitely recommend to check +/- 1 as part of spillover assessment, if only to confirm that the mass calibration was appropriate.
Btw: In the same paper we also tried to graphical capture the different sources of 'spillover' in mass cytometry in Figure 1a:
https://www.ncbi.nlm.nih.gov/pmc/articl ... gure/fig1/Cheers, Vito