How soft is your ESI-MS anyway?

30 January 2024, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


Electrospray ionization mass spectrometry (ESI-MS) is a powerful tool for identifying and characterizing organometallic and coordination compounds. However, detection of fragile structures bound by weaker intermolecular forces can be significantly limited in ESI-MS owing to the use of relatively harsh instrument conditions and configurations. In this study, a set of tests was developed to assess the softness of ESI-MS systems. Two variants are presented: positive ion mode, utilizing a mixture of sodium ions and triphenylphosphine oxide producing [Na(OPPh3)n]+ ions (n = 1-4), and negative ion mode utilizing Pd(PPh3)4 and sulfonated triphenylphosphine producing [Pd(L)(PPh3)n]– ions (n = 0-2), where softer instrument conditions preserve a higher proportion of the high-coordinate ions and harsher conditions will result in increased detection of products of ion fragmentation. The results revealed notable variations in instrument softness, which were influenced by a combination of instrument design and experimental parameters. Meticulously optimizing experimental conditions and ESI-MS parameters is essential to achieving the softest ionization possible, ensuring reliable analysis where applicable. This study offers valuable insight through straightforward tests that can be employed to assess the suitability of an instrument for specific research needs.


coordination complexes
electrospray ionization
instrument optimization
ion fragmentation
mass spectrometry

Supplementary materials

Supporting Information for "How soft is your ESI-MS anyway?"
The supporting information is available free of charge. This document includes: materials and chemicals used; ESI-MS instruments and operating parameters (Table S1–8); general test procedures; stoichiometric effects on [Na(OPPh3)n]+ spectra (Figure S1); effect of parameter optimization on [Na(OPPh3)n]+ spectra; fragmentation by ion trapping for System 3 (Figure S2).


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