The effects of modifiers on electrospray ionization for small‐to‐medium‐sized molecules in comparison with solution‐phase ionization

Modifiers are essential additives in electrospray ionization (ESI) mass spectrometric analysis to enhance the production of ionic species in the gas phase for the detection of small‐to‐medium‐sized molecules in biological and aqueous samples. Therefore, an appropriate modifier, in conjunction with the right ionization polarity, is crucial for sensitive ESI mass spectrometric analysis. However, the charge carried by basic and acidic functional groups in solution is neither quantitatively nor qualitatively reflected in the MS spectrum. For example, adding an acidic modifier will cause sufficient protonation of a basic functional group in solution; however, this may result in inferior positive ESI compared to using a basic modifier. Another example is when the MS spectrum does not reflect the charge distribution of multiply charged molecules in solution. Therefore, this review will summarize and discuss the proposed mechanisms by which modifiers induce and influence ESI. It will also cover molecular characteristics that affect ESI, such as charge state and polarity, which result in a difference between solution‐phase ionization and ESI.