On the mechanisms by which the charged droplets produced by electrospray lead to gas phase ions

Abstract

Abstract A brief description of the process leading to the formation of the very small droplets which ultimately produce gas phase ions is followed by a discussion of the ion evaporation model (IEM) and the charge residue model (CRM). The IEM developed for small ions by Iribarne and Thomson is a well developed model which provides quantitative predictions for the rates of evaporation of ions. The CRM attributed to Dole and extended by Rollgen does not provide a detailed consideration as to how the ‘final’ droplets containing only one ion are formed. Several experimental tests of IEM and CRM are discussed: The rates of formation of gas phase alkali ions M + (Li + , Na + , K + , Cs + ) determined from mass spectra, are compared with predicted rates by the Iribarne and Thomson equation. Unfortunately, the available thermochemical data required for the evaluation of the theoretical rates are of insufficient accuracy. Therefore, the theoretically predicted rates exhibit a large scatter. However, also, the experimentally determined relative rates on the basis of mass spectra are not in agreement. Relative rates for the alkali ions from this laboratory predict nearly equal rates for the alkali ions while data by Leize et al. predict an increase from Li + to Cs + . Both sets of data are compatible with the theoretical predictions of the IEM equation. The CRM, when extended to include the effect of surface activity of ions, is compatible with the results of nearly equal rates, but not compatible with the increasing rates of Leize et al . It is very desirable to establish in future work as to which set of experimental data is correct. If the increasing rates of Leize et al. are found to be true, a strong argument in favor of the IEM will be provided. Mass spectrometric observations of the intensities of the ions Na + and Na(NaCl) n + obtained from aqueous solutions of NaCl at different concentrations are compared with ion distributions expected on the basis of IEM and CRM. In general, larger intensities of Na + relative to Na(NaCl) n + would be expected on the basis of IEM. The experimental data are found to correspond much more closely to the predictions of IEM. However, various assumptions had to be made in order to be able to make the predictions. Most important of these is the history of the droplets as they undergo Rayleigh fission, and in particular, the size, charge and number of offspring droplets. Since accurate values for these quantities are not available, the conclusions in favor of IEM are not definitive. The final stages of CRM where droplets have radii of a few nanometers and several charges and solute molecules cannot be treated with the Rayleigh equation. Loss of single charges (ions) is possible in this stage. If this is the case, the last stage of CRM will be IEM-like. Such a stage will lead to a blurring of the distinction between IEM and CRM, for small ions. Fernandez de la Mora has provided very strong evidence that globular, not denatured, proteins are produced by CRM. More open, multiply-charged macroions could be produced by either IEM or CRM. The ions produced in the gas phase are not necessarily those present in the solution. Stable ions like the alkali ions are transferred without change to the gas phase. However, protonated bases may undergo changes as a consequence of the different basicity orders in solution and in the gas phase and other processes.