A conversation with John B. Fenn. Interview by M. Samy El-Shall.

Editor's Note: Every year, the Editorial Committee of each Annual Reviews series asks an internationally distinguished researcher to prepare a prefatory article for the volume being planned. It was an easy choice for us to seek a contribution from John B. Fenn (b. 1917), who since 1994 has been a professor of analytical chemistry at Virginia Commonwealth University in Richmond, Virginia. Few people have achieved the originality and the impact that John Fenn has shown in his development of new instruments for chemical analysis. Anyone using supersonic jet expansions or electrospray ionization owes a huge debt to this special individual, who was awarded the Nobel Prize in Chemistry in 2002. A full account of the development of electrospray ionization may be found in Fenn's 2002 article in the Journal of Biomolecular Techniques (see Related Resources at the end of the interview). What follows is a transcript of an interview that Fenn's colleague Professor Samy El-Shall kindly conducted on our behalf. This interview captures the spirit and the imagination of this singular individual. It also shows that the path to discovery is seldom straight and narrow. -Richard N. Zare.

Gas-phase ions of solute species from charged droplets of solutions.

Charged droplets, produced by electrostatic dispersion of solutions of amino acids and peptides are driven by a potential difference a countercurrent to a flow of heated nitrogen bath gas. Evaporation of solvent from those droplets increases surface charge density, resulting in subdivision into smaller charged droplets. Each smaller droplet repeats that sequence until the ultimate result is a dispersion of solvent-free solute ions in the bath gas. Surprisingly, mass spectrometric analyses of the final ion-bath gas mixtures showed that the relative abundances of the desolvated ions were substantially higher when the nitrogen bath gas contained vapor of a polar solvent species than when no such solvent vapor was present. Adding solvent vapor to the background bath gas can certainly not increase, but must decrease, the net rate of solvent evaporation from the charged droplets. Consequently, the observed enhancement of ion formation by the presence of solvent vapor in the bath gas constitutes persuasive evidence that the observed solute ions cannot have been produced by the charged residue mechanism originally suggested by Dole et al. [Dole M, et al. (1968) J Chem Phys 49:2240-2249 and Dole M, Rheude A, Mack LL (1970) J Chem Phys 52:4977-4986]. It is therefore concluded that electrospray ions are most likely produced by the ion evaporation mechanism of Iribarne and Thomson [Iribarne JV, Thomson BA (1975) J Chem Phys 64:2287-2294]. Moreover, and probably as important, this observed signal enhancement constitutes a welcome increase in detection sensitivity.