John B. Fenn

Ion formation from charged droplets: roles of geometry, energy, and time

The formation of ions from the charged droplets produced in the several spray ionization techniques is viewed as an activated rate process involving field-assisted desorption, in accord with the ideas first set forth by Iribame and Thomson. The novel features of the present treatment are particularly relevant to the unique ability of electrospray ionization to transform large molecules in solution to free ions in the gas phase, with extensive multiple charging. These new features stem mainly from the realization that the spacing of charges on a desorbed ion must relate to the spacing of charges on the surface of the droplet whence it came. The consequences of this “rule” can account for the existence of maxima and minima in the number of charges on the ions of a particular species as well as the nature of the distribution of ions among the intervening charge states. They also explain the dependence of charge state on the configuration in solution of the parent molecule of the desorbed ion. In addition, they provide insight into the sequence in time at which ions in the various charge states leave an evaporating droplet.

Clustering of water on hydrated protons in a supersonic free jet expansion

This paper presents results concerning size distribution and growth in the gas phase of ions in the series H+(H2O)n. The measurements span a range of cluster sizes up to n =28, a domain not previously investigated quantitatively. The molecular beam sampling technique developed for this study promises to be a powerful tool in the study of ion nucleation phenomenon. Noteworthy observations include the unusual stability of the cluster with n =21 and the absence of trapped molecules of carrier gas within a cluster.

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this months Special Feature. We particularly hope that younger scientists will find value in this months Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.

Of protons or proteins

Mass analyses have been carried out on ions produced by an Electrospray (ES) source from dilute solutions of protein molecules with molecular weights (M) in the range from 5000 to nearly 40000. Each spectrum comprises a sequence of peaks corresponding to multiply charged intact parent species. The ions of each peak differ from those of their adjacent neighbors by one unit charge, H+ in these experiments. The maximum number of charges per ion generally increases with the molecular weight of the parent molecule, reaching a value of 45 in the case of alcohol dehydrogenase, atM=39830 the largest species in this study. Thus the resulting values ofm/z are within reach of a simple quadrupole mass filter whose nominal upper mass limit is 1500 daltons! The immediate application for the ES source is in mass spectrometric analysis of large fragile molecules of biochemical importance. But the multiply charged ions it produces are newcomers to the laboratory scene that constitute interesting subjects for study.

Electrospray mass spectrometry of fossil fuels

Abstract Exploratory experiments with electrospray ionization mass spectrometry have been carried out on crude oil, jet fuel, gasoline, and coal. The resulting mass spectra contain a remarkable amount of information on the composition and character of these sometimes very complex materials. They suggest that electrospray ionization may have found a new arena in which to exercise its powers.

Relaxation rates from time of flight analysis of molecular beams

In the free jet generated when a gas is expanded from a reservoir at stagnation temperature T0 and pressure P0 into a vacuum chamber, substantial lags can occur in the conversion of internal energy into directed kinetic energy of the expansion. An analytical technique is presented for determining rotational energy relaxation rates from velocity distributions measured on the axis of the free jet at a fixed distance from the sonic throat. The agreement with rates obtained by other methods is good. Some sample experimental and analytical results for carbon dioxide, oxygen, hydrogen, and methane are presented.

Rotational relaxation of molecular hydrogen

The rotational relaxation rate in molecular hydrogen has been determined experimentally between 300 and 1900°K by time of flight analysis of molecular beams from supersonic free jets. Over their common temperature range from 295 to 1073°K our results are in reasonable agreement with the ultrasonic measurements of Winter and Hill which show a rotational collision number Zr between 300 and 400. In terms of momentum transfer collisions Zr is relatively insensitive to temperature. This observation suggests that rotation‐rotation transfers play an important role.

The exciting oxidation of CO on Pt

Abstract An uncollimated molecular beam (free jet) of CO and O 2 molecules incident on a polycrystalline Pt surface at 775 K in vacuo produces vibrationally hot CO 2 molecules at a density sufficient for infrared emission spectrometry. Analysis of spectra at a resolution of 0.1 cm −1 clearly shows that nascent product molecules have much more internal excitation than would be the case for equilibrium at the surface temperature.

Electrospray Mass Spectrometry

The objective in this chapter is to introduce the principles and methods of electrospray mass spectrometry (ESMS), a relatively new technique for the analysis of complex, polar, and labile molecules. Based on electrospray (ES) ionization at atmospheric pressure of solute species in volatile solvents, ESMS has attracted widespread interest only since 1988, when it was shown capable of producing and weighing intact ions of very large biomolecules (1). Along with laser desorption (LD) (2–5), ES has now become an ionization method of choice for MS analysis of molecules in the mass range above 10 kDa. It is also finding increasing application in the analysis of smaller species and seems destined to become an indispensable tool in the emerging discipline of biological mass spectrometry (6). In this account we describe the essential features of ESMS in an attempt to provide some perspectives on how it works and how it can be used. The interested reader is referred to several recent reviews (7–10) for additional information on results that have been obtained, comparisons with other methods, and references to the literature.

Mass spectrometric implications of high-pressure ion sources

Abstract The nature of supersonic free jet expansion of a gas from high pressure into vacuum is reviewed and characterized in this article. The increasingly widespread applications and implications of this jet expansion process in mass spectrometry are described. Particular attention is paid to prospective advantages and possible problems when such jets are used to transport ions from a source at high pressure into the vacuum environment of a mass analyzer.