Time-of-flight mass spectrometry (concepts, achievements, and prospects)

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Abstract

A brief account is given of the history of dynamic mass spectrometers from the time of their advent. The reasons for their relatively slow development in the 1930s–1940s of the last century are discussed. The general principle underlying the operation of all dynamic mass spectrometers, the difference between the velocities of motion of ions differing in mass but having the same energy, is pointed out. In accordance with its title, the review considers in more detail the history of development of time-of-flight (TOF) mass spectrometers and stresses their two major advantages over the static (magnetic) instruments, namely, the unlimited mass range and the possibility of obtaining extremely rapidly mass spectra over a broad ion mass range. This accounts for the fast progress in their performance and broad application in the period of 1960s–1980s, when investigation of heavy, thermally unstable organic molecules was started and efficient methods of producing their ions were developed. A serious limitation on the resolution of TOF instruments was largely removed after the formulation of the reflectron principle of TOF mass-spectrometer design. The problem of a further increase of the resolution of TOF systems is discussed. The illustrations of TOF reflectron applications are presented, and a possibility of analyzing molecules and clusters with masses of up to millions of daltons is pointed out. These merits have resulted in a sharp increase of production and a broadening of application potential of TOF mass spectrometers compared to the magnetic setups, particularly in the field of biophysics, biochemistry, and organic chemistry. In conclusion the broad horizons opened by the TOF magnetic-resonance mass spectrometers (MRMS) in the area of high-precision mass spectrometric measurements are pointed out.

Introduction

The history of mass spectrometry started with the work of J.J. Thomson published as far back as 1913 [1], which put forward the principal idea underlying mass spectrometric analysis of the composition of a substance, namely, transformation of all components of the sample to ions, their acceleration by an electric field, and separation of ions with different mass to charge ratios by means of a magnetic field. E.W. Aston built a prototype of modern static magnetic mass spectrometers in 1919 [2]. Thomson and Aston were the first to make by mass spectrometry a great discovery by revealing the existence of isotopes of stable elements.

Subsequent progress in the development of static instruments due to A.J. Dempster [3], A.O. Nier [4], J. Mattauch and H. Herzog [5], K.T. Bainbridge and E.B. Jordan [6], G.P. Barnard [7], V.M. Kelman, I.V. Rodnikova, and L.M. Sekunova [8]; the enormous number of publications dealing only with the improvement of analyzers, ion sources, ion detection techniques; reviews and monographs on the application of mass spectrometry—all this has raised the mass spectrometer to the level where it may be considered one of the most universal instruments for compositional analysis of substances, and the mass spectrometry, one of the most important tools of research in nuclear and atomic physics, chemistry, and biology.

The wealth of scientific achievements amassed in the nearly 90 years of continuous improvements and application of static mass spectrometers, as well as their mass production organized by many instrument-making companies in all technologically advanced countries, became eventually an obstacle to development of dynamic mass spectrometers.

Progress in the field of dynamic mass spectrometers during the first decades after their advent was impeded also by the fact that their principal analytical parameters, such as the resolution and the sensitivity, were inferior to those of the static instruments. This was partially due to the poor level of rf and pulsed technology at the time.

It should be pointed out that many potential merits of magnet-free dynamic instruments, such as, for instance, the unlimited ion mass range they were capable of analyzing, could not be appropriately recognized, because even the methods of obtaining ions of complex thermally unstable molecules were unknown in the 1950s–1960s.

The separation of ions with different mass/charge ratios (m/q) at an equal energy (qU) in static mass spectrometers is based on the difference of the radii of their trajectories in a uniform dc magnetic field. No combination of static electric fields is capable of separating a flux of such ions in mass. This can be done only by applying periodically changing (in particular, sine wave) or pulsed fields.

The basic idea consists here in that monoenergetic ions with different m/q ratios have different velocities, and, hence, they take different times to traverse a given path in the instrument. By applying a periodically varying or a pulsed electric field in certain parts of the path, one can impart different energies to ions with different velocities, thus making possible their separation. If the electric field varies periodically, ions with a given m/q (i.e. a given time taken by them to pass a certain path) will acquire the largest (or the smallest) increment of energy (provided the field frequency was chosen accordingly). By properly varying the frequency, one can measure consecutively the whole mass spectrum. By applying a pulse one can create a short ion packet, and ions with different velocities will separate in their subsequent propagation in individual packets, so that their consecutive arrival to the detector can be recorded in the form of a mass spectrum.

Dynamic (similar to static) instruments make use of various methods of focusing angularly divergent ions. In contrast to the static devices, however, in dynamic instruments one can employ many methods of producing output signals (a narrow slit with a collector or microchannel plate, energy analyzer, method of induced ac potential, Fourier analyzer, absorption of driving ac field energy).

The rich diversity of possible ways to combine ion separation, focusing, and detection in dynamic mass spectrometers accounts for the large number of instrument designs proposed. The monograph by E.W. Blauth, for instance, considered more than 50 such arrangements [9]. Although most of them did not find application because of the limited range of their analytical and operating characteristics, some ideas turned out to be feasible and enjoy an ever increasing use.

In accordance with the main idea underlying ion separation in dynamic instruments, namely, using differences in the time taken to pass a certain path in the instrument, they all can be called “time-of-flight,” however this term has stuck in the literature only to the mass spectrometers, in which short ion packets are produced by a pulsed source and the ions with different mass/charge ratios are separated due to their different flight times to the detector in the field-free drift space and in the correcting parts of the static fields.

It is such instruments that we are going to discuss in the following.

Section snippets

Simple time-of-flight mass spectrometer

The first TOF instrument with a mass-spectrum display was built by A.E. Cameron and D.F. Eggers in 1948 [10]. It contained all components of a simple time-of-flight instrument, namely, a pulsed ion source, a field-free drift space, and an oscillograph as detecting device. However the resolution of the instrument was extremely low (the Hg+, Hg++, and Hg+++ ions were barely detectable). Therefore this work played to a certain extent a negative role in development of the concept of TOF

TOF instruments with compensation of ion energy spread on the resolution (TOF reflectrons)

To increase the resolution of TOF mass spectrometers, many scientists in the 1950s–60s tried to design ion sources with a smaller energy spread, however no substantial progress has been reached in this field. My analysis of the problem has led to the conclusion that one should devote efforts not to combating the energy spread (particularly after the advent of laser ionization techniques) but rather to neutralization of their effect outside the source. Recollection of children’s games (whose

Progress in TOF spectrometry

Although TOF mass spectrometers without reflectors had a resolution only of a few hundreds, they found broad application in research already starting with the late 1950s, because they made possible obtaining without any inertia the full mass spectrum of the ions produced in the ionization chamber of the source. This permitted one to use them to advantage in studies of the sorption and desorption processes on various surfaces (the flash-filament technique) [46], [47]. The oxidation of

Conclusion

The objective pursued by this paper has been not to make an exhaustive account of the history of development of time-of-flight mass spectrometry up to the year 2000, because the wealth of publications on improvements in instruments and on the studies made with them is so large that such a task would have required a monograph. This is evidenced by the thick volumes containing Proceedings of Conferences of the American Society for Mass Spectrometry (ASMS), which cover thousands of papers. The

Acknowledgements

Professor Jürgen Grotemeyer, who kindly proposed to me to write this manuscript for a special issue of the International Journal Mass Spectrometry, suggested a broad freedom in my selecting the material to be included. I express to him my deep gratitude for this proposal and ask him to excuse me big-heartedly if I have used this freedom with too large a liberty.

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