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The Time of Flight Mass Spectrometer (TOF) was invented many years ago but, due to the factors controlling resolution not being clearly recognized as a result of certain design defects that occurred in the first models and also due to the electronic components available at that time being significantly inferior to those available today, it exhibited limited performance. Consequently, the TOF was rapidly eclipsed by other developing mass spectrometric techniques. However, with improved design, modern fabrication methods and the introduction of Fourier transform techniques, the performance has been vastly improved. As a result, there has been a resurgence of interest in this particular form of mass spectrometry. A diagram of the time of flight mass spectrometer is shown in figure 80.
In a Time of Flight Mass Spectrometer the following relationship holds,
T = L((m/2zeV)0.5)
is the time taken for the ion to travel a distance (L)
is the accelerating voltage applied to the ion,
is the distance traveled by the ion to the ion sensor.
It follows that for a given system, the mass of the ion is directly proportional to the square of the transit time to the sensor. The sample is volatilized into the space between the first and second electrodes and then a burst of electrons (over about a microsecond period) is allowed to produce ions. The extraction voltage, (E) is then applied for another short period of time which, as those further from the second electrode will experience a greater force than those closer to the second electrode, this will focus the ions. The ions farther from the electrode will experience a greater accelerating potential than those closer to the electrode and, thus, will catch them up
After focusing, the accelerating potential (V) is applied for a much shorter period than that used for ion production (ca 100 nsec) so that all the ions in the source are accelerated almost simultaneously. The ions then pass through the third electrode into the drift zone and are then collected by the sensor electrode. One particular advantage of the time of flight mass spectrometer lies in the fact that it is directly and simply compatible with direct desorption from a surface. Consequently, it can be employed with laser desorption and plasma desorption techniques which have already been discussed.
The first commercial Time-of-flight mass spectrometer was manufactured by the Bendix Corporation, which was based on the original design of Wiley McLaren. However, the model suffered from poor resolution largely due to the relatively slow electronics that were available at that time. In the 1970’s Kore Technology Ltd. revived the instrument employing new designs and faster electronics. Over the years as the speed of electronic systems became greater so did the performance of the time-of-flight mass spectrometer and today the instrument has a wide field of application in both research and analytical laboratories. The Proton Transfer Reaction Time-of-Flight Mass spectrometer is shown in figure 80. Chemists interested in the detection and identification of trace contaminants in the atmosphere find the TOF fast and efficient. The Proton Transfer Reaction Time-of-Flight Mass spectrometer shown in figure 80 was designed for the universities of Leicester and York and employed proton transfer ionization source using H3O+ to transfer a proton to the analyst stream. This ionization procedure ionized the molecules efficiently at the same time minimizing molecular fragmentation. The major constituents of the atmosphere do not react with H3O+ and so only the minor components are detected and measured. The TOF system can detect atmospheric contaminants at levels down to one part per trillion.
Courtesy of Kore Technology Ltd.
The instrument shown in figure 81 was developed by Kore for the Physics Department of Leicester University to characterize the masses and distribution of metal cluster produced by their proprietary metal cluster source.
Courtesy of Kore Technology Ltd.
The beam of metal cluster particles pass through the source region of the TOF-MS where they are ionized and then orthogonally pulsed out into the mass spectrometer. Heavy masses such as metal clusters have low velocities and consequently low detection efficiencies. The effect is counteracted by floating the detector at a high voltage to post-accelerate the ions, which increases their velocity of impact and, consequently, the detection efficiency.
As a result of the basic layout of the TOF mass spectrometer and its high sensitivity it can be designed in a compact shape that can be manufactured in a portable form. A photograph of Kore’s MS-200 portable mass spectrometer is shown in figure 82. As extremely small quantities of sample are required, low powered, zero maintenance vacuum pumps are used The MS-200 employs electron impact ionization giving spectra that correspond well to established reference data. The vacuum system is kept permanently sealed and contained in the portable case. The vacuum is maintained by an ion pump and a non-evaporable getter pump which can cope with all gases. Sample gases are drawn though a heated inlet by a pump and enter the source through a concentrating membrane inlet.
Courtesy of Kore Technology Ltd.
Detection limits are in the ppb range for most compounds. Analysis time are about one minute at the highest sensitivities. The general specifications of the mass spectrmeter are,
Sensitivity < 5 ppb in 10 seconds (Benzene)
Mass range 1 – 1000 amu
Dynamic Range 6 decades
Linearity better than 5%.
An excellent discussion on general organic mass spectrometry is given in Practical Organic Mass Spectrometry edited by Chapman .
About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.