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The ion trap detector is a modified form of the Quadrupole Mass Spectrometer, but was originally designed more specifically as a chromatography detector than as a instrument for structure elucidation or identification. Nevertheless the combination of the Ion Trap Mass Spectrometer with the chromatograph is another powerful tandem technique. The electrode orientation of the quadrupole Ion Trap Mass Spectrometer is shown in figure 76.
It was shown in figure 68 that the quadrupole spectrometer contains four, rod electrodes. The ion trap mass spectrometer has a quite different electrode arrangement and consists of three cylindrically symmetrical electrodes comprised of two end caps and a ring. The device can be made very small the opposite internal electrode faces being only 2 cm apart. Each electrode has accurately machined hyperbolic internal faces.
In a similar manner to the quadrupole spectrometer, an rf voltage together with an additional DC voltage is applied to the ring and the end caps are grounded. In the same way as the Quadrupole Mass Spectrometer, the rf voltage causes rapid reversals of field direction, so any ions are alternately accelerated and decelerated in the axial direction and vice versa in the radial direction. Again the operating parameters, (a), and (q), define the conditions of oscillation which are analogous to those for the Quadrupole Mass Spectrometer but, in this case, (r0) is the internal radius of the ring electrode.
The ion trap is small and (ro) is typically about 1 cm. At a chosen voltage, ions of a specific mass range are held oscillating in the trap. Initially, the electron beam is used to produce ions and after a given time the beam is turned off. All the ions, except those selected by the magnitude of the applied rf voltage, are lost to the walls of the trap, and the remainder continue oscillating in the trap. The potential of the applied rf voltage is then increased, and the ions sequentially assume unstable trajectories and leave the trap via the aperture to the sensor. The ions exit the trap in order of their increasing m/z values.
The first Ion Trap Mass Spectrometers were not very efficient, but it was found that the introduction traces of helium to the ion trap significantly improved the quality of the spectra. The improvement appeared to result from ion–helium collisions that reduced the energy of the ions and allow them to concentrate in the center of the trap. The spectra produced are quite satisfactory for solute identification by comparison with reference spectra. However, the Spectrum produced for a given substance will probably differ considerably from that produced by the normal Quadrupole Mass Spectrometer.
The device incorporates a new enhanced charge capacity system that extends the number of ions that can be stored in resonation which, in turn, increases the over all signal-to-noise ratio of the system. The spectrometer is fitted with an electro-spray ionization and atmospheric pressure ionization facilities, the latter being temperature programmable and, thus, improves the ionization efficiency of thermally labile materials. The two ionization systems can be exchanged in less than a minute.
Courtesy of Varian Inc.
The ‘electro spray’-‘ion trap’ system readily produces multiple charged ions and as the mass analyzer discriminates on the basis of (m/z), ((m) is mass and (z) is the charge) and not just mass, then a doubly charged ion of mass 1000 will appear on the m/z scale at 500. Thus, multiple charged ions in effect extends the mass range of the mass spectrometer.
The heights of peaks having multiple charges are linearly relate to one another and, thus, by fitting a linear function to the data the presence of multiple charge ions can be exposed. An example of multiple charges is shown by the spectra in figure 78.
Courtesy of Varian Inc.
The Ion Trap Mass Spectrometer is extremely sensitive as shown by the chromatogram that was monitored by the spectrometer given in figure 79. It is seen that the peak represents a sample size of 500 fg. It should also be noted that the signal-to-noise ratio is 52. Now, assuming the peak is discernable when its height is twice the noise, then the ultimate sensitivity will be about ca. 19 fg.
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.