The FTMS Analyzers

ETMS is associated with very high mass resolution and mass accuracy. ET Orbitrap instruments generate resolutions up to 200 000, while ETICR instruments can generate resolution above 1000000. 

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Figure 4. Ions undergoing coherent cyclotron motion induce image currents in the plates of the FTMS analyzer cell. Reproduced with permission from Ref. 18. Copyright 1985, North-Holland Physics Publishing.

BCD is a method used to fragment ions in FT MS/MS by allowing them to capture low-energy electrons emitted by a heated dispenser cathode (31). Ions generated in the ESI ion source are injected into the FTMS analyzer cell, where they stay close to the center axis of the cell and oscillate rapidly back and forth between the two trapping plates. A low-energy (1-10 eV) electron beam with a narrow energy bandwidth (<1 eV) is directed into the ICR cell and irradiates the trapped ions. 

Figure 7b also illustrates the high detection sensitivity of the FTMS instrument. We calculate that the CO peak corresponds to approximately 5000 ions in the analyzer cell. In Figure 7a, the number of ions with m/z 43 was calculated to be approximately 20 million. A point to note is that In FTMS the sensitivity increases with resolution whereas it decreases with other mass spectroscopies. 

Figure 7.10 A commercial FTMS system with a front-end nanoflow LC system. The schematic cutaway view depicts the sequential vacuum stages that are required to transfer ions in the ESI source (formed at atmospheric pressure) to the mass analyzer cell of the FTMS (10 ° mbar).

All experiments were performed using a Nicolet Analytical Instruments FTMS-2000 dual-cell Fourier transform mass spectrometer with optional GC and laser desorption interfaces. The FTMS-2000 dual cell is specially constructed of stainless steel with low magnetic susceptibility. This permits very efficient ion transfer between the source and analyzer cells, if the cells are properly aligned in the magnetic field. 

For experiments requiring higher pressures of Cl reagent gas, a second experiment sequence was written which incorporates the use of the pulsed valves supplied with the FTMS. In this sequence, the pulsed valves are opened for 6 msec, allowing a high pressure pulse of reagent gas to enter the source cell. A variable delay is introduced between the time the valves are opened and the time the electron beam is turned on. This enables the pressure at which the initial ionization occurs to be varied. Again, after ion formation in the source, the products may be transferred into the analyzer cell for detection if desired. 

Note Occasionally, FT-ICR-MS is inaccurately referred to as FTMS. Of course, ICR without Fourier transformation would not have become as successful as it has, but Fourier transformation alone cannot separate ions according to m/z. With the advent of the orbitrap analyzer (below), there is a second system that makes use of Fourier transformation. Hence, the acronym FT-MS is proposed for all FT-based methods such as FT-ICR and orbitrap. 

The abihty to desorb and cool ions even from such a hot matrix as a-cyano-4-hydroxycinnamic acid suggests that another approach may also work. While only a half dozen matrix preparation methods are routinely used in MALDl, all of these techniques have been developed initially for MALDl-TOF instruments. Since MALDl-TOFMS requires very flat desorption surfaces for the best resolving power, many potential matrices have not been explored. However, systems where the external ion source is decoupled from the mass analyzer, as is the case with MALDI-QTOF, MALDI-QIT, and MALDl-FTMS instruments, do not have this limitation. Indeed, some of the best signals from MALDl-FTMS systems are generated from large, irregular DHB crystals. Thus, there is a high probability that exploration of matrices with the decoupled MALDl sources will identify new matrices with favorable properties, such as improved ion abundance or reduced matrix adduction. 

Fourier transform mass spectrometry (FTMS) offers the highest mass resolution and mass measurement accuracy of all mass analyzers. FTMS, also referred to as Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS), can be adapted to a wide variety of ion sources and ion dissociation methods.The fundamental behavior of ions in FTMS instmments is based on the principle of ion cyclotron resonance (ICR), conceived and developed by E. O. Lawrence in the 1930s to build ion accelerators for nuclear physics experiments. ICR was first implemented in mass spectrometry in an instrument called the omegatron, developed by scientists at the National Bureau of Standards in the 1950s. Advances such as the application of Fourier transform methods to ICR spectrometry and the trapped analyzer cell resulted in the development of a powerful analytical instmment. 

A third component of the FTMS instrument is the vacuum system. During detection, ions travel several kilometers as they orbit the analyzer cell, and it is important that they do not undergo collisions with background neutral gases. FTMS instruments require ultrahigh... 

A Pt(s)[7(111) X (100)] crystal is positioned in front of a hole in one of the plates of the analyzer cell. Ions formed by electron impact are trapped in the analyzer cell and detected by FTMS. An exclmer laser, having a pulse width of 20 nsec, is used to desorb molecules from the Pt crystal. 

Figure 6 shows the sequence of events in a laser desorption FTMS experiment. First, a focused laser beam traverses the analyzer cell and strikes the crystal normal to the surface. Molecules desorbed by the thermal spike rapidly move away from the crystal and are ionized by an electron beam which passes through the cell parallel to the magnetic field and 3 cm in front of the crystal. 

Figure 6. The sequence of events in a laser desorption FTMS experiment, (a) The laser beam enters the cell and strikes the crystal, (b) Some of the desorbed molecules are ionized by an electron beam, (c) Ions are trapped in the analyzer cell by the magnetic and electric fields, (d) Ions are accelerated by an RF pulse and the resulting coherent image current signal is detected. Reproduced with permission from Ref. 18. Copyright 1935, North-Holland Physics Publishing.

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