Transmitter plates

The theory and instrumentation of Fourier transform mass spectrometry (FTMS) have been discussed extensively in this book and elsewhere [21-23]. All experiments were performed on a Nicolet prototype FTMS-1000 Fourier transform mass spectrometer previously described in detail [24] and equipped with a 5.2 cm cubic trapping cell situated between the poles of a Varian 15 in. electromagnet maintained at 0.85 T. The cell was constructed in our laboratory and utilizes two 80 transmittance stainless steel screens as the transmitter plates. This permits irradiation with a 2.5 kW Hg-Xe arc lamp, used in conjunction with a Schoeffel 0.25 m monochromator set for 10 nm resolution. Metal ions are generated by focusing the beam of a Quanta Ray Nd YAG laser (either the fundamental line at 1064 nm or the frequency doubled line at 532 nm) into the center-drilled hole (1 mm) of a high-purity rod of the appropriate metal supported on the transmitter screen nearest to the laser. The laser ionization technique for generating metal ions has been outlined elsewhere [25]-... 

Transmitter Plates Plates in the FTMS cell which are used to... 

During the trapping time as defined in Fig. 2, ion/molecule reactions can take place where the ions have nearly thermal velocities and the molecules thermal velocities. Unwanted ions can be removed from the cell during this time by application to the transmitter plates of the cell of a series of ion-ejection pulses (see Fig. 2), which are appropriate to increase the radii of the ion orbits so much that the ions strike the walls of the cell, are discharged and pumped away. Other methods of removing unwanted ions from the cell... 

Fig. 3.6. Schematic representation of an ion cyclotron resonance ion trap. Ions are trapped in the x-y plane by the magnetic field B) and held in the z plane by a trapping potential applied to the end plates. Excitation occurs by applying broadband radio frequency voltage pulse to the transmitter plates, followed by detection of the image current on the same plates. Figure adapted from Fig. 7 in Marshall etal. (1998).

To obtain a mass spectrum over the desired m/z interval, all ions within this interval are excited simultaneously by a rapid frequency sweep of the voltage on the transmitter plates. The excitation pulse increases the orbital radii of all ions and puts ions of the same m/z ratio in phase. The orbiting ions create a complex wave signal in the circuit connecting the receiver plates, which is monitored over time as the coherent motion of the ions is destroyed by collisions (Figure 1.25). Fourier-transformation of this time-domain signal furnishes the individual cyclotron frequencies and, hence, the m/z values (Eq. 1.17) of the ions (Figure 1.25). 

Ions travel in a circular path. Their proximity to the receiver plates is controlled by pulses from transmitter plates. 

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