Receiver Plate x z
Fig. 1.29 Diagram of an ion cyclotron resonance instrument. The magnetic field is oriented along the z-axis and ions (•) are trapped according the same axis. Due to the cyclotronic motion the ions rotate around the z-axis in the x—y plane.
Cyclotron motion is characterized by its cyclotron frequency (f; from 5 kHz to 5 MHz) which depends on: (i) the magnetic field (B), (ii) the charge on the ion (z) and (iii) the mass of the ion (m). In contrast to other types of mass spectrometers, detection is performed in a non-destructive way. The ions are detected by excitation applying a coherent broadband excitation. The ions undergo cyclotron motion as a packet with a larger radius. When the ion packet approaches the detection plates it generates an alternating current named image current. The resulting signal is generally called the transient free induction decay (FID). Ions of any mass can be detected simultaneously with Fourier transform mass spectrometry (FTMS). The image current is composed of different frequencies and amplitudes which are converted by applying a Fourier transformation to frequency components and further to a mass spectrum. Mass resolution is best with high field strength, decreases when the mass increases and is dependent on acquisition time. The mass resolution is strongly dependent on the length of the transient time. Typical transient times are in the range 0.1-2.0 s. With commercial instruments a mass resolving power of 100000 or more can be routinely achieved. Collision induced dissociation can also be performed in the FT-ICR cell. The transient signal decreases with collision of ions and neutral gas molecules. It is therefore essential to work at very high vacuum (1.3 x 10~8 Pa). The dynamic range of a FT-ICR mass spectrometer is relatively poor because the instrument suffers from the fact that the number of ions in the trap must be in a specified range. Over- and underfilling of the trap results in mass shifts towards high and low values, respectively. To have a better control of the ion population in the cell, a commercial hybrid instrument (LTQ-FTMS, Thermo) was developed by combining a linear ion trap (LIT) with a FT-ICR mass spectrometer . Because the LIT is equipped with two detectors data can be recorded simultaneously in the ion trap and in the FT-ICR mass spectrometer. In this way the FT-ICR operates only as a high resolution detector for MS or MSn experiments performed in the linear ion trap.
Makarov  invented a novel type of mass spectrometer based on the orbital trapping of ions around a central electrode using electrostatic fields named orbi-trap. Kingdon had already described the orbiting of ions around a central electrode using electrostatic fields in 1923, but the device had been only used for ion capturing and not as a mass analyzing device. The orbitrap (Fig. 1.30) is formed by a central spindle-like electrode surrounded by an electrode with a barrel-like shape to create an electrostatic potential. The m/z is a reciprocal proportionate to the frequency (o) of the ions oscillating along the z-axis. There is no collisional cooling inside the orbitrap, which operates at very high vacuum (2 x 10~8 Pa). Detection is performed by measuring the current image of the axial motion of the ions around the inner electrode. The mass spectrum is obtained after Fourier transformation of the image current. The mass resolving power depends on the time constant of the decay transient. The orbitrap provides a mass resolving power exceeding 100 000 (FWHM) and a mass accuracy < 3 ppm. To be opera-
tional as a mass spectrometer the orbitrap requires external ion accumulation, cooling and fragmentation. The setup of the LIT-orbitrap from Thermo is depicted in Fig. 1.30. The instrument consists of a linear ion trap with two detectors connected to the orbitrap via a C-trap. With the LIT various MS or MSn experiments can be performed. When the orbitrap is used as a detector the ions are transferred into the C-trap where they are collisionally damped by nitrogen at low pressure. The C-trap acts as a trapping and focusing device. Injection from the C-Trap into the orbitrap is then performed with short pulses of high voltages.
The particularity of the LIT-orbitrap instrument is the independent operation of the orbitrap and the LIT. Because high resolution requires longer transient time, further data can already be collected in the LIT at the same time. As an example accurate mass measurements of the precursor ion can be performed in the orbitrap while MS2 and MS3 spectra are recorded with the linear ion trap. The LIT-orbitrap has less resolution than a FT-ICR instrument with similar duty cycle, but its maintenance costs are far lower than for the FT-ICR. Both instruments will have a major impact in mainly qualitative analysis of low molecular weight compounds and macromolecules.
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