Dynamic field mass analyzers

If one wishes to carry ont gas-phase experiments, that is, to manipulate mass-selected ions inside the mass spectrometer, ion-trap analyzers offer the broadest arsenal of experiments including unimolecular fragmentations as well as bimolecular reactions with sufficiently volatile neutral reagents. Consequently, the choice of analyzer is also an important point. Mass analyzers use static or dynamic electric or magnetic fields to separate the ions either in time or in space. Sector-field mass analyzers use magnetic (B) and electrostatic (E) sectors to separate the ions... 

The basic instrumental set-up for dynamic SIMS is the same as for SSIMS (Sect. 3.1.2). Depending on the intensity, beam diameter, and ion species needed, dif ferent ion sources are used. Several mass analyzers with different characteristics enable a broad field of applications. 

Apart from the quadrupole and TOP analyzers described in Sect. 3.2.2, the most important types of mass analyzer used in common dynamic SIMS instruments employ a magnetic-sector field. 

Dynamic ion separation systems are based on another physical principle and use the different flight time of ions with different masses and different velocity (e.g., in ToF mass analyzers). In addition, in dynamic ion separation systems there is a time dependent variation of one or more system parameters, e.g., changing of electrical or/and magnetic field strengths, which means the ion motion during the measurement procedure is crucial for the mass spectrometric analysis. 

Dynamic mass separation systems use the fact that ions with different masses (accelerated with the same voltage) possess several velocities and consequently their flight times are different. There are about 50 dynamic separation systems known2 using several types of ion movements (linear straight ahead, linear periodic or circular periodic as a function of the electric or magnetic sector field applied). The simplest dynamic mass separation system is a linear time-of-flight (ToF) mass analyzer, and a widely applied mass separation system is the quadrupole analyzer. 

Since 2000, the field has moved increasingly toward hybrid FT-ICR instruments in which the FT-ICR is interfaced with a front-end mass analyzer. The groups of Marshall [46,47] and Smith [48,49] introduced the quadrupole-FT-ICR. That configuration is available commercially. The hybrid linear ion trap FT-ICR [87] was introduced commercially in 2003. Hybrid instruments offer greater versatility in terms of mass-selective external accumulation with the associated increase in sensitivity and dynamic range. 

Dynamic SIMS typically uses a quadrupole mass analyzer however, a TOF mass analyzer offers much higher sensitivities (with limitations, vide infra) and mass ranges. In a TOF-SIMS, an ion of known electrical charge, but unknown mass, is accelerated by an electrical field. As a result, all ions of the same charge will have identical kinetic energies. However, the velocity of the ion will depend on the... 

The ionization typically proceeds in two steps. In the first step (primary ion formation), the matrix absorbs the laser energy. Together with intact macromolecules, the formed matrix ions desorb into the gas phase. This process is very fast and happens in a few nanoseconds. A dense plume is formed in which the second step, the charge transfer from the matrix ions to the maaomolecules, occurs. This is mostly done by a gas phase cation (H, Na, K ) transfer. A quantitative two-step rate equation model of the ionization process was developed by Knochenmuss. This approach was extended by introducing a quantitative molecular dynamics model. According to Karas et al.. ..single charged ions are the lucky survivors.... These ions are accelerated in an electric field of several kilovolts and introduced into the mass analyzer. 

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