RF voltages applied

Mass analysis is based on mass selective instability. DC and RF voltages applied to the trap electrodes are such that ions over the entire m/e range of interest can be trapped within the field imposed by the electrodes. After this storage period, the parameters U and V0 are changed so that the trapped ions of consecutive values of m/e become... 

Ion trap with an RF voltage applied to the ring electrode, providing the fundamental frequency v and its associated variable amplitude V. Instead of injecting ions, electrons may be injected for internal ionization. Variable RF voltage can be applied to the end caps for ion excitation or ion ejection. 

A second example describes the use of resonant ejection of ions by selected-waveform inverse Fourier transform (SWIFT). Figure 2.26 describes an MS/MS experiment with an instrument using RF voltages applied to the caps, but no DC voltage. In this example, the final analysis of the fragments is performed by the stability limit method. 

The quadrupole ion-trap, usually referred to simply as the ion-trap, is a three-dimensional quadrupole. This type of analyser is shown schematically in Figure 3.5. It consists of a ring electrode with further electrodes, the end-cap electrodes, above and below this. In contrast to the quadrupole, described above, ions, after introduction into the ion-trap, follow a stable (but complex) trajectory, i.e. are trapped, until an RF voltage is applied to the ring electrode. Ions of a particular m/z then become unstable and are directed toward the detector. By varying the RF voltage in a systematic way, a complete mass spectrum may be obtained. 

FIG. 9. Relation between dc self-bias voltage Vj,. and applied RF voltage for different gases (Ar. H2. SiHa) and gas mixtures 0.1 < ISiHal/tlHTl-FlSiHal) < 0.9. Note that the slope is independent of the gas used. 

The substrate is radiatively heated by heaters that are placed outside the vacuum. A backing plate ensures a laterally homogenous temperature profile. In the same chamber also PECVD can be carried out. The backing plate then is the grounded electrode, and the RF voltage is applied to the counter electrode. 

By applying the supplementary RF voltage during injection (without scanning the RF voltage on the ring electrode) it is possible to prevent certain ions from being trapped. 

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