Orthogonal acceleration time-of-flight instruments

TOF analysers are directly compatible with pulsed ionization techniques such as plasma or laser desorption because they provide short, precisely defined ionization times and a small ionization region. However, to take advantage of TOF analysers, it is interesting to combine such powerful analysers with continuous ionization techniques. These ionization techniques can be compatible with TOF analysers but require some adaptations to pulse the source or to transform a continuous ion beam into a pulsed process. For instance, the coupling of an ESI (or any other API) source with a TOF mass spectrometer is difficult, because ESI yields a continuous ion beam, whereas the TOF system works on a pulsed process. 

Schematic description of orthogonal acceleration with linear mass analyser. Pulses of ions are injected orthogonally from a continuous ion beam into a TOF analyser. 

Orthogonal injection provides a high-efficiency interface for sampling ions from continuous beam to a TOF analyser. The TOF analyser allows simultaneous transmission of all ions and therefore all the ions formed are analysed. However, the duty cycle is far from 100 % for the oa-TOF spectrometer and it is lower than for the TOF spectrometer. This is due to the orthogonal accelerator that samples to the analyser only a part of the ions produced in the source. The duty cycle, despite this fact, is between 5 and 50 %. This is a considerable improvement over the conventional techniques described for coupling a continuous source to a TOF spectrometer. 

As already mentioned, the orthogonal accelerator is filled with new ions from the ion source while the sampled ions are simultaneously analysed in the field-free region. New ions cannot be injected until the ions from the previous injection have reached the detector. 

Another important advantage of orthogonal injection is its ability to minimize the initial velocities in the TOF direction and, simultaneously, to reduce the dispersion of these velocities. This is due to the formation of a parallel ion beam with orthogonal orientation to the flight tube. This favourably affects mass resolution, mass accuracy and mass calibration stability. This makes easier to achieve resolutions of more than 10000. 

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