a) Describe the differences in setup of linear and reflector TOF instruments.
An instrumental setup where the ions are travelling on a straight line from the point of their creation to the detector is called linear TOF.
In the reflector TOF analyzer – often abbreviated ReTOF – the reflector acts as an ion mirror that focuses ions of different kinetic energies in time. Commonly, reflector instruments are also equipped with a detector behind the reflector allowing linear mode operation simply by switching off the reflector voltage.
b) Compared to the precursor ions, at what times do fragments from metastable dissociations reach the detector of a linear TOF analyzer?
Metastable decompositions during the flight do not reduce the intensity of a molecular ion signal, because the fragments formed conserve the velocity of the fragmenting ion and are therefore detected at the same time-of-flight as their intact precursors. In such a case, the ionic and the neutral fragment cause a response at the detector.
c) What is the „trick“ to make CID fragment ions comply with the comparatively small energy acceptance of a ReTOF used as TOF2 in TOF-TOF instruments?
Although differing in detail, the basic idea of TOF-TOF instruments is to employ a short linear TOF as MS1 and a high resolving ReTOF as MS2 which is separated from TOF1 by a timed ion selector for precursor ion selection. TOF1 is operated at comparatively low acceleration voltage, whereas TOF2 is desigend to analyze ions of about 20 keV kinetic energy. The difference in kinetic energy is provided by a second acceleration stage located behind the collision cell. By lifting all ions by a certain amount of kinetic energy, their relative spread in kinetic energy is reduced, e.g., a precursor ion of 5 keV yields fragments having 0.5–5 keV. Addition of 15 keV to all fragment ions lifts them to 15.5–20 keV. Provided the reflector offers sufficient energy acceptance they can be analyzed without stepping of the reflector voltage.