Application of Ion Valve Technology in an Orthogonal-Injection TOF Mass Spectrometer

It is known that continuous ion beams generated by normal ion sources cannot be detected completely by an orthogonal-injection time of flight mass spectrometer (O-TOF MS). A duty cycle is defined as by the ratio of the ions accelerated by a TOF pusher to continuous ion beams introduced to the pusher. Typically, the duty cycle of the O-TOF MS is between 5% to 20% depending on the instrument geometry and the m/z value of the ion, because most of the ions are lost while they pass through the pusher during the period among the acceleration pulses of the pusher. To solve the low duty cycle problem, a new technique has been put forward to improve the duty cycle of an O-TOF MS to some degree. The core of the technique relies on an ion valve placed on the skimmer2, or the end-cap of the RF quadrupole, that has the function to transmit or stop the ion beam. By using the ion valve technology, the RF quadrupole is turned into an ion trap, which ejects ions axially into the TOF synchronously with the TOF pusher. A delay between the ejection timing of the RF quadrupole and the push timing of the O-TOF MS leads to the mass synchronization. The ions can be detected with a duty cycle of near 100% in a certain mass range and a much higher sensitivity can be realized. It is noted that the synchronous mode is only suitable for the circumstances where recording the complete mass spectrum is not necessary but a higher sensitivity is expected for a certain mass range. The limitation of a mass range is caused by the spatial spread which occurs during the flight from skimmer2 to the pusher, because the lower m/z ions would move faster than the higher m/z ions in the same acceleration DC field if ions obtained the same kinetic energy. Our future effort will be try to improve a duty cycle in a larger mass range. Here, we presented the simulation results for several ion valve configurations and their comparison with experimental measurements. The experiment results showed that the application of an optimized ion valve could improve the peak intensities of some masses more than an order of magnitude. The ion valve technology could be a promising synchronous technique for O-TOF MS.