Tandem Mass-Selective Cryogenic Digital Ion Traps for Enhanced Cluster Formation

Abstract

We present the implementation of tandem mass-selective cryogenic ion traps, designed to enhance the range of ion processing capabilities that can be performed prior to spectroscopic interrogation. We show that both the formation of ion clusters and mass filtering steps can be combined in a single cryogenic linear quadrupole ion trap driven by RF square waves. Mass filtering and mass isolation can be achieved by manipulation of the RF frequency and duty cycle. Very importantly, this scheme circumvents the need for high-amplitude RF voltages that can be incompatible with typical cryogenic ion processing conditions. In addition, proper adjustment of the stability boundaries during the clustering process allows for the preferential formation of a specific cluster size rather than a broad distribution of sizes. Lastly, we show that a specific cluster size can be formed, mass-selected, and then transferred to another ion trap for a second completely separate ion processing step. The instrumentation and modular design developed here expand the scope of ionic species and clusters that can be accessed by processing electrosprayed ions.

Square Parametric Excitation: A Digital Resonant Method for the Quadrupole Ion Trap

“…The square parametric excitation waveform can also be used to isolate a selected m / z species. Mass isolation in a digital ion trap by RF frequency and duty cycle manipulation has been reported previously , and utilized by our group . In short, this method works by moving the boundaries of the Mathieu stability diagram so that the desired m / z ion(s) are isolated in the apex of the stable region.…”

“…The LIT is driven by fixed amplitude 100 V pp square waves with variable frequency and duty cycle produced by home-built circuitry. First, a waveform generator, based on the design of Hoffman et al and described previously, produces TTL level (5 V) waveforms. This generator is controlled by an Arduino DUE microcontroller which allows for software-based adjustment of the frequency and duty cycle of the waveforms.…”

“…This generates a pair of out-of-phase high-voltage RF square waves which switch between the positive and negative floating outputs (+HV, -HV) of a 4 W, 100 V DC power supply (Acopian Model A100HT400) and are applied to the X and Y rod pairs of the LIT. The frequency and duty cycle of these driving square waves can be varied in order to control the location of ions in the Mathieu stability zones and make them unstable for the mass-selective radial ejection scheme described here. ,, Note that we define duty cycle ( d = (t/T) *100 ) as the percentage of time the square wave spends in the high voltage state ( t ) relative to the full period of the waveform ( T ), and the duty cycles for the X and Y rods are independently adjustable. The notation d x : d y is used throughout this paper to denote the duty cycles applied to the X and Y rod pairs.…”

“…The ability to vary waveform frequency instead of amplitude also allows for lower voltage requirements . This is advantageous for quadrupolar devices operating at higher pressure such as in cryogenic clustering applications or in portable mass spectrometers. , Most importantly, this frequency variability extends the high mass range of RF square wave-driven quadrupoles due to the inverse relationship between frequency and m / z . Here we present the addition of an auxiliary square waveform to a quadrupole linear ion trap driven by digital RF waves which we use for resonant parametric excitation of the ion motion.…”

“… − In typical digital wave applications, the DC float voltage ( U ) is kept to zero but the duty cycle of the waveform is varied. Combined manipulation of the square wave frequency and duty cycle has been shown to change the shape of the quadrupolar Mathieu stability zones, introducing mass filtration and isolation capabilities. ,, …”