Methodology and Characterization of Isolation and Preconcentration in a Gas-Filled Digital Linear Ion Guide.

Digital operation of linear ion guides allows them to operate as traps and mass filters by modulating the duty cycles of the two driving waveforms. A gas-filled (5 mTorr) digitally driven quadrupole ion guide was used to demonstrate ion isolation and preconcentration. These abilities allow ion trapping mass spectrometers to be filled to capacity with only ions in the range of interest at essentially any value of m/z. Due to the unique performance characteristics of digitally operated quadrupoles, isolation with purely duty cycle enhanced waveforms was developed with three increasingly sophisticated isolation methods. First, the guide was used as a gas-filled transmission mass filter using the waveform duty cycle to generate a narrow mass window. The second method used broadband trapping to collect ions and translationally cool along the transmission axis before shifting the duty cycle to filter the trapped ions. A subsequent duty cycle change axially ejected the filtered population for measurement. The third method improved resolution by shifting the operating frequency during isolation. The resolving power was optimized with the shift frequency to yield a device limited resolving power of 400 (m/Δm). It is the temporal control of the duration of the isolation process that sets digital waveform based isolation apart from the current technology and that minimizes ion loss even when the mass is very large. Preconcentration by repeated trapping and isolation of an individual charge state was also demonstrated to saturate the ion guide with that charge state. These digital isolation and preconcentration techniques will permit the same isolation resolution (m/Δm) at any value of mass or m/z without significant ion loss as long as the secular frequencies do not significantly overlap while in the trapping mode. It is therefore ideal for the isolation and preconcentration of single charge states of large proteins and complexes.

Using Digital Waveforms to Mitigate Solvent Clustering During Mass Filter Analysis of Proteins.

With advances in the precision of digital electronics, waveform generation technology has progressed to a state that enables the creation of m/z filters that are purely digitally driven. These advances present new methods of performing mass analyses that provide information from a chemical system that are inherently difficult to achieve by other means. One notable characteristic of digitally driven mass filters is the capacity to transmit ions at m/z ratios that vastly exceed the capabilities of traditional resonant systems. However, the capacity to probe ion m/z ratios that span multiple orders of magnitudes across multiple orders of magnitude presents a new set of issues requiring a solution. In the present work, when probing multiply charged protein species beyond m/z 2000 using a gentle atmospheric pressure interface, the presence of solvent adducts and poorly resolved multimers can severely degrade spectral fidelity. Increasing energy imparted into a target ion population is one approach minimizing these clusters; however, the use of digital waveform technology provides an alternative that maximizes ion transport efficiency and simultaneously minimizes solvent clustering. In addition to the frequency of the applied waveform, digital manipulation also provides control over the duty cycle of the target waveform. This work examines the conditions and approach leading to optimal digital waveform operation to minimize solvent clustering. Graphical Abstract ᅟ.

A comparison based digital waveform generator for high resolution duty cycle.

A comparison-based digital waveform generator has been developed that directly enables purely duty cycle controlled digital mass filters. This waveform generator operates by the comparison of a periodic waveform and a DC level to produce a digital waveform. The improved duty cycle realized by this method of waveform generation is demonstrated by producing a mass spectrum of electrosprayed lysozyme by varying the duty cycle of a digital waveform applied to a quadrupole rod set. Operation and control of the waveform generator using an inexpensive open-source microcontroller is discussed.

Digital Waveform Technology and the Next Generation of Mass Spectrometers.

Ion traps and guides are integral parts of current commercial mass spectrometers. They are currently operated with sinusoidal waveform technology that has been developed over many years. Recently, digital waveform technology has begun to emerge and promises to supplant its older cousin because it presents new capabilities that result from the ability to instantaneously switch the frequency and duty cycle of the waveforms. This manuscript examines these capabilities and reveals their uses and effects on instrumentation. Graphical Abstract ᅟ.