Considerations for Generating Frequency Modulation Waveforms for Fourier Transform-Ion Mobility Experiments

Abstract

By casting the information regarding an ion population’s mobility in the frequency domain, the coupling of time-dispersive ion mobility techniques is now imminently compatible with slower mass analyzers such as ion traps. Recent reports have detailed the continued progress toward maximizing the efficiency of the Fourier transform ion mobility-mass spectrometry (FT-IM-MS) experiments, but few reports have outlined the intersection between the practical considerations of implementation against the theoretical limits imposed by traditional signal processing techniques. One of the important concerns for Fourier-based multiplexing experiments is avoiding signal aliasing as a product of undersampled signals that may occur during data acquisition. In addition to traditional considerations such as detector sampling frequency, the limitations (i.e., maximum measurable drift time) imposed by experimental mass scan duration and the frequency sweep used for ion gate modulation must also be assessed. This work aims to connect the fundamental underpinnings of FT-IM-MS experiments and the associated experimental parameters that are encountered when coupling the comparatively fast separations in the mobility domain with the slower m/z scanning common for ion-trap mass analyzers. In addition to stating the relevant theory that applies to the FT-IM-MS experiment, this report highlights how aliased signals will manifest post Fourier transform in reconstructed arrival time distributions and calculated mobilities.

Improving Ion Mobility Mass Spectrometry of Proteins through Tristate Gating and Optimization of Multiplexing Parameters

“…A baseline sweep rate of 16.6 Hz/s was chosen for additional optimization and assessment of gating performance because it provided the most consistent balance between attaining high-quality mass spectra and reasonable experimental throughput. It is worth noting, however, that there is a maximum sweep rate that can be applied during the FT-IM experiment, above which aliasing may occur . Overall, increasing the sweep rate to 16.6 Hz/s resulted in no significant decrease in IM S/N or resolving power, demonstrating the ability to approach the Nyquist limit while maintaining data quality.…”

“…It is worth noting, however, that there is a maximum sweep rate that can be applied during the FT-IM experiment, above which aliasing may occur. 46 Overall, increasing the sweep rate to 16.6 Hz/s resulted in no significant decrease in IM S/N or resolving power, demonstrating the ability to approach the Nyquist limit while maintaining data quality.…”

Implementation of an Open-Source Multiplexing Ion Gate Control for High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

“…6,11,12 The sweep rate of the FT experiment must be carefully defined to ensure a Gaussian peak shape (i.e., longer experiments ensure at the bare minimum that the Nyquist frequency is preserved) and can be further fine-tuned to increase resolution and accuracy of ion mobilities. 13 One major problem is the interpolation of the exact frequency the gates are operating at to the ion signal, although recent efforts show changing the sweep function can mitigate this issue. 13−15 The other multiplexed technique is Hadamard transform as first shown by Clowers et al 16 and shortly followed by Szumlas and Hieftje.…”

“…Fourier transform is most commonly used on IMS-MS instruments with two ion gates since two gates are pulsed in tandem at increasing frequency over time called the sweep rate. ,, The sweep rate of the FT experiment must be carefully defined to ensure a Gaussian peak shape (i.e., longer experiments ensure at the bare minimum that the Nyquist frequency is preserved) and can be further fine-tuned to increase resolution and accuracy of ion mobilities . One major problem is the interpolation of the exact frequency the gates are operating at to the ion signal, although recent efforts show changing the sweep function can mitigate this issue. − The other multiplexed technique is Hadamard transform as first shown by Clowers et al and shortly followed by Szumlas and Hieftje. , The HT experiment only requires one ion gate and uses a set of pseudorandom binary sequences (PRBS) to pulse the ion gate promising a duty cycle approaching 50% . The PRBS can be generated either through a series of algorithms, such as those from Harwit and Sloane, Barker codes, almost-perfect sequences, or a random number generator. − …”

FTflow: An Open-Source Python GUI for FT-IM-MS Experiments

“…Data shown in Figure 2 is an example data set drawn from previous experiments, the details of which have been described previously. 14 Briefly, data was collected using an atmospheric pressure PCB drift tube ion mobility spectrometer directly coupled to a Thermo-Finnigan LTQ linear ion trap. Frequency sweeps used ranged from 5 to 7505 Hz over 2000 scans.…”