Hadamard Transform Ion Mobility Spectrometry Based on Matrix Encoding Modulation

Ion mobility spectrometry (IMS) has been widely used for the on-site detection of trace chemicals, but continue to suffer from a low duty cycle of ion injection. The Hadamard transform ion mobility spectrometry (HT-IMS) technique was employed to address the problem with increased signal-to-noise ratio (SNR). However, in this work, through simulation, a certain deviation between the mathematical principle of Hadamard transform and actual data collection process was found, which resulted in a distortion of the baseline in the spectrum. The reason behind this problem was analyzed and a novel IMS based on Sylvester-type Hadamard matrix encoding modulation (Sylvester-HT-IMS), together with a set of date collection and processing technique, was proposed. Sylvester-HT-IMS offered much improved quality of deconvoluted spectrum and overall performance in the simulation. In experimental verification, with reactant ions and product ions characterized, Sylvester-HT-IMS showed improved SNR and ion discrimination over both conventional signal-averaged IMS (SA-IMS) and HT-IMS, providing an alternative method for multiplexed IMS.

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

With ion mobility spectrometry increasingly used in mass spectrometry to enhance separation by increasing orthogonality, low ion throughput is a challenge for the drift-tube ion mobility experiment. The High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) is no exception and routinely uses duty cycles of less than 0.1%. Multiplexing techniques such as Fourier transform and Hadamard transform represent two of the most common approaches used in the literature to improve ion throughput for the IMS experiment; these techniques promise increased duty cycles of up to 50% and an increased signal-to-noise ratio (SNR). With no instrument modifications required, we present the implementation of Hadamard Transform on the HiKE-IMS using a low cost, high-speed (600 MHz), open source microcontroller, a Teensy 4.1. Compared to signal average mode, 7- to 10-bit pseudorandom binary sequences resulted in increased analyte signal by over a factor of 3. However, the maximum SNR gain of 10 did not approach the theoretical 2n-1 gain largely due to capacitive coupling of the ion gate modulation with the Faraday plate used as a detector. Even when utilizing an inverse Hadamard technique, capacitive coupling was not completely eliminated. Regardless, the benefits of multiplexing IMS coupled to mass spectrometers are well documented throughout literature, and this first effort serves as a proof of concept for multiplexing HiKE-IMS. Finally, the highly flexible Teensy used in this effort can be used to multiplex other devices or can be used for Fourier transform instead of Hadamard transform.

Almost perfect sequence modulated multiplexing ion mobility spectrometry

RATIONALE

Multiplexing ion mobility spectrometry with multiple ion injection pulses was used to achieve a high duty cycle and thus improve the signal-to-noise (S/N) ratio while maintaining high resolving power compared with the traditional single-pulse signal averaging method. Historically, an ion mobility spectrum was reconstructed by various multiplexing methods including Fourier transform ion mobility spectrometry (FT-IMS), Hadamard transform ion mobility spectrometry (HT-IMS), and linear frequency modulation correlation ion mobility spectrometry (LFM-CIMS) sequence or Barker code.

METHODS

To achieve an artifact-free multiplexing ion mobility spectrum, an almost perfect sequence (APS) with correlation technique was proposed to modulate the Bradbury-Nielson ion gate and was compared with FT-IMS, HT-IMS, LFM-IMS, and the traditional single-pulse signal averaging method.

RESULTS

Experimental results showed that there are no artifact peaks in the APS-IMS spectra except an inverted mirror peak, and the S/N ratio was improved 5-8 times with a repetition time of 40-60 ms, corresponding to the improvement in the duty cycle. With the same duty cycle and similar acquisition time, APS-IMS showed a higher S/N ratio than HT-IMS for its unique autocorrelation response.

CONCLUSIONS

The APS-IMS technique offered a higher duty cycle and relatively shorter modulation period compared with reported multiplexing methods and is suitable to track rapidly changing signals without losing information and adding extra transformation artifact peaks.

Characterization of a Modulated X-ray Source for Ion Mobility Spectrometry

As a highly deployed field instrument for the detection of narcotics, explosives, and chemical warfare agents, drift tube ion mobility spectrometry relies heavily upon the performance of the ionization source and mechanism of ion beam modulation. For this instrumental platform, ion chemistry plays a critical role in the performance of the instrument from a sensitivity and selectivity perspective; however, a range of instrumental components also occupy pivotal roles. Most notably, the mechanism of ion modulation or ion gating is a primary contributor to peak width in a drift tube ion mobility experiment. Unfortunately, physical ion gates rarely perform perfectly, and in addition to serving as physical impediments to ion transmission, their modulation also has undesirable field effects. Using a recently developed modulated, non-radioactive X-ray source, we detail the performance of an ion mobility spectrometry (IMS) system that is free of a gating structure and utilizes the pulsed nature of the modulated X-ray source (MXS) for both ion generation and initiation of the IMS experiment. After investigating the influence of pulse duration and spatial X-ray beam width on the analytical performance of the instrument, the possibility of using multiplexing with a shutterless system is explored. By increasing ion throughput, the observed multiplexing gain compared to a signal-averaged spectrum approaches the theoretical maximum and illustrates the capability of the MXS-IMS system to realize significant signal to noise improvements.

Simulating, Predicting, and Minimizing False Peaks for Hadamard Transform Ion Mobility Spectrometry

Multiplexing techniques, including the Hadamard transform, are widely used in the recovery of weak signals from high-level noise. Hadamard transform ion mobility spectrometry (HT-IMS), however, can suffer serious drawbacks due to false peaks. False peaks in HT-IMS are generally attributed to nonperfect gating behavior. This paper confirmed that the origin of false peaks in HT-IMS is not generally due to ion gating but rather to peak shifts by Coulombic repulsion of the ion packets inside the drift tube. The amplitudes of these false peaks are determined by the number of ions inside the ion packets. This phenomenon is simulated and confirmed by the convolution of the spectrum with a shifted s-sequence to reproduce the artifact peaks with the exact position, amplitude, and profile. Two approaches, including preoffset sequence modulation and post-data processing, were evaluated to mitigate the false peaks in HT-IMS, and both methods can work effectively.

Fourier transform ion mobility spectrometry with multinozzle emitter array electrospray ionization

Fourier transform ion mobility spectrometry (FT-IMS) is a useful multiplexing method for improving the duty cycle (DC) of IMS from 1 to 25% when using an entrance and exit ion gate to modulate the ion current with a synchronized square wave chirp.