Normal-inverse bimodule operation Hadamard transform ion mobility spectrometry

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.

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.

Dopant for detection of methamphetamine in the presence of nicotine with ion mobility spectrometry

Methamphetamine (MA) is a highly addictive and illegal psychostimulant drug and is currently one of the most commonly abused illicit drugs in the world. The on-site rapid detection of trace amounts of MA and screening illicit drugs in clandestine laboratories is important for drug enforcement agencies and the forensic community in general. However, detecting methamphetamine in the presence of nicotine and cigarette smoke by ion mobility spectrometry faces difficulty due to the overlapped spectral peaks of methamphetamine and nicotine. In this work, a new method was developed to detect MA using pyridine as a dopant in the presence of nicotine by a homemade ion mobility spectrometry. The reduced mobilities of MA and nicotine were measured under the temperatures of the drift tube from 40 to 120 °C and doping with pyridine. The result shows that the temperature of 100 °C is beneficial to resolve the two substances. The concentration of doped pyridine is optimized to be 18 ppm. In this doped experiment, the reaction rate of nicotine is higher than that of MA by measuring the instrumental responses of MA and nicotine. No matter how high the nicotine content is, the interference of nicotine can be eliminated in the detection of MA doped with pyridine. This method is also successfully applied for the determination of MA and nicotine simultaneously in real saliva samples. The limit of detection of MA was measured to be about 0.5 ng/μL. The promising results in this work provide an effective method for on-site detection of MA.

Ion Mobility Spectrometry with High Ion Utilization Efficiency Using Traveling Wave-Based Structures for Lossless Ion Manipulations

Ion packets introduced from gates, ion funnel traps, and other conventional ion injection mechanisms produce ion pulse widths typically around a few microseconds or less for ion mobility spectrometry (IMS)-based separations on the order of 100 milliseconds. When such ion injection techniques are coupled with ultralong path length traveling wave (TW)-based IMS separations (i.e., on the order of seconds) using structures for lossless ion manipulations (SLIMs), typically very low ion utilization efficiency is achieved for continuous ion sources [e.g., electrospray ionization (ESI)]. Even with the ability to trap and accumulate much larger populations of ions than being conventionally feasible over longer time periods in SLIM devices, the subsequent long separations lead to overall low ion utilization. Here, we report the use of a highly flexible SLIM arrangement, enabling concurrent ion accumulation and separation and achieving near-complete ion utilization with ESI. We characterize the ion accumulation process in SLIM, demonstrate >98% ion utilization, and show both increased signal intensities and measurement throughput. This approach is envisioned to have broad utility to applications, for example, involving the fast detection of trace chemical species.

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.

Influence of multiplexing conditions on artefact signal and the signal-to-noise ratio in the decoded data in Hadamard transform ion mobility spectrometry

In Hadamard transform ion mobility spectrometry (HT IMS), the signal-to-noise ratio is always lower for non-modified pseudorandom sequences than for modified sequences. Since the use of non-modified modulating pseudorandom sequences is strategically preferable from a duty cycle standpoint, we investigated the change in the interference signal when transitioning from non-modified modulating sequences to sequences modified by the addition of 1,3,5 and 7 zeros. The interfering signal in HT IMS with modified pseudorandom sequences was shown to be mainly random noise for all the cases except for modifying by incorporation of 1 zero. For standard samples of tetraalkylammonium halides, modulation by non-modified pseudorandom sequences is beneficial in the case of small numbers of averaged spectra (below ∼40 averaged spectra compared to any modified pseudorandom sequences except for 1 zero modified and below ∼200 averaged spectra compared to signal averaging ion mobility spectrometry) and worsens the signal-to-noise ratio in the case of large numbers of averaged spectra. Contrarily, modulation by modified pseudorandom sequences is beneficial for any number of averaged spectra, except for very small ones (below 15 averaged spectra compared to modulation by non-modified sequences). Pseudorandom sequence modified with 1 zero incorporation is beneficial in the case of below ∼400 averaged spectra compared to any modified and non-modified pseudorandom sequences. The signal-to-noise ratio in conventional signal averaging mode ion mobility spectrometry is affected by random noise, whereas the HT IMS with non-modified pseudorandom sequences was demonstrated to be primarily affected by a systematic noise-like artefact signal. Because noise-like artefact signals were found to be reproducible, predicting models for interference signals could be generated to improve signal-to-noise ratio. This is significant because non-modified modulating sequences are limited by their poor signal-to-noise ratio. This improvement would increase the viability of non-modified modulating sequences which are preferred because of their higher sample utilization efficiency.