Drift Time Corrections Based on a Practical Measurement of the Depletion Zone to Allow Accurate and Reproducible Determination of the Reduced Mobility of Ions in DT-IMS

Tristate Bradbury-Nielsen Gate for Enhancing the Sensitivity of High-Resolution Ion Mobility Spectrometry

Resolution and sensitivity are crucial for standalone ion mobility spectrometry (IMS) used in the field detection of hazardous chemicals. However, it is challenging to achieve a high resolution and high sensitivity simultaneously. For commonly used Bradbury-Nielsen gate (BNG)-based IMS, increasing the gating voltage difference (GVD) between the two sets of BNG wires enhances the temporal compression effect and improves the resolving power; however, this also significantly diminishes IMS sensitivity due to the accompanying enhanced ion depletion effect. In this work, an ion focusing effect induced by increasing the GVD of BNG was first illustrated, and a nearly 200% increase in ion number density before BNG injection was achieved. To fully utilize the temporal compression effect and the ion focusing effect, a new tristate BNG working mode was proposed by inserting a compressing state between the normal BNG opening and closing states. The extra compressing state was able to reduce the mobility discrimination between ions with large mobility difference of ∼0.90 cm2/V·s to 1/19 of the original. Using the tristate BNG mode to analyze hazardous chemical mixtures, the ion current of less mobile di-2-ethylhexyl phthalate ions (DEHP)H+ and methyl salicylate ions (MS·O2)- increased 22 and 26 times, respectively, compared to normal bistate BNG mode, and their resolving powers still remained around 100. The corresponding estimated limits of detection for DEHP and MS were lowered to 228 and 143 ppt, respectively, demonstrating a promising potential for developing high-performance IMS systems that function over wide mobility ranges.

Ultrahigh Resolving Power Ion Mobility Spectrometry with a Simple Pulser Circuitry

The pulsing circuitry for high resolving power drift-tube ion-mobility spectrometry is based on three avalanche photodiodes. These are switched on by illumination through optical fibers, which provide electrical insulation of the driver circuitry from the high voltage. The setup was tested with a series of quaternary ammonium ions introduced with an electrospray ion source. Two instruments with drift tubes of 10 and 30 cm length were employed and a separation voltage of up to 23.7 kV. Resolving powers above 200 could be achieved for the longer tube, which are comparable to those obtained with a previously employed more elaborate electrically floating pulser. The new pulser allows the creation of common two-state injection pulses as well as tristate pulses known to reduce the discrimination of low mobility ions. A comparison between the two pulsing regimes showed that, as predicted by theory, for the longer tube, the discrimination of low-mobility ions in the two-state shutter mode was less significant than for the shorter tube.

Accurate Prediction of Ion Mobility Collision Cross-Section Using Ion's Polarizability and Molecular Mass with Limited Data

The rotationally averaged collision cross-section (CCS) determined by ion mobility-mass spectrometry (IM-MS) facilitates the identification of various biomolecules. Although machine learning (ML) models have recently emerged as a highly accurate approach for predicting CCS values, they rely on large data sets from various instruments, calibrants, and setups, which can introduce additional errors. In this study, we identified and validated that ion's polarizability and mass-to-charge ratio (m/z) have the most significant predictive power for traveling-wave IM CCS values in relation to other physicochemical properties of ions. Constructed solely based on these two physicochemical properties, our CCS prediction approach demonstrated high accuracy (mean relative error of <3.0%) even when trained with limited data (15 CCS values). Given its ability to excel with limited data, our approach harbors immense potential for constructing a precisely predicted CCS database tailored to each distinct experimental setup. A Python script for CCS prediction using our approach is freely available at https://github.com/MSBSiriraj/SVR_CCSPrediction under the GNU General Public License (GPL) version 3.

Electrospray Ion Mobility Spectrometer Based on Flexible Printed-Circuit Board Electrodes with Improved Resolving Power

An easily built drift tube instrument with ring electrodes made of rolled-up flexible printed circuit boards is reported. Its resolving power was maximized by careful attention to the drift tube geometry and the response time of the detector amplifier and by employing a high separation field strength. The separation of singly charged aliphatic quaternary ammonium ions introduced by electrospray was performed, and the measured resolving power was between 86 and 97% of the theoretical limit for three different drift tube lengths investigated. For the longest drift length of 30 cm, a resolving power of up to 228 was obtained. Three benzalkonium chlorides were also separated with resolving powers of over 210. The tristate injection scheme can also be used, with only a small loss of the separation performance compared to the two-state injection.