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Abstract
Ion mobility spectrometry is widely used for the detection of illegal substances and explosives in airports, ports, custom, some stations and many other important places. This task is usually complicated by false positives caused by overlapping the target peaks with that of interferents, commonly associated with samples of interest. Shift reagents (SR) are species that selectively change ion mobilities through adduction with analyte ions when they are introduced in IMS instruments. This characteristic can be used to discriminate false positives because the interferents and illegal substances respond differently to SR depending on the structure and size of analytes and their interaction energy with SR. This study demonstrates that ion mobility shifts upon introduction of SR depend, not only on the ion masses, but on the interaction energies of the ion:SR adducts. In this study, we introduced five different SRs using ESI-IMS-MS to study the effect of the interaction energy and size on mobility shifts. The mobility shifts showed a decreasing trend as the molecular weight increased for the series of compounds ethanolamine, valinol, serine, threonine, phenylalanine, tyrosine, tributylamine, tryptophan, desipramine, and tribenzylamine. It was proved that the decreasing trend was partially due to the inverse relation between the mobility and drift time and hence, the shift in drift time better reflects the pure effect of SR on the mobility of analytes. Yet the drift time shift exhibited a mild decrease with the mass of ions. Valinol pulled out from this trend because it had a low binding energy interaction with all the SR and, consequently, its clusters were short-lived. This short lifetime produced fewer collisions against the buffer gas and a drift time shorter compared to those of ions of similar molecular weight. Analyte ion:SR interactions were calculated using Gaussian. IMS with the introduction of SR could be the choice for the free-interferents detection of illegal drugs, explosives, and biological and warfare agents. The suppression of false positives could facilitate the transit of passengers and cargos, rise the confiscation of illicit substances, and save money and distresses due to needless delays.
Keywords: Adduction, ion mobility spectrometry, mass spectrometry, shift reagent, valinol, buffer gas modifier
Supplementary materials
Title
Instrument, reagent structures, Electrostatic surface potential maps, average shifts in drift time versus the average interaction energy for different SR-ion clusters, Mobility shifts, %ΔK0, for selected ions, Interaction energies and proton affinities
Description
Fig. S1 Photograph of the IMS-MS instrument.
Fig. S2 Structures of compounds used in this investigation.
Fig. S3 Electrostatic surface potential map (ESPM) for the adduct trifluoromethyl benzyl alcohol-methionine
Fig. S4 Mobility shifts of ethanolamine, valinol (blue solid arrow), serine, threonine, methionine (red dotted arrow), phenylalanine, tyrosine, tributylamine, tryptophan, desipramine, and tribenzylamine, when different SR were introduced into the buffer gas at 150 °C.
Fig. S5 Valinol with a five-membered intramolecular bond
Fig. S6 The potential energy surface map shows that when valinol protonation takes place an intramolecular bond of 9.22 kcal/mol is formed.
Fig. S7 The average shifts in drift time versus the average interaction energy for different SR-ion clusters, using data in Tables 2 and 3.
Table S1. Mobility shifts, %ΔK0, for selected ions when different SR were introduced into the buffer gas
Table S2. Interaction energies (IE) and proton affinities (PA) for the compounds and adducts.
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