Which parameters influence the quantitative determination of halogenated substances? A summary of systematic investigations

Rapid Comm Mass Spectrometry

Schaefer

et al. 2021

Rationale: Ion mobility spectrometry (IMS) instruments are typically equipped with atmospheric pressure chemical ionization (APCI) sources operated at ambient pressure. However, classical APCI-IMS suffers from a limited ionization yield for nonpolar substances with low proton affinity (PA). This is mainly due to ion clustering processes, especially those that involve water molecules, inhibiting the ionization of these substances. Methods: High Kinetic Energy (HiKE)-IMS instruments are operated at decreased pressures and high reduced electric field strengths. As most clustering reactions are inhibited under these conditions, the ionization yield for nonpolar substances with low PA in HiKE-IMS should differ from that in classical APCI-IMS. To gain first insights into the ionization capabilities and limitations of HiKE-IMS, we investigated the ionization of four model substances with low PA in HiKE-IMS using HiKE-IMS-MS as a function of the reduced electric field strength. Results: The four model substances all have proton affinities between those of H 2 O and (H 2 O) 2 but exhibit different ionization energies, dipole moments, and polarizabilities. As expected, the results show that the ionization yield for these substances differs considerably at low reduced electric field strengths due to ion cluster formation. In contrast, at high reduced electric field strengths, all substances can be ionized via charge and/or proton transfer in HiKE-IMS. Conclusions: Considering the detection of polar substances with high PAs, classical ambient pressure IMS should reach better detection limits than HiKE-IMS. However, considering the detection of nonpolar substances with low PA that are not detected, or only difficult to detect, at ambient pressure, HiKE-IMS would be beneficial. 1 | INTRODUCTION Due to their high sensitivity, fast response times, and compact design, ion mobility spectrometers are commonly used in safety and security applications such as the detection of chemical warfare agents, 1,2 toxic industrial chemicals, 3,4 drugs, 5,6 and explosives. 7-9 Basically, ion mobility spectrometry (IMS) instruments can be divided by their principle of ion separation. In this work, a drift tube (DT) ion mobility spectrometer is used. In DT-IMS, ions are separated by their motion along the axis of a drift tube driven by a homogeneous static electric field. To initiate the measurement, an ion packet is injected into the drift tube. During their motion, the ions are separated based on the absolute value of their ion mobility in the present drift gas. At the end of the drift tube, the ions are captured by a detector that converts

Section: Introductionmentioning

confidence: 99%

Formation of positive product ions from substances with low proton affinity in high kinetic energy ion mobility spectrometry

Allers

Rapid Comm Mass Spectrometry

Schaefer

et al. 2021

“…For fast online and onsite measurements of trace gases in ambient air, ion mobility spectrometry is an often used technique. , Ion mobility spectrometry provides limits of detection in the low ppb V (parts-per-billion by volume) and even ppt V (parts-per-trillion by volume) range within measurement times of 1 s. Therefore, ion mobility spectrometers (IMS) are mainly used in safety and security applications, e.g., for the detection of chemical warfare agents, − toxic industrial chemicals, , drugs, − or explosives. − In an IMS, the formed ions are separated by their individual drift motion in a drift region being the acceleration in an applied electric field and repeated deceleration through collisions with neutral gas molecules. In most cases clean, dry air is used as neutral gas at an operating pressure of about 1000 mbar.…”

Section: Introductionmentioning

confidence: 99%

A High Kinetic Energy Ion Mobility Spectrometer for Operation at Higher Pressures of up to 60 mbar

Schlottmann

Schaefer

J. Am. Soc. Mass Spectrom.

et al. 2023

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at absolute pressures around 20 mbar in order to reach high reduced electric field strengths of up to 120 Td for influencing reaction kinetics in the reaction region. Such operating points significantly increase the linear range and limit chemical cross sensitivities. Furthermore, HiKE-IMS enables ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to additional reaction pathways and fewer clustering reactions. However, operation at higher pressures promises increased sensitivity and smaller instrument size. In this work, we therefore study the theoretical requirements to prevent dielectric breakdown while maintaining high reduced electric field strengths at higher pressures. Furthermore, we experimentally investigate influences of the pressure, discharge currents and applied voltages on the corona ionization source. Based on these results, we present a HiKE-IMS that operates at a pressure of 60 mbar and reduced electric field strengths of up to 105 Td. The corona experiments show shark fin shaped curves for the total charge at the detector with a distinct optimum operating point in the glow discharge region at a corona discharge current of 5 μA. Here, the available charge is maximized while the generation of less-reactive ion species like NO x + is minimized. With these settings, the reactant ion population, H3O+ and O2 +, for ionizing and detecting nonpolar substances like n-hexane is still available even at 60 mbar, achieving a limit of detection of just 5 ppbV for n-hexane.

“…Ion mobility spectrometry (IMS) is a well-known technique for fast online trace gas detection. , Providing limits of detection in the low parts-per-billion by volume (ppb v ) or even parts-per-trillion by volume (ppt v ) range within a second of measurement time, it is commonly used in safety and security applications for the fast detection of chemical warfare agents, , toxic industrial chemicals, , drugs, , or explosives. − Ion mobility spectrometers separate ions by their motion through a neutral gas under the influence of an electric field. Classical ion mobility spectrometers in these applications are operated at an ambient pressure of about 1000 mbar.…”

Section: Introductionmentioning

confidence: 99%

High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS) at 40 mbar

Schlottmann

J. Am. Soc. Mass Spectrom.

Allers

et al. 2020

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are usually operated at an absolute pressure of 20 mbar reaching high reduced electric field strengths of up to 125 Td for controlled reaction kinetics. This significantly increases the linear range and limits chemical cross sensitivities. Furthermore, HiKE-IMS enables the ionization of compounds normally not detectable in ambient pressure IMS, such as benzene, due to new reaction pathways and the inhibition of clustering reactions. In addition, HiKE-IMS allows the observation of additional orthogonal parameters related to an increased ion temperature such as fragmentation and field-dependent ion mobility, which may help to separate compounds that have similar ion mobility under low field conditions.Aiming for a hand-held HiKE-IMS to carry its benefits into field applications, reducing size and power consumption of the vacuum system is necessary. In this work, we present a novel HiKE-IMS design entirely manufactured from standard printed circuit boards (PCB) and experimentally investigate the analytical performance in dependence of the operating pressure between 20 mbar and 40 mbar. Hereby, the limit of detection (LoD) for benzene in purified, dry air (1.4 ppmV water) improved from 7 ppbV at 20 mbar down to 1.8 ppbV at 40 mbar. Furthermore, adding 0.9 ppmV toluene the signal of the benzene B + peak decreases by only 2 % at 40 mbar. Even in the presence of high relative humidity in the sample gas above 90 % or toluene concentrations of up to 20 ppmV, the LoD for benzene just increases to 9 ppbV at 40 mbar.