Towards a hand-held, fast, and sensitive gas chromatograph-ion mobility spectrometer for detecting volatile compounds
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RESEARCH PAPER
Towards a hand-held, fast, and sensitive gas chromatograph-ion mobility spectrometer for detecting volatile compounds André Ahrens 1
&
Stefan Zimmermann 1
Received: 2 September 2020 / Revised: 7 November 2020 / Accepted: 10 November 2020 # The Author(s) 2020
Abstract Ion mobility spectrometers can detect gaseous compounds at atmospheric pressure in the range of parts per trillion within a second. Due to their fast response times, high sensitivity, and limited instrumental effort, they are used in a variety of applications, especially as mobile or hand-held devices. However, most real-life samples are gas mixtures, which can pose a challenge for IMS with atmospheric pressure chemical ionization mainly due to competing gas-phase ionization processes. Therefore, we present a miniaturized drift tube IMS coupled to a compact gas chromatograph for pre-separation, built of seven bundled standard GC columns (Rtx-Volatiles, Restek GmbH) with 250 μm ID and 1.07 m in length. Such pre-separation significantly reduces chemical cross sensitivities caused by competing gas-phase ionization processes and adds orthogonality. Our miniaturized GC-IMS system is characterized with alcohols, halocarbons, and ketones as model substances, reaching detection limits down to 70 pptv with IMS averaging times of just 125 ms. It separates test mixtures of ketones and halocarbons within 180 s and 50 s, respectively. The IMS has a short drift length of 40.6 mm and reaches a high resolving power of RP = 68. Keywords Ion mobility spectrometer . IMS . Gas chromatograph . GC . Bundled GC columns . Hand-held GC-IMS
Introduction Drift tube ion mobility spectrometers (IMS) often consist of three axially arranged regions: the reaction, drift, and detector regions. Once a gaseous sample has been ionized in the reaction region by an ionization source, the ions are shifted into the drift region through an ion gate. A neutral drift gas flow at atmospheric pressure continuously flushes the drift region. Here, driven by a homogeneous electric field, the ions are separated on their way along the axis of the drift region based on their ion mobility. The low field ion mobility is defined as the proportionality factor between the mean drift velocity and the electric drift field at low reduced electric drift fields [1]. Low field conditions are typically given for reduced electric fields between 2 and 10 Td depending on the ion species [2]. The low field ion mobility can be determined by the length of
* André Ahrens [email protected] 1
Institute of Electrical Engineering and Measurement Technology, Department of Sensors and Measurement Technology, Leibniz University Hannover, Appelstr. 9A, 30167 Hannover, Germany
the drift region L, the drift voltage Ud, and the drift time td. However, since the low field ion mobility depends on the ionneutral pair collision cross section and the ion mass, often the reduced ion mobility K0 is given for comparable results [1, 3]. Here, the influence of a changing number of neutral gas particles due t
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