Electrostatic Switching and Selection of H3O+, NO+, and O2+

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Electrostatic switching and selection of HO, NO and O reagent ions for selected ion flow-drift tube mass spectrometric analyses of air and breath Patrik Spanel, Anatolii Spesyvyi, and David Smith Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00530 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Analytical Chemistry

Electrostatic switching and selection of H3O+, NO+ and O2+• reagent ions for selected ion flow-drift tube mass spectrometric analyses of air and breath. Patrik Španěl, Anatolii Spesyvyi, David Smith J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences, Dolejškova 3, 18223 Prague 8, Czech Republic. ABSTRACT: Soft chemical ionization mass spectrometry techniques, particularly the well-established proton transfer mass spectrometry, PTR-MS, and selected ion flow tube mass spectrometry, SIFT-MS, are widely used for real time quantification of volatile organic compounds in ambient air and exhaled breath with applications ranging from environmental science to medicine. The most common reagent ions H3O+, NO+ or O2+• can be selected either by quadrupole mass filtering from a discharge ion source, which is relatively inefficient, or by switching the gas/vapour in the ion source, which is relatively slow. The chosen reagent ions are introduced into a flow tube or flowdrift tube reactor where they react with analyte molecules in sample gas. This paper describes a new electrostatic reagent ion switching, ERIS, technique by which H3O+, NO+ and O2+• reagent ions, produced simultaneously in three separate gas discharges, can be purified in post-discharge source drift tubes, switched rapidly and selected for transport into a flow-drift tube reactor. The construction of the device and the ion-molecule chemistry exploited to purify the individual reagent ions are described. The speed and sensitivity of ERIS coupled to a selected ion flow-drift tube mass spectrometry, SIFDT-MS, is demonstrated by the simultaneous quantification of methanol with H3O+, acetone with NO+ and dimethyl sulphide with O2+• reagent ions in single breath exhalations. The present ERIS approach is shown to be preferable to the previously used quadrupole filtering as it increases analytical sensitivity of the SIFDT-MS instrument whilst reducing its size and the required number of vacuum pumps.

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Real time direct analyses of trace concentrations of volatile organic compound, VOC, vapours and other gases are of interest in many areas1 ranging from air quality monitoring, environmental research,2 food,3-4 industrial process monitoring,5-6 health and safety7 to medicine as represented by non-invasive breath analyses for disease diagnostics and therapeutic monitoring.8-9 Two techniques are widely used for this purpose: selected ion flow tube mass spectrometry, SIFTMS,10-11 and proton transfer reaction mass spectrometry, PTR-MS.12-14 They both exploit soft chemical ionization by reactions of specific reagent ions occurring in a flow tube reactor (SIFTMS) or a flow/drift tube reactor (PTR-MS). Concentrations (volume mixing ratios) of the trace gas analytes are determined from the count rates of the reagent and analyte ions detected by a downstream mass spectrometer. In SIFT-MS, the reagent ions can be either H3O+, NO+ or O2+•, which are created collectively in a microwave discharge through humid air and individually injected into the carrier gas via a quadrupole mass filter, QMF. The gas sample to be analysed is introduced into the carrier gas, usually helium, at a known flow rate. In PTR-MS, as originally conceived, H3O+ reagent ions only are generated in a hollow cathode discharge and directly introduced into the carrier gas, which is the air sample containing the analyte trace gases. A subsequent innovation of PTR-MS is the switchable reagent ion, SRI, version in which a different gas is introduced into the discharge to generate individually the H3O+, NO+ and O2+•, but this procedure takes several seconds.15 Very recently developments in PTR-MS have been presented16 of a triple ion source (TRION) that facilitates rapid, split second switching between H3O+, NO+ and O2+• reagent ions from three separate discharges arranged perpendicularly to the drift tube axis. Similar development has been recently implemented17 also in a chemical ionization mass spectrometer based on the PTR3 instrument (Ionicon, Innsbruck, Austria) that allows switching between the NH4+(H2O)n, (n=0, 1, 2) and H3O+(H2O)n, (n=0, 1) ionization modes in two minutes using two corona discharge ion sources. In SIFT-MS, the reagent ions are routinely rapidly switched by QMF and analysis of the same sample can be achieved using the three reagent ions without changing the discharge gas composition. Recently, other developments in the soft chemical ionization MS techniques have been introduced to increase versatility, sensitivity and selectivity, including negative reagent ions,6 the orthogonal accelerated TOF mass spectrometer,15, 18-19 RF fields within the drift tube reactor,20-22 or fast E/N switching23. The strengths and weaknesses of SIFT-MS and PTR-MS have been detailed previously.24-25 Whilst SIFT-MS benefits from the rapid switching of reagent ions, PTR-MS benefits from higher reagent ion count rates and hence a higher analytical sensitivity. Recently, we have described the selected ion flow-drift tube mass spectrometry, SIFDT-MS, technique that combines the best features of SIFT-MS and PTR-MS, by retaining the QMF reagent ion selectivity and rapid switching, realizing higher reagent ion count rates and introducing accurate determination of the reaction time.26-27 Intube collision-induced dissociation in SIFDT-MS has been described for the identification of isomeric ions,28 and the energy dependence of reaction kinetics has been investigated.29 Here, we present a further development of SIFDT-MS in the form of a new electrostatic reagent ion switching system, ERIS, to replace the QMF. It is based on rapid selection of the reagent ions from three separate, continuously running parallel gas discharges30 by changing the transport electric fields in an arrangement of three post-discharge source drift tubes, SDT. One of the objectives was to achieve the speed of reagent ion switching comparable to the QMF to enable analyses of several compounds by different reagent ions with a time resolution sufficient for online analysis of single breath exhalations and for food flavour release studies.25, 31 The second objective was to increase the reagent ion signal significantly above that attained by QMF injection.


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The construction and operation of ERIS are described and it is demonstrated how it can be used in SIFDT-MS for sensitive and versatile trace gas analysis. The use of ERIS is preferable because it allows significant simplification of the SIFDT instrumentation by removing the QMF and associated radiofrequency electronics and, very significantly, also eliminating the turbomolecular pump that is needed for its evacuation. These simplifications are shown to be possible without any loss of sensitivity.

EXPERIMENTAL SECTION Construction of the Electrostatic Reagent Ion Switching system, ERIS. The ERIS system consists of three separate hollow cathode discharges32-33 each producing one of the H3O+, NO+ or O2+• reagent ion types from dedicated source gas (H2O vapour, N2/NO mixture and O2). The structural geometry of ERIS is shown in Figure 1 (see also Figure S1 for a detailed drawing). Each of the three discharges has its own hollow cathode and a plate anode with a 1 mm diameter aperture at its centre through which the generated ions can pass. Each anode is immediately followed by a 45 mm long drift tube consisting of a stack of twenty-three 0.5 mm thick stainless steel electrodes each with a central circular aperture of 4 mm diameter, separated by 1.5 mm thick PTFE insulator with 5 mm diameter central aperture. Voltages are applied to the stack electrodes using a resistor network (twenty-two 1 M), thus creating a drift tube.

Figure 1. Schematic view of the cross section of the ERIS assembly, where just one of the three ion source channels is visible. The denoted parts are polypropylene chamber (1); PEEK flange with mounted ERIS (2); hollow cathodes (3) and anodes (4); source drift tube (SDT) electrodes, first (5) and last (6); focusing lenses (7) and (8); focusing and gating lens (9); resistive glass drift tube reactor DTR (10). The PTFE insulators are in green. The figure is to actual scale with the drift tube reactor (10) having an internal diameter of 1 cm. Also shown are the directions of flow of the ion source gases, the helium purge gas, and the sample gas and DTR helium buffer gas.


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The post-discharge source drift tubes, SDTs, are essential components that allow ion chemistry to occur between the ions generated in the gas discharge and the particular source gas in order to produce the desired current of pure H3O+, NO+ or O2+• reagent ions (see the Ion chemistry and reagent ion purity section). Switching between reagent ions is accomplished by simultaneously changing the voltages in the three SDTs. The E-field in one chosen SDT is set to allow positive ions from the discharge to pass through whilst the other two SDTs are biased to prevent the passage of ions. Each SDT is terminated with a focusing electrode F1 (No. 7 in Figure 1) with a 4 mm diameter aperture and an additional focusing lens F2 (No 8). A further focusing and gating lens F3 (No 9) with 6 mm aperture allows ion current modulation for the Hadamard method of determining the reagent ion residence times in the drift tube reactor, DTR (No 10).26 Note that the volume between F1, F2 and F3 electrodes is purged by clean helium. Figure S2 in the Supporting Information shows their detailed arrangement together with SIMION34 calculated trajectories of the drifting ions. The DTR is in the form of a cylindrical resistive glass tube (10 mm internal diameter; 145 mm length) along which a uniform axial electric field is established. It is important to note that the helium carrier/buffer gas and the air containing the neutral analyte molecules are introduced at the end of the DTR opposite to the reagent ion entry position. This “counterflow” is intended to prevent the passage of discharge source gases into the DTR. All three discharge gases and the helium/sample gas are pumped via the surrounding chamber (No 1 in Figure 1) by a single vacuum pump to maintain a common pressure throughout the ERIS and DTR, which is normally about 1 mbar. Drift tube ion current is measured at an electrode placed at the end of DTR, which has the ion sampling orifice (0.5 mm diameter) at its centre. The reagent and analyte ions are sampled via this orifice into the analytical quadrupole mass spectrometer, QMS, with a pulse counting electron multiplier detector. The QMS chamber is differentially pumped to an appropriately low pressure (~ 10-5 mbar) by a turbo pump.26-27

Ion source conditions. H2O vapour (taken from pure water) is used in the H3O+ channel; a 1% mixture of NO in N2 (Messer Group GmbH) in the NO+ channel; pure O2 (Messer 99.999%) in the O2+• channel. These gases flow separately through the hollow cathode discharges and associated SDTs where ion chemistry occurs that “purifies” the reagent ions. The flow rates of the ion source gases (90% for O2+•. The newly introduced ERIS as the reagent ion selector is preferable to the traditional QMF for SIFDT-MS as it: (i) (ii) (iii)

avoids the QMF, which simplifies the electronics by using DC voltage supplies rather than RF supplies uses low power DC electrical discharges as the ion sources instead of a 100 W magnetron with its high voltages eliminates one turbomolecular pump and functions with a smaller fore-vacuum pump.


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Along with all these obvious benefits, the analytical performance of the ERIS/SIFDT-MS combination is significantly better than of conventional SIFT-MS and SIFDT-MS instruments (as demonstrated by calibration tests and breath analysis) by virtue of higher reagent ion count rates, yet retaining the rapid switching between the reagent ions. ERIS principles could be adopted in other soft chemical ionisation analytical techniques, including SIFT-MS and PTR-MS. The only apparent undesirable feature of this first ERIS arrangement compared to QMF ion selection is the introduction of minor impurity ions into the DTR, including fluorocarbon ions from the PTFE insulators exposed to active discharges. Thus, the next version of ERIS will be constructed using less surface reactive insulating materials (e.g. ceramics). Other ideas for future developments and operation of the ERIS/SIFDT-MS combination include the use of ambient air sample in the DTR (avoiding helium as the carrier gas), thus facilitating air analysis in remote locations, and the use of a TOF-MS analyser, by which an entire analytical spectrum could be obtained during each ERIS channel switched-on period. These developments open up new vistas of a generation of smaller, more sensitive and lower-cost SIFDT instruments.

Author Information Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We would like to thank Kristýna Sovová for useful discussions and for providing the motivation for the present work. We gratefully acknowledge financial support from The Czech Science Foundation (GACR Project No. 17-13157Y). References 1. Bylinski, H.; Gebicki, J.; Dymerski, T.; Namiesnik, J., Direct Analysis of Samples of Various Origin and Composition Using Specific Types of Mass Spectrometry. Crit. Rev. Anal. Chem. 2017, 47 (4), 340-358. 2. Materic, D.; Bruhn, D.; Turner, C.; Morgan, G.; Mason, N.; Gauci, V., Methods in plant foliar volatile organic compounds research. Applications in Plant Sciences 2015, 3 (12), 10. 3. Wang, Y.; Li, Y. X.; Yang, J. L.; Ruan, J.; Sun, C. J., Microbial volatile organic compounds and their application in microorganism identification in foodstuff. TrAC 2016, 78, 116. 4. Deuscher, Z.; Andriot, I.; Sémon, E.; Repoux, M.; Preys, S.; Roger, J.-M.; Boulanger, R.; Labouré, H.; Le Quéré, J.-L., Volatile compounds profiling by using proton transfer reactiontime of flight-mass spectrometry (PTR-ToF-MS). The case study of dark chocolates organoleptic differences. J. Mass Spectrom. 2019, 54 (1), 92-119.


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Electrostatic Switching and Selection of H3O+, NO+, and O2+

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