Soft Argon–Propane Dielectric Barrier Discharge Ionization

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Cite This: Anal. Chem. 2018, 90, 3537−3542

Soft Argon−Propane Dielectric Barrier Discharge Ionization Alexander Schütz,† Felipe J. Lara-Ortega,‡ Felix David Klute,† Sebastian Brandt,† Michael Schilling,† Antje Michels,† Damir Veza,§ Vlasta Horvatic,∥ Juan F. García-Reyes,‡ and Joachim Franzke*,† †

Leibniz-Institut für Analytische WissenschaftenISASe.V., Bunsen-Kirchhoff-Straße 11, 44139 Dortmund, Germany Analytical Chemistry Research Group, University of Jaén, Campus Las Lagunillas, 23071 Jaén, Spain § Department of Physics, Faculty of Science, University of Zagreb, Bijenicka 32, 10000 Zagreb, Croatia ∥ Institute of Physics, Bijenicka 46, 10000 Zagreb, Croatia ‡

S Supporting Information *

ABSTRACT: Dielectric barrier discharges (DBDs) have been used as soft ionization sources (DBDI) for organic mass spectrometry (DBDIMS) for approximately ten years. Helium-based DBDI is often used because of its good ionization efficiency, low ignition voltage, and homogeneous plasma conditions. Argon needs much higher ignition voltages than helium when the same discharge geometry is used. A filamentary plasma, which is not suitable for soft ionization, may be produced instead of a homogeneous plasma. This difference results in N2, present in helium and argon as an impurity, being Penning-ionized by helium but not by metastable argon atoms. In this study, a mixture of argon and propane (C3H8) was used as an ignition aid to decrease the ignition and working voltages, because propane can be Penning-ionized by argon metastables. This approach leads to homogeneous argon-based DBDI. Furthermore, operating DBDI in an open environment assumes that many uncharged analyte molecules do not interact with the reactant ions. To overcome this disadvantage, we present a novel approach, where the analyte is introduced in an enclosed system through the discharge capillary itself. This nonambient DBDIMS arrangement is presented and characterized and could advance the novel connection of DBDI with analytical separation techniques such as gas chromatography (GC) and high-pressure liquid chromatography (HPLC) in the near future. ielectric barrier discharge ionization (DBDI) was first coupled to mass spectrometry approximately ten years ago; the DBDI source was used for simultaneous desorption and ionization.1 In contrast, a dielectric barrier discharge can be used as an ambient soft ionization source for organic mass spectrometry (DBDI-MS) techniques such as atmospheric pressure chemical ionization (APCI).2−6 Analysis of solid, liquid, or gaseous samples can be performed directly in front of the mass spectrometer inlet and produces [M + H]+ and [M − H]−7,8 ions as well as other species.9,10 A common design is to therefore use a glass capillary with a noble gas inside and two ring electrodes outside or one ring electrode outside and a pin electrode inside. In most cases, high-purity helium is used as a plasma gas for DBDI. Atmospheric pressure ionization (API) techniques such as DBDI8 and other techniques such as the use of low-temperature plasma (LTP),11,12 direct analysis in real time (DART),13 atmospheric pressure solids analysis probes (ASAPs),14 and flowing atmospheric pressure afterglow (FAPA)15 are operated in ambient air. In contrast, active capillary dielectric barrier discharge plasma ionization can be mounted at the nozzle of a mass spectrometer.16 This possibility is desirable for DBDI-MS as well, because it could lead to an improved sample supply system and the highest probability for reactions between reactant ions and analytes.

D

© 2018 American Chemical Society

However, ambient air is necessary for helium DBD, resulting in a nitrogen impurity and limiting the application of DBDI in enclosed systems. The ionization mechanism for helium-based DBDI is shown in Figure 1 (left, blue background); excited helium atoms (He*) are generated by a collision of accelerated electrons with ground-state helium atoms. Helium metastables (HeM) with longer lifetimes are then formed by transitions from He*. Next, N2+ is generated by Penning ionization of N2 and HeM. The N2+ ions have an optical transition under an emission band at 391 nm. Finally, the classical reaction pathway is the ionization of water clusters, which leads to protonation of analyte molecules.17 The ionization mechanism for argon-based DBDI18 is illustrated in Figure 1 (right, yellow background). Understanding that the same discharge geometry requires a much higher voltage to ignite an argon plasma than a helium plasma is essential. Such higher voltages can result in a filamentary plasma dramatically decreasing the soft ionization efficiency.19,20 In contrast to a helium discharge, argon discharge Received: December 22, 2017 Accepted: February 20, 2018 Published: February 20, 2018 3537

DOI: 10.1021/acs.analchem.7b05390 Anal. Chem. 2018, 90, 3537−3542

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Article

EXPERIMENTAL SECTION

Safety Considerations. The use of high voltage mandates the use of precautions such as adequate isolation for energized cables, warning signs, and working without voltage when not necessary. Working with pressurized gas bottles can also be dangerous. The bottles have to be fixed at the wall at all times and closed if no gas is needed. Propane (C3H8) is an extremely flammable gas and has a low flammability limit in air. Chemicals and Solvents. Argon 5.0 (purity 99.999%) and propane 3.5 (purity 99.95%) were purchased from Messer Industriegase GmbH, Germany. For liquid chromatography/ mass spectrometry (LC/MS) experiments at the University of Jaén, a premixed bottle of argon 5.0 containing 3000 ppm of propane was bought from Air Liquide España S.A. Menthone (C10H18O) was obtained from Sigma-Aldrich and kept in a glass flask with a polytetrafluoroethylene (PTFE) lid. Plasma Gas. Argon with propane (500−10 000 ppm) was used in the experiments at ISAS. The gas mixtures were prepared in-house by using an evacuated gas bottle. First, the propane was injected into the empty gas bottle, while the concentration was calculated by a fixed gas flow rate. The gas bottle was then filled with argon until a pressure of 5 bar was reached, producing a parent solution of argon with 10 000 ppm of propane. Two mass flow controllers (Analyt-MTC Serie 358) were used: the first was connected to the argon supply in the lab, while the second was connected to the cylinder with the parent solution. Any specific concentration of propane in argon under 10000 ppm can thus be mixed by using pure argon with the parent solution. Atmospheric Pressure Ionization. The application of dielectric barrier discharge ionization for mass spectrometry (DBDI-MS) can be divided into two categories: direct desorption/ionization from a surface or its use as an API source.17 The focus of this study is only the use of DBDI as an API source. A dielectric barrier discharge (DBD) was built inhouse and used as the API source in the mass spectrometry experiments. Briefly, the DBD is powered by an in-house highvoltage square-wave generator and an integrated function generator that can generate a voltage of up to 3.5 kV and a variable, asymmetric pulse with a time base between 200 μs (5 kHz) and 33.33 μs (30 kHz). A homogeneous plasma can, depending on parameters, be ignited between two dielectric barrier ring electrodes (distance: 10 mm) that are sputtered on a glass capillary (id: 450 μm, od: 900 μm). Copper electrodes were sputtered on glass using magnetron sputtering (Balzers BAK-604) at the Lehrstuhl für Hochfrequenztechnik at Technische Universität Dortmund. Such a typical DBDI setup is shown in Figure 2.

Figure 1. Graphical illustration of energy levels for Penning ionization processes with helium−N2 and argon−propane mixtures.

is based on another ionization pathway: N2+ is not involved, because N2 molecules cannot be Penning-ionized by ArM. In principle, Ar+ ions have sufficient energy to ionize N2, but the density of ArM is approximately 5 orders of magnitude higher than that of Ar+.21 The Ar+ ions are thus negligible for the generation of N2+. Other dopant-assisted argon discharges have been previously reported: for example, Cody et al.22 used doped argon for DART. In the mass range of m/z 10−800, no ions were detected with pure argon; however, the use of a dopant led to the generation of the appropriate ions. Markowski et al.23 used argon−propane with a high-pressure ion source and a quadrupole mass spectrometer to study ion− molecule reactions. In contrast, propane was primarily used in this study as an ignition aid to decrease the ignition voltage of an argon− propane gas mixture and was furthermore responsible for the ionization. This use has also been shown by Heylen:24 argon doped with propane lowers the ignition voltage by a factor of 3.5, while argon doped with methane increases the ignition voltage. Therefore, the ignition aid needs to have an ionization energy lower than that of ArM (11.54 eV). Propane fulfills this requirement with its ionization energy of 10.94 eV. This work must also be considered in the context of chemical ionization (CI). Historically, CI was one of the first soft ionization techniques for mass spectrometry.2,3 Methane (CH4), ethane (C2H6), and ammonia (NH3) were typically used as chemical dopants, while propane seems to have been an exception.2,4−6 Methane is commonly used for CI and has an ionization energy of 12.7 eV, which is higher than the ArM energy level. Tal’roze and Frankevich reported “the formation of hydronium ion from propane ion and water” in 1956.25 A recent publication by Wolf et al.26 supplemented the commonly understood mechanism of the formation of hydronium clusters with a radical-mediated pathway for plasma-based ion sources. This pathway might bring an alternative explanation of soft ionization with DBDI in general and specifically for argon−propane mixtures. Furthermore, argon has some advantages as a carrier gas with regard to vacuum; the vacuum pumps of a Thermo-Fisher LCQ Deca XP and a Thermo-Fisher LTQ XL performed much better with argon than helium. The vacuum conditions are now comparable with ambient air and might prevent secondary limitations. Further details are included in the supplementary data.

Figure 2. Experimental setup of a DBD ion source in front of a mass spectrometer (DBDI-MS) with coincident plasma and plasma jet. The analyte (green arrow) is supplied directly into the plasma jet via headspace. 3538

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Analytical Chemistry Optical Emission Spectrometer. An Ocean Optics USB4000 spectrometer was used for optical emission spectroscopy (OES). The waveguide was installed perpendicular to the discharge capillary in between the high-voltage and ground electrodes. Ion-Trap Mass Spectrometer. An ion-trap mass spectrometer (Thermo-Fisher LCQ Deca XP) with activated automatic gain control (AGC) was used. An in-house-built housing made of polyether ether ketone (PEEK) was attached to the inlet of the mass spectrometer. Figure 3 contains a schematic of an adapter that was used in this study.

coincident plasma is switched off by using an asynchronous square-wave signal.28 In contrast to the setup in Figure 2, argon−propane DBDI must be used in this case. This necessity is because helium DBDI is based on a specific mixture of He and N2 even though argon 5.0 contains 4 ppm of N2 as an impurity. Furthermore, the reduced amounts of N2 and H2O lead to decreased ionization efficiency. In addition, the development of the plasma jet is a time-dependent process that is sensitive to the concentrations of these two gases.27 When no coincident plasma is present, the analyte can be supplied via the carrier gas through the glass capillary.

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RESULTS AND DISCUSSION An argon−propane-based DBD was initially ignited so that its OES could be studied. In Figure 4, the emission spectrum of

Figure 3. Experimental setup of an enclosed DBDI-MS mounted on the inlet of the mass spectrometer: The coincident plasma is switched off, because the analyte is then supplied via the carrier gas through the discharge capillary itself.

Implementation of DBDI with LC/TOFMS. An Agilent 1290 Infinity ultrahigh-performance liquid chromatography (UHPLC) system (Agilent Technologies, Santa Clara, CA, U.S.A.) equipped with a C18 UHPLC column (100 × 3.0 mm, 1.8 μm particle size (ZORBAX Eclipse Plus C18)) was used for calibration. For each determination, 20 μL of sample extract was injected. Mobile phases were Milli-Q water (A) and methanol (B), both with 0.1% (v/v) formic acid. The gradient program was as follows: at the beginning, 10% B; 10% B (3 min); 70% B (5 min); 100% B (15 min); and 100% B (17 min). Finally, a post run of 10% B for 3 min was applied to return to initial conditions, resulting in a total of 20 min per sample. The flow rate was 0.4 mL min−1. The UHPLC system was connected to an Agilent 6220 time-of-flight (TOF) mass spectrometer (TOFMS) using a dielectric barrier discharge ionization (DBDI) probe coupled to a commercial APCI source housing with the corona needle removed. A photograph of the experimental setup can be found in the supplementary data as Figure S-1. The method was developed in positive iondetection mode, and the following operation parameters were used: gas temperature: 325 °C; vaporizer: 350 °C; drying gas flow rate: 3.4 L min−1; nebulizer pressure: 40 psi; capillary voltage: 3500 V; fragmentor voltage: 190 V, skimmer voltage: 65 V; and octopole rf: 250 V. High-resolution TOFMS mass spectra were recorded in the range from m/z 50−1000. The full-scan data was processed with Agilent MassHunter Software (version B.04.00). Different DBDI-MS experiments were designed for this work. Figure 2 illustrates DBDI in front of an MS inlet at ambient conditions in an open environment. The plasma consists of two parts: the coincident plasma between the electrodes and the plasma jet, which is outside of the glass capillary. A detailed description of the mechanism can be found in previous publications.27,28 Here, the analyte is introduced into the plasma jet to generate analyte ions. This setup can be used for helium or an argon−propane DBDI, although helium DBDI requires nitrogen and water from the ambient air. Figure 3 shows the second type of DBDI-MS experiment. An adapter built in-house was mounted on the mass spectrometer inlet to sequester the DBDI from the ambient air. The

Figure 4. Optical emission spectrum of argon with 1000 ppm of propane measured between electrodes (3.5 kV, duty cycle 50/50).

the discharge is shown, and its characteristic spectral lines are marked. The strongest emission lines and molecular bands belong to N2 (3 ppm present as an impurity in argon 5.0), CH, and C2 (Swan bands). The high voltage of 3.5 kV and duty cycle (50/50) caused high fragmentation and dissociation of the molecule. This plasma is clearly not favorable for soft ionization. The propane fragments will certainly form novel clusters, which might be the reason for the appearance of ions in the mass spectra. On the other hand, the propane and its fragments might be the proton donor in the ionization process. Analogous to CI at low pressure, which is usually based on methane (CH4), the propane may contribute H+ for protonation and possibly generate further reactive species. Figure 4 shows that a significant amount of CH and C2 are present in the discharge. Nevertheless, propane ions were also simultaneously found in the low-mass range with a mass spectrometer (see Figure S-2). Klute et al.18 used a mixture of argon with 1000 ppm of propane to mostly prove the operation of a DBD by OES. However, this study was a proof-of-concept experiment showing that propane molecules in an argon DBD play an analogous role as the N2 molecules in a helium DBD. A comparison to helium was made without any further studies of the advantages of such gas mixtures as soft ionization sources for organic mass spectrometry. In the second part of the experiment, the argon−propane DBD was operated in front of a mass spectrometer under ambient conditions as shown in Figure 2. Menthone (C10H18O) was introduced as an analyte into the plasma jet 3539

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Analytical Chemistry via the headspace using an argon gas flow of 10 mL/min to test this setup’s ionization capabilities. The resulting mass spectrum is in Figure S-3 and shows that the protonated peak of menthone is the base peak of the spectrum at m/z 155; many ions are present near the peak in the mass range from m/z 50− 500. The appearance of such clusters was predicted when propane fragments were observed by OES (Figure 4). A strong fragmentation of propane is assumed to occur when the ionmj m trap was overloaded. The distance between peaks, Δ( z i − z ), inside the pattern is m/z 14, which might be related to the predicted fragments. These fragments might produce the new ionized molecules that are present in the upper mass spectrum in Figure 5.

system may be used instead of ambient air. Furthermore, the DBDI setup can now be mounted in front of a mass spectrometer, and the analyte can now be introduced via carrier gas through the glass capillary as shown in Figure 3.27,28 The place of ionization is shielded from ambient air to prevent disturbing background ions in the spectra. The analyte is carried through the whole capillary with the plasma gas itself. Here, the idea is to increase interactions between uncharged analyte molecules and the reacting species from the plasma to gain sensitivity. The mass spectrum is further optimized by varying the propane concentration. With a square-wave high voltage of 3.5 kV at a frequency of 20 kHz and a 50/50 duty cycle, the ignition started at a propane concentration of 500 ppm (with a 300 mL min−1 gas flow of the carrier gas). The optimal concentration of propane in argon was in the range from 1000−3000 ppm. Higher concentrations quenched the signal intensity, while lower concentrations did not function as an ignition aid. The results from the enclosed system can be seen in Figure 6, where menthone was directly introduced into the plasma gas. The top mass spectrum shows the initial spectrum under hard plasma conditions.

Figure 5. Mass spectra of menthone (C10H18O) with an argon DBD with 1000 ppm of propane. Upper part: the mass spectrum shows a strong fragmentation of propane by its characteristic pattern. Bottom: the optimization of the voltage to 3.2 kV and of the duty cycle to 25/ 75 (“tuning”) leads to a significantly improved signal-to-noise ratio.

Previous publications showed that optimization of parameters such as the high voltage and duty cycle can strongly influence the resulting mass spectrum.28 This relation is due to the fact that the plasma consists of temporally (early plasma and coincident plasma) and spatially (plasma jet and interelectrode plasma) separated events. For soft ionization, the analyte may be exclusively introduced in the plasma jet.27,28 This technique is the basis for using an argon−propane mixture to prevent dissociation, fragmentation, or the building of clusters from the analyte molecules during soft ionization. The upper part of Figure 5 illustrates the nonoptimized (“untuned”) mass spectrum. The fragmentation of propane is observed when high voltages of 3.5 kV are applied at 20 kHz with a 50/ 50 duty cycle (25 μs/25 μs). The spectrum obtained using optimized plasma conditions is shown in the bottom part of the figure. Here, the tuning of the plasma makes it suitable for soft ionization in organic mass spectrometry: using a lower voltage of 3.2 kV at 20 kHz with a 25/75 duty cycle led to a much higher signal intensity, prevented the formation of cluster ions, and resulted in an improved signal-to-noise ratio (SNR). While the dissociative part of the plasma is suppressed to increase the soft ionization efficiency, a tiny part of the propane will still dissociate to deliver enough H+ for protonation. In a previous publication, the DBD plasma was mainly tuned to improve the SNR, but it was also tuned to prevent undesirable reactions.28 Since an argon−propane DBD does not depend on the presence of nitrogen molecules from ambient air,18 an enclosed

Figure 6. Mass spectra of menthone (C10H18O) in an enclosed system, in which the analyte was directly introduced into the plasma gas. In the first panel, the plasma conditions are not optimal for soft ionization. In the second and third panels, a decrease in the voltage leads to higher intensities at two typical concentrations of propane in argon. The fourth panel shows that operating with optimized parameters (voltage, duty cycle, and propane concentration) yields an improved mass spectrum. In addition, the mass spectrum shows a monomer and dimer of menthone when the plasma conditions were optimized. The label “CP: off” refers to when the coincident plasma was switched off.

The second mass spectrum shows that a decrease in the voltage leads to better signal intensities. In addition, a slight increase of the signal was observed when 1000 ppm of propane was used instead of 1500 ppm of propane, while the last mass spectrum, which was obtained with optimized parameters, had the best results. A protonated dimer [2M + 1]+ = m/z 309 was detected in addition to the protonated monomer [M + 1]+ = m/z 155 due to the high concentrations of the model analyte. The same concentration was used in all cases, while DBDI without coincident plasma yielded the highest intensities of the monomer and dimer signals. These conditions increased the sensitivity for the detection of lower concentrations. 3540

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Analytical Chemistry An enclosed system also increases the probability of interaction between the reactive species and the analyte molecules. This benefit and the high concentrations that were used might be the reasons for the presence of dimer, which was not observed when the open setup was used. Finally, all of the gas mixture with the ionized analyte molecules is pumped into the mass spectrometer, which is a huge advantage over the open system in Figure 2. At this point, the results from the argon−propane DBDI-MS experiments have to be compared to those obtained with wellestablished ion sources such as APCI and electrospray ionization (ESI) to determine the advantages of using of argon−propane DBDI. The experiment is based on a UHPLC/ TOFMS (Agilent 6220/Agilent 1290 Infinity) instead of an ion-trap-based mass spectrometer. The LC/TOFMS setup was used in a previous publication for pesticide residue analysis with ESI,29 and a photograph of the experiment is included in the supplementary data as Figure S-1. The technical limitations of LC/TOFMS, i.e., the high flow rates of the thermovaporizer from the HPLC, rendered experiments with enclosed environments impossible. However, this study uses a direct and enclosed coupling approach for novel DBDI-MS-based experiments that offers new combinations with gas chromatography, which is comparable to the GC/DBDI-MS setup by Mirabelli et al.30 Here, 11 analytes were tested to compare each configuration with helium and argon−propane DBDI against ESI and APCI. A set of three different parameters were used for both gases (Table 1). The precision data of the DBDI experiments with argon−propane and helium can be found in Table S-1.

Figure 7. LC/TOFMS calibration curve on a double-log scale with helium (he3) and argon−propane (ar3) DBDI for atrazine (C8H14ClN5). The dashed line represents a linear function.

a

parameters (Table 1) for helium and argon−propane experiments on the resulting slopes are shown in Tables S-2 and S-3, respectively. Finally, the analytical performances of argon− propane DBDI, ESI, APCI, and helium DBDI are compared in Table S-4. The results generally agree with the previous finding that suppression of the coincident plasma (he3, ar3) yields higher peak areas in the chromatogram than the he1, he2, ar1, and ar2 experiments. Moreover, peak areas with ar3 were better than those from APCI in all cases. Additionally, the slopes of the peak areas of argon−propane DBDI of aflatoxin (C17H12O7) and propachlor (C11H14ClNO) against their concentrations were higher than those of ESI with these compounds. In most cases, the optimized argon−propane DBDI (ar3) slope is slightly smaller compared to that of helium DBDI with optimized parameters (he3). Argon−propane DBDI can be more sensitive than helium DBDI without coincident plasma suppression. However, argon−propane is advantageous for an enclosed environment, because helium will not work there due to the lack of nitrogen and water from the ambient air.

The supplementary data contains high-resolution m/z values of the detected species and calibration curves as well as further details about the experiments. Figure S-4 shows a comparison of two representative compounds to show that the mass spectra do not significantly differ. Both helium and argon−propane DBDI result in the same mass spectra. Figure S-5 contains overlapped extracted ion chromatograms (XIC) of propachlor and yields a precision of ca. 5%, which is similar to that of ESI and APCI. Exemplary calibrations with helium and argon−propane DBDI for atrazine (C8H14ClN5) are presented in Figure 7. For the calibration with LC/TOFMS, the optimized plasma parameters he3 and ar3 were used. The concentration of the analytes ranged from 1−1000 ppb. In the double-log plot, the dashed line represents a linear function (slope = 1) to compare the atrazine calibration curves when helium and argon− propane were used. The influences of the main DBDI

CONCLUSION This study showed that argon−propane DBD can be used as a soft ionization source for organic mass spectrometry. While the mechanism for a helium DBD seems to be completely understood, this study questions whether this pathway is correct and complete. The results with this gas mixture point to a more complex mechanism that is responsible for any plasmabased ionization. While these results are proof that argon− propane DBDI is an alternative for helium DBDI, the ionization pathway itself cannot be proven here. In the case of helium, the ionization of analyte molecules by a DBD is usually explained by the formation of hydronium clusters, but this reason is not valid for argon-based ion sources. Nevertheless, experiments with argon−propane gas mixtures mainly show the same mass spectra as those with helium, while the ionization mechanism has to be completely different. Mass spectra from the positive low-mass range show that propane is an ignition aid, and its ion simultaneously acts in CI. Finally, the LC/TOFMS data proves that tuned argon−propane DBDI can be applied for an analytical approach, and its performance is comparable to that

Table 1. Varying Experimental Conditions Employed for Each Gas Typea abbreviation

gas type

voltage [V]

duty cycle

frequency [kHz]

coincident plasma

he1 he2 he3 ar1 ar2 ar3

He He He Ar + C3H8 Ar + C3H8 Ar + C3H8

3500 2500 2000 3500 2500 2070

50/50 50/50 3/97 50/50 50/50 3/97

20 20 20 20 20 20

on on off on on off

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A fixed concentration of 3000 ppm of C3H8 in argon was used, while the parameters voltage and duty cycle were varied. The coincident plasma was present or switched off depending on these parameters.

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of helium DBDI. A secondary benefit of using an enclosed argon−propane DBDI could be the formation of smaller amounts of oxidized species that appear often with API sources. To conclude, the ionization mechanisms in plasma are likely a composite of different individual processes that need to be elucidated.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b05390. Photographs of the DBDI interface with the LC/ TOFMS instrument using the housing from the APCI source; positive mass spectrum in the low-positive-mass range with propane ion; mass spectrum of an argon DBD with 1000 ppm of propane; comparison of LC/DBDITOFMS spectra using helium and argon−propane; overlapped extracted ion chromatograms of propachlor using LC/DBDI-TOFMS with argon−propane; calibration plots of: (a) propachlor and (b) atrazine using LC/ DBDI-TOFMS with argon−propane; identification parameters and precision data (RSD (%) (n = 8) for a suite of 11 compounds studied by LC/DBDI-TOFMS with argon−propane as discharge gas; optimization of main parameters (amplitude voltage and duty cycle) for Ar propane DBDI using multiclass representative compounds; optimization and comparison of helium DBDI using different voltages and duty cycles; comparison of the analytical performance of optimized argon−propane DBDI with ESI, APCI, and heliumDBDI; additional information about vacuum conditions of mass spectrometer and helium/argon−propane DBDI (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +49 231 1392-174; Fax: +49 231 1392-120 (J.F.) ORCID

Alexander Schütz: 0000-0002-2741-7397 Felix David Klute: 0000-0002-4694-3093 Sebastian Brandt: 0000-0002-6938-3033 Michael Schilling: 0000-0002-3505-686X Joachim Franzke: 0000-0003-0419-1898 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein-Westfalen, the Senatsverwaltung für Wirtschaft, Technologie und Forschung des Landes Berlin, the Bundesministerium für Bildung und Forschung, and the Deutsche Forschungsgemeinschaft is gratefully acknowledged. F.J.L-O. and J.F.G-R. also acknowledge funding from Junta de Andalucia (Project ref. FQM-2242) and the Spanish Ministerio de Economia y Competitividad through Projects ref. CTQ 2012-34297 and CTQ-2015-71321P. Special thanks go to Mikheil Gogiashvili for the translation of the original Russian publication from Tal’roze and Frankevich into German. 3542

DOI: 10.1021/acs.analchem.7b05390 Anal. Chem. 2018, 90, 3537−3542