Glow Discharge Plasma Ionization Mass Spectrometry for

ANALYTICAL SCIENCES FEBRUARY 2020, VOL. 36 269

Introduction

Suspended particulate matter of PM2.5 and PM10 aerosol, which are said to be harmful substances on human health, have attracted attention worldwide. Volatile organic compounds (VOCs) and semi-VOCs (SVOCs) arising from anthropogenic and natural emissions are oxidized photochemically by sunlight in the atmosphere.1,2 These oxidized products may coagulate due to their low vapor pressures, then forming secondary organic aerosols (SOA). The SOA is mainly composed of various organic components, which have not been completely explained on their source emission processes.3 Therefore, the real-time monitoring of VOCs as a precursor of SOA is very useful for the emission reduction of VOCs.

VOCs in ambient air are normally analyzed using off-line techniques by solvent extraction or thermal desorption coupled with chromatographic separation using gas chromatography (GC) or liquid chromatography (LC). The measurement of formaldehyde is carried out after derivatization with 2,4-dinitrophenylhydrazine (DNPH) is determined by high-performance liquid chromatography (HPLC).4,5 GC/mass spectrometry (MS) combined with solid-phase adsorption/

solvent extraction6,7 or solid-phase adsorption/thermal desorption8–10 is widely used for the determination of other VOCs. The DNPH-HPLC and GC-MS techniques need complicated pretreatment processes with high cost and are time-consuming, leading to low reproducibility. Therefore, novel direct mass spectrometry should be developed for the real-time analysis of VOCs by direct sample introduction without any pretreatment.

Direct analysis in real time (DART), which ionizes analytes by excited (metastable) He atoms, is an ionization technique based on an atmospheric pressure chemical ionization (APCI) method, which was developed by Cody et al.11,12 It was reported that excited He atoms in glow discharge plasma generated between a needle electrode and an annular disc electrode were released to air, where the analyte was protonated via water.13 This method is capable of detecting organic components for on-line monitoring regardless of solid, liquid or gaseous sample forms without any complicated pretreatment processes.2 However, DART ionization mainly takes place by proton attachment to analytes, resulting in the formation of protonated ions, [M+H]+, in the mass spectrum. In DART, the analyte can be ionized via proton transfer when the analyte has a higher proton affinity.

We have successfully developed a soft plasma ionization (SPI) source for the purpose of direct analysis in mass spectrometry using glow discharge plasma.14–17 Glow discharge tube is mostly

2020 © The Japan Society for Analytical Chemistry

† To whom correspondence should be addressed.E-mail: [email protected]

Glow Discharge Plasma Ionization Mass Spectrometry for Direct Detection of Oxygenated Organic Compounds in the Gas-phase

Yoko NUNOME,*† Kenji KODAMA,** and Kazuaki WAGATSUMA***

* Graduate School of Integrated Sciences for Life, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739–8521, Japan

** X-ray Instrument Division, Rigaku Corporation, 14-8 Akaoji, Takatsuki, Osaka 569–1146, Japan ***Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi 980–8577, Japan

This study describes a direct analysis of oxygenated organic compounds, such as ketones, esters and ethers, rapidly and easily using a soft plasma ionization (SPI) source combined with a Q-mass spectrometer. A related molecular ion, [2M+H]+, in which a sample molecule (M) can undergo protonation via water clusters, such as [(H2O)n+H]+ and [N2(H2O)n+H]+, in an ambient air glow discharge plasma, can be dominantly detected as a base peak with little or no fragmentation at a pressure of several kPa. Oxygenated organic compounds with high proton affinity were found to generate their dimers through the hydrogen bonding interaction at higher pressures. A deuterated solvent was used to examine whether or not the adduct ion of analyte was derived from the solvent. The formation of [2M+H]+ strongly depended on the time. A two-dimensional spectrometer was used to obtain the distribution of several excited species and then to confirm the ionization reactions of the analyte in the SPI source. The sample molecule would be readily ionized through Penning-type collisions with excited N2, which causes fragmentation for oxygenated compounds due to the lower pressures (approx. 1.0 kPa) while it is ionized by an attachment with protons from water clusters at higher pressures (several kPa). The SPI source can be a new and powerful tool for soft ionization in direct analysis of volatile organic compounds (VOCs).

Keywords Glow discharge plasma, oxygenated organic compounds, mass spectrometry, two-dimensional emission images, ambient air

(Received July 3, 2019; Accepted September 30, 2019; Advance Publication Released Online by J-STAGE October 11, 2019)

270 ANALYTICAL SCIENCES FEBRUARY 2020, VOL. 36

classified into a parallel flat plate type and a Grimm type (an anode plate and a hollow cathode), while the SPI source has newly designated coaxial cylindrical electrodes composed of a Cu-hollow anode and a Cu-mesh cathode. The glow discharge plasma generated between these electrodes ionizes organic components. Fast electrons released from the cathode would be restrained between the electrodes, while excite/ionize gas species (G*, G+) generated by the fast electrons can be distributed through the mesh cathode. Therefore, sample molecule can be softly ionized by Penning ionization (PI) with excited gas species (G*) and/or adduct ionization with an attachment of ionized gas species (G+) because the sample is introduced into the central portion of the discharge tube. The glow discharge plasma can stably generate several kinds of reactive ions, and a plasma using ambient air can be obtained at pressures of several kPa in the SPI source. Our previous work has shown that aromatic hydrocarbons can be ionized through the attachment of NO+, which would be derived from ambient air.16 The reactions of aromatic hydrocarbons with these ions easily occured at a higher pressure (several kPa) in an ambient air discharge. Another study of ours was conducted in an oxidation reaction with saturated hydrocarbons in an air plasma at a pressure of several kPa, indicating that the saturated hydrocarbons were observed as molecular ions, [M+13]+, which could be assigned to an oxidation product, [M+O–3H]+.17 The saturated hydrocarbons could be ionized via step-wise reactions, such as hydride abstraction with O2

+•, a dehydrogenation reaction of [M–H]+ and subsequently an oxidation reaction of [M–3H]+ with O3. The ionization reactions of the saturated hydrocarbons are proven by using a deuterated solvent. Unlike DART, in the SPI ionization technique it can be expected that various organic compounds would be ionized independent of their structure.

In this study, we select oxygenated organic compounds, such as ketones, esters and ethers, as the target material in the SPI method, in order to investigate how these compounds are ionized and cause fragmentation in the SPI source. It is especially interesting to compare the ionization mechanism in the SPI source between organic compounds having different functional groups. Furthermore, the emission image of chemical species in the discharge tube is also observed with a two-dimensional (2D) spectrometer, which would provide useful information on the spatial distribution of excited species to consider the ionization processes in more detail.

Experimental

ChemicalsOrganic solvents for typical VOCs, including ketones, esters

and ethers, were prepared. 2-Pentanone, 3-pentanone, 2-hexanone, 3-hexanone, 4-methyl-2-pentanone (methyl isobutyl ketone: MIBK), 2-heptanone, 3-heptanone, 4-heptanone, methyl acetate, methyl propionate, ethyl propionate, dipropyl ether and diisopropyl ether were purchased from FUJIFILM Wako Pure Chemical Corp. (Japan). A deuterated chemical, acetone-d6 was also obtained from FUJIFILM Wako Pure Chemical Corp. (Japan). Diethyl ether was from Katayama Chemical Industries Co., Ltd. (Japan). Acetone and ethyl acetate were obtained from Nacalai Tesque, Inc. (Japan).

SPI-MS apparatusThe experimental apparatus of direct current (dc)-pulsed

SPI-MS was described in our previous paper.17 The SPI source consists of a Cu-hollow anode (16 mm i.d., 19.5 mm o.d., 40 mm length) and a co-axially-arranged inner Cu-mesh cathode

(12 mm i.d., 73 mm length).14 A glow discharge plasma, which is generated between the electrodes, makes it possible to ionize organic components.15–17 Fast electrons released from the mesh cathode are accelerated towards the anode, and then excite/ionize gas species (G*, G+) in the plasma region, as shown in Fig. S1 (Supporting Information). On the other hand, the excite/ionize gas species could diffuse into the central portion of the hollow electrode through the mesh electrode. Because the introduced gas species would have relatively low kinetic energies, the sample molecule could be softly ionized through collisions with excite gas species (PI) and/or ionize gas species (adduct ionization).

Vapor of the gas sample was introduced into the SPI source filled with ambient air, which was used as the discharge gas (carrier gas) for practical purpose. This procedure enabled a large amount of gas sample (approx. 100 mL min–1) to be introduced into the SPI source regardless of the exhaust capacity of the vacuum system, which could generate a stable discharge plasma at pressures of up to several kPa in the SPI source.16

A DC pulse power supply system comprised a high-speed pulse switching circuit (FHVS-2000, Chubu R & D Co., Ltd., Japan) and a DC power supply (HVR-2K150P/FG/100, Chubu R & D Co., Ltd., Japan), which restricted the average supplied power. Current–voltage characteristic of the pulsed discharge plasma indicated that the discharge voltage remained almost constant regardless of the current values, so that a constant-current mode was conducted. The duty ratio was 50% and a repetition rate of 2.5 kHz (pulsed delay time of 200 μs) was employed.

A quadrupole MS (ZQ-2000, Waters) was operated in the full-scan mode. The voltage of a cone, an extractor and a radio frequency (RF) lens (a hexapole ion guide) was set to 3, 0 and 0.1 V, respectively. The full-scan mass spectrum was acquired over a mass scan range of m/z 2 – 202 at a scan time of 0.2 s, or m/z 2 – 402 at a scan time of 0.4 s and an interscan delay of 0.1 s. The integration time of the mass spectra obtained was 1.0 min. Each mass spectrum was corrected by subtracting a blank spectrum of ambient air. All data were recorded in a continum mode and processed using MassLynx software (Ver. 3.5, Waters Corp., USA).

2D imaging spectrographAn imaging spectrograph (IMS-250TKW, Bunkoukeiki Co.,

Ltd., Japan) was used to record a two-dimensional (2D) emission image through a quartz window for viewing the discharge plasma in the SPI source. The emission spectra were taken under the following discharge conditions: a discharge current of 30 mA and ambient air pressures of 0.8, 1.0 and 2.5 kPa.

Emission signals from the SPI were collected through a collimator optics onto the entrance slit of the imaging spectrograph, and the emission image was then dispersed and detected on an ICCD (Intensified Charge Coupled Device) detector (DH734-18F-0, Andor Technology Ltd., UK), where the 2D image of a particular emission line could be observed. The width of the entrance and exit slits were set to be 30 μm. The exposure time for recording one image and the multichannel plate (MCP) gain value were 0.5 s and 100. The observed band heads were N2* 337.1 nm, N2

+ 391.4 nm and NO 247.8 nm for air plasma gas.

Results and Discussion

Mass spectra of backgroundFigure S2 (Supporting Information) shows mass spectra of the


background for the SPI source obtained at discharge pressures of (a) 1.0 kPa, (b) 1.5 kPa and (c) 2.5 kPa. The peaks at m/z = 30, 32 and 46 corresponding to NO+, O2

+ and NO2+ were

detected at a discharge pressure of 1.0 kPa, as shown in Fig. S2(a). These background ions can be ionized by electron ionization (EI), or by charge-transfer ionization (CT) and Penning-type ionization (PI) with excited N2 molecules.16,18–20 The NO+ and O2

+ which are mainly produced are the most active precursor ions because they do not easily react with remaining species of air gas.21

At a pressure of 1.5 kPa (Fig. S2(b)), the intensities of two peaks at m/z = 55 and 73 strongly increased, leaving NO+, O2

+ and NO2

+ ions. The peaks at m/z = 55 and 73 were identified as being protonated water clusters [(H2O)n+H]+ (n = 3, 4) with the maximum intensity of m/z = 55 (n = 3). Protonated water clusters are routinely observed in glow discharge ionization spectra of ambient air involved a small amount of water in the discharge ion source.18,22–26 It has shown that the [(H2O)n+H]+ ions are formed from the primary ions of the discharge (N2

+, O2

+, etc.) through a sequence of ion/molecule reactions.20,22,27,28 The formed [(H2O)n+H]+ ions serve as proton-transfer reagents.18

The signals of NO+, O2+ and NO2

+ completely disappeared, while the background ions of m/z = 36, 65 and 83 were still observed in addition to m/z = 55 and 73 when the discharge pressure increased up to 2.5 kPa (Fig. S2(c)). The small peak at m/z 36 was assigned to [(H2O)2]+• because a sequence of [(NH3)(H2O)n+H]+ was not observed in the mass spectra.29,30 The remaining peaks at m/z = 65 and 83, exhibiting a mass difference of 18 u, were identified as being N2-water clusters of the formation [N2(H2O)n+H]+, where n = 1, 2.27 Not only [(H2O)n+H]+, but also [N2(H2O)n+H]+, may act as proton-transfer regent ions.

Dependence of discharge pressure of backgroundFigure S3 (Supporting Information) shows a pressure

dependence of the mass-signal intensity for the background species in a range from 0.8 to 3.0 kPa at a discharge current of 30 mA. Only the mass spectral intensities of NO+ and O2

+ are scaled on the right vertical axis. The intensities for NO+ and O2

+ tended to decrease with increasing the discharge pressure. On the other hand, the intensities of [(H2O)2]+•, [(H2O)n+H]+ (n = 3, 4), and [N2(H2O)n+H]+ (n = 1, 2), especially [(H2O)2]+• and [(H2O)n+H]+ (n = 3, 4), drastically increased with increasing the background air pressure from a discharge pressure of approximately 2.0 kPa. This suggests that NO+ and O2

+ would be consumed through a sequence of ion/molecule reactions for the formation of various water clusters, because the mean free path of them may become reduced with increasing gas pressures. Such a change in background species on the discharge pressures would largely affect the ionization of sample molecules in the SPI source (described later).

Dependence of discharge current of backgroundIntensity variations of the background species at a discharge

pressure of 2.5 kPa are plotted as a function of the discharge current, as shown in Fig. S4 (Supporting Information). Only the mass spectral intensities of NO+ and O2

+ are scaled on the right vertical axis. The intensities of NO+, O2

+ and [(H2O)2]+• increased with increasing the discharge current. A higher discharge current would provide a higher number density of electrons, leading to a larger probability of EI in the outer region (see Fig. S1). In contrast, the intensities of [(H2O)n+H]+ (n = 3, 4), and [N2(H2O)n+H]+ (n = 1, 2) had the maximum at approximately 25 mA and tended to decrease at discharge currents of more than 25 mA. This suggests that the protonation of analytes is likely to occur effectively at low discharge currents.18

Mass spectra of acetoneFigure 1 shows the mass spectra of acetone when the discharge

pressure was set to be (a) 1.0 kPa and (b) 2.5 kPa. As shown in Fig. 1(a), the peak at m/z = 58 was detected as a base peak corresponding to the molecular ion peak, [M]+•, while the peak at m/z = 43 was also apparent as a fragment ion, [M–CH3]+, which was considered to be a major dissociation product in EI. The result shows that the following three ionization reactions for acetone would occur: (i) Excited N2 (C3Πu) can generate [M]+• by a collision with M by PI (Eq. (1)).15,16,31 If the analyte is a carbonyl compound, the C–C bond between carbonyl and α-C can be easily cleaved, and then a resonance-stabilized acylium ion is produced as a fragment ion (Eq. (2)). In contrast, (ii) PI with excited O2 may give a small contribution compared with PI with the excited N2, as shown in Eq. (3). The excited O2, which acts as an energy donor for PI, is generally unstable and dissociated into oxygen atoms simultaneously with the excitation. (iii) M can be ionized by N2

+• and/or O2+• with CT

(Eqs. (4) and (5)), and the monocular ion, [M]+•, may be successively dissociated into fragment ions,12,21 as shown in Eq. (2). CT in collisions with N2

+• and/or O2+• can contribute to

ionization of the sample molecule. However, the excited N2 (C3Πu), whose excitation energy is 11.1 eV, is extremely stable in the metastable state. Nitrogen molecule could possess not only this metastable state, but also several other excited states; therefore, PI with the excited N2 could cause predominant ionization reaction in air discharge.

N2* + M ⎯→ N2 + M+• + e–, (1)

M+• ⎯→ fragment+ + R•, (2)

Fig. 1　Mass spectra of acetone for (a) 1.0 kPa, (b) 2.5 kPa and acetone-d6 for (c) 2.5 kPa. Modulation frequency, 2.5 kHz; duty ratio, 50%; discharge current, 30 mA; sample/air flow ratio (R), 4.0%.


O2* + M ⎯→ O2 + M+• + e–, (3)

N2+• + M ⎯→ N2 + M+•, (4)

O2+• + M ⎯→ O2 + M+•, (5)

where M is a sample molecule, and the superscripted asterisk denotes an excited state and R• denotes an alkyl radical. The excited energies of N2 (C3Πu), N2

+ (B2∑u) and O2+ (a4Πu) are 11.1,

18.7532,33 and 16.1 eV,34,35 respectively, whereas most of the organic molecules have ionization potentials in a range of 7 – 10 eV.

At a discharge pressure of 2.5 kPa, the peak at m/z 134 was detected as a base peak (Fig. 1(b)). This peak was identified to a radical cation of water adduct, [2M+H2O]+•, for an acetone dimer. Additional peaks at m/z = 76, 175 and 192 with low intensities were identified to the water adduct monomer, [M+H2O]+•, a protonated trimer, [3M+H]+, and a water adduct trimer, [3M+H2O]+•, respectively. Clusters of acetone, [(CH3)2CO]n (n = 1 – 3), which are well known, are formed via the C=O···H–C hydrogen-bonding interaction, where the clusters of n = 2 and 3 are linked by four and six C=O···H–C hydrogen-bonds, respectively.36,37 The inner mesh cathode is grounded (see Fig. S1), so that there is no potential gradient in the central part of the cathode where the kinetic energy of the H2O+• would be small. Besides, H2O+• could be deaccelerated by third-body collisions at higher discharge pressures of ambient air.16,38 Moreover, the formation of water adduct ions at higher pressures is required for third-body collisions to lose their excess energy.16,38 Therefore, stable water adduct ions would be formed without any fragmentation when the SPI source is operated at higher discharge pressures, according to the following equations:

[(H2O)2]+• + M ⎯→ [M+H2O]+• + H2O, (6)

[M+H2O]+• + M ⎯→ [2M+H2O]+•, (7)

[2M+H2O]+• + M ⎯→ [3M+H2O]+•, (8)

Deuterated acetone (MW = 64) was used so as to examine whether the adduct ion of the acetone dimer is water. The peak intensity ratio at m/z = 146:147:148 was approximately 10:5:1, as shown in Fig. 1(c). Trace moisture of heavy water may be invariably mixed in the acetone-d6 solvent during the production and storage processes. Thus, three species of H2O, HDO and D2O are normally present in the acetone-d6 solvent, and H2O and HDO were reported to be often observed in proton magnetic resonance (1H-NMR) spectrum.39 The peak at m/z = 146, corresponding to [2M+H2O]+•, can be regarded as the dominant peak; its isotope peaks at m/z = 147 and 148 were observed as [2M+HDO]+• and [2M+D2O]+•, respectively. Accordingly, the water adduct ions, such as [2M+H2O]+•, [2M+HDO]+• and [2M+D2O]+•, can be attributed to H2O, HDO and D2O contained in the sample solvent.

Dependence of sample gas pressureFigure 2 shows the total ion currentgram (TIC) and MS

iongrams of several mass peaks for (a) acetone and (b) acetone-d6 at a discharge pressure of 2.5 kPa. The intensity of TIC was drastically elevated for both samples at a time of 2.0 min and for a period of 1.0 min after the sample vapor was introduced, implying an increment of the gas pressure of the samples.40 During introduction of the sample vapor, the intense peak of m/z = 134 ([2M+H2O]+•) appeared dominantly (Fig. 2(a)). In Fig. 2(b), the intense peaks of [2M+H2O]+• (m/z = 146) and [2M+HDO]+• (m/z = 147) were also clearly observed while the

sample gas was introduced. From the behavior of these species, it was found that water originated from the organic solvent would contribute to the attachment of the sample molecule (see Mass spectra of acetone).

Just after introduction of the sample gas, the protonated dimers [2M+H]+ at m/z = 117 and 129 began to be observed apparently (Figs. 2(a) and 2(b)). The protonation for the sample molecule seemed to occur once aspiration of the sample vapor was complete in the SPI source. Ionization of the sample molecule would not be affected by moisture in the sample solvent after sample introduction. It was reported that the protonated dimer was observed for ketones if they were ionized,25,41 and it is produced continuously with increasing the pressure by a reaction with M.42,43 The proton added to the acetone dimer would be originated from ambient air, because the [2M+H]+ at m/z = 129 can be observed for acetone-d6, and the discharge plasma can produce several cluster ions of the mass number at m/z = 55, 65, 73 and 83 as the proton source, as shown in Fig. S2(c). The water clusters would generate the protonated molecule [M+H]+ if the analyte molecule has a higher proton affinity than the ionized water clusters (Eq. (9)),12,40 which could subsequently give the protonated dimer [2M+H]+ for clustering of the analyte (Eq. (10)).26 The third-body collisions would contribute to more efficient stabilization of the protonated molecules when the gas pressure becomes higher, as described in Mass spectra of acetone.

[(H2O)n+H]+ + M ⎯→ [M+H]+ + nH2O, (9)

[M+H]+ + M ⎯→ [2M+H]+. (10)

Fig. 2　Total ion currentgram (TIC) and the MS iongrams for (a) acetone and (b) acetone-d6. Modulation frequency, 2.5 kHz; duty ratio, 50%; ambient air pressure, 2.5 kPa; discharge current, 30 mA; sample/air flow ratio (R), 4.0%.


Mass spectra of ketonesFigure 3 shows the mass spectra of the ketones for (a) acetone

(MW = 58), (b) 2-pentanone (MW = 86), (c) 3-pentanone (MW = 86), (d) 2-hexanone (MW = 100), (e) 3-hexanone (MW = 100), (f ) MIBK (MW = 100), (g) 2-heptanone (MW = 114), (h) 3-heptanone (MW = 114) and (i) 4-heptanone (MW = 114) when the discharge pressure was 2.5 kPa. The mass spectra of (b), (d), (e), (g), (h) and (i) included many peaks of [2M–H]+, [M–H]+ and the other unknown ions, when the accumulation time was fixed to be 1.0 min for an interval of from 2.5 to 3.5 min (during sample introduction), as is shown for example in Fig. S5 (Supporting Information) for the total ion currentgram (TIC) and MS iongrams of the mass peaks for 4-heptanone. These compounds include the γ-hydrogen atom (position 4) in common. It would seem likely that hydrogen atom abstraction from the various C–H bonds may occur by reacting with an alkyl radical (R•),44 which could be slightly formed even at higher discharge pressures (see Mass spectra of acetone), when the alkyl chain is longer than 4 carbon atoms. Therefore, to avoid these unfavorable side reactions, the mass spectra of (a) – (i) ketones were obtained under an accumulation time of 1.0 min for an interval of from 4.0 to 5.0 min (after sample introduction). It was found that these ketones were detected to be only the corresponding protonated dimer [2M+H]+ with little or no fragmentation by the SPI method.

Mass spectra of esters and ethersThe mass spectra of esters and ethers were also obtained under

the same conditions of Fig. 3. Figure 4 shows mass spectra of esters for (a) methyl acetate (MW = 74), (b) methyl propionate (MW = 88), (c) ethyl acetate (MW = 88) and (d) ethyl propionate (MW = 102). Similar to the case of ketones, the [2M+H]+ ions of (a) – (d), which appeared at m/z = 149, 177, 177 and 205, respectively, were observed as a base peak with little or no fragmentation. Figure 5 shows mass spectra obtained

from ethers of (a) diethyl ether (MW = 74), (b) dipropyl ether (MW = 102) and (c) diisopropyl ether (MW = 102). These compounds also stably provided [2M+H]+ as a dominant ion with little or no fragmentation. Esters and ethers involving O atom in the molecule, as is the case with ketones (Eqs. (9) and (10)), would generate protonated species, [M+H]+, which subsequently combined with M into protonated dimers ([2M+H]+) at higher air discharge pressures, due to the short mean free path.

2D emission imagesIn our previous paper, 16 the emission spectra of the 2nd-

positive system of N2 (C3Πu – B3∑g; 297.7 – 405.9 nm), the 1st-negative system of N2

+ (B2∑u+ – X2∑g

+; 391.4 – 470.9 nm) and the nitric oxide (NO) γ-system (A2∑+ – X2Π; 215.5 – 272.2 nm) were clearly observed. The most intense band head in each molecular system was selected as the following: N2

* 337.1 nm (11.1 eV), N2

+ 391.4 nm (15.6 eV) and NO 247.8 nm (9.26 eV).32,45

Figure 6 shows typical pictures of the discharge tube with a size of 400 × 400 pixels, where 1 pixel corresponds to ca. 0.042 mm. Figure 6(a) is a 0-oder non-dispersed image, which presents a photograph of the tube, itself; (b), (c) and (d) are 2D emission images in which the intensities of the N2*, N2

+ and NO band heads are all included at discharge pressures of 0.8, 1.0 and 2.5 kPa, respectively. All of the images are somewhat enlarged toward the perpendicular axis due to the slit collimation of the spectrograph, although they should have circle shapes of the hollow electrode. It was found the emission maps looked to change to be ring-shaped, half ring-shaped and quarter ring-shaped images as the discharge pressure increased from 0.8 to 2.5 kPa.

Figure 7 shows 2D emission images based on the difference spectrum, in which the normalized intensity of the N2

+ (left column) or the NO (right column) is subtracted from that of the

Fig. 3　Mass spectra of ketones for (a) acetone (R = 4.0%), (b) 2-pentanone (R = 0.4%), (c) 3-pentanone (R = 4.0%), (d) 2-hexanone (R = 1.6%), (e) 3-hexanone (R = 0.8%), (f ) MIBK (R = 4.0%), (g) 2-heptanone (R = 4.0%), (h) 3-heptanone (R = 4.0%) and (i) 4-heptanone (R = 4.0%). The accumulation time was 1.0 min at a time of between 4.0 and 5.0 min (after sample introduction). Modulation frequency, 2.5 kHz; duty ratio, 50%; ambient air pressure, 2.5 kPa; discharge current, 30 mA.


N2*, at discharge pressures of 0.8 kPa ((a) and (b)), 1.0 kPa ((c) and (d)) and 2.5 kPa ((e) and (f )), respectively. It indicated that the N2* emission was widely distributed outward over the N2

+ emission, and that the NO emission was more widely distributed outward regardless of the discharge pressure. The plasma

distribution changed to be halfshaped with increasing the discharge pressure from 0.8 to 1.0 kPa, whereas there were no significant differences in this pressure range in the mass spectra (data not shown). The excited N2* species, which were distributed through the mesh electrode (Figs. 7(a) and 7(c)), could diffuse towards the inner portion and then would react with sample molecules, resulting in the formation of [M]+• due to PI by N2* species (Eqs. (1) and (2)). The N2

+ species, which was observed principally in the outer region of the mesh electrode in their emission images, was not detected in the background of the mass spectra at a discharge pressure of 1.0 kPa (Fig. S2(a)), so that the ionization of M by N2

+• would not occur, as shown in Eq. (3). In contrast, the NO species, which were widely distributed in the SPI cell (Fig. 7(d)), would be ionized by N2* species, resulting in the formation of NO+. The NO+ was detected in the mass spectra (Figs. S2(a) and 1(a)); therefore, it would be also distributed in the inner region of the electrode.

At a discharge pressure of 2.5 kPa, the emission images of these chemical species became shirked, probably because their lifetimes were generally shorter, as shown in Figs. 7(e) and 7(f ). In the plasma region between two electrodes, the N2

+ species would contribute to generate various water clusters by reacting with water in ambient air (Fig. 7(e)) (see Background spectra). The water clusters react with analyte molecules, resulting in the formation of protonated molecular ions for oxygenated compounds due to the high proton affinity (Eqs. (9) and (10)). On the other hand, the widely-distributed NO (Fig. 7(f )) would further generate NO+ by reacting with N2* at higher discharge pressures (Fig. S3). If the analyte is aromatic compounds, the formation of [M+NO]+ easily occurred at higher discharge pressures.16 However, in the case of oxygenated compounds, the attachment of NO+ to the analyte would hardly occur; therefore, the NO+ adduct ion, [M+NO]+ was not observed in the mass spectra (Figs. 3, 4 and 5).

Fig. 4　Mass spectra of esters for (a) methyl acetate (R = 4.0%), (b) methyl propionate (R = 0.4%), (c) ethyl acetate (R = 4.0%) and (d) ethyl propionate (R = 4.0%). The experimental conditions are the same as in Fig. 3.

Fig. 5　Mass spectra of ethers for (a) diethyl ether (R = 0.4%), (b) dipropyl ether (R = 4.0%) and (c) diisopropyl ether (R = 4.0%). The experimental conditions are the same as in Fig. 3.


Fig. 6　(a) Axial-view photograph of the discharge tube (0-order image), and 2D emission images for the integrated intensity of the N2*, N2

+ and NO band heads at gas pressures of (b) 0.8 kPa, (c) 1.0 kPa and (d) 2.5 kPa, when the discharge current is fixed to be 30 mA.

Fig. 7　2D emission images (400 × 400 pixels) obtained from the intensity difference of N2* – N2+

(left column) and N2* – NO (right column) at gas pressures of 0.8 kPa (a, b), 1.0 kPa (c, d) and 2.5 kPa (e, f ), when the discharge current is fixed to be 30 mA. The intensity of the band heads is each normalized for the corresponding maximum value.


Conclusions

The direct detection of oxygenated organic compounds, such as, ketones, esters and ethers, has been successfully performed by using pulsed dc SPI-MS. The mass spectra of the oxygenated organic compounds comprised their protonated dimer, [2M+H]+, at a gas pressure of several kPa in air discharge plasma, where an intense base peak dominantly appeared with little or no fragmentation. The proton for the analyte molecules would be supplied by water clusters, such as [(H2O)n+H]+ and [N2(H2O)n+H]+, which were formed in ambient air discharge plasma, because the [2M+D]+ was not detected, even if a deuterited acetone, acetone-d6, was employed. The formation of [2M+H]+ was found to strongly depend on the time. To consider the ionization reactions in detail, the distribution of chemical species in the discharge tube was also examined with a 2D spectrometer, whose experiment has not been investigated in combination with the SPI source, as far as we know. From the data of the mass spectra and 2D images, a sample molecule would be readily ionized by PI with excited N2, which made the fragmentation for oxygenated compounds having polar carbonyl groups due to the lower pressures (approx. 1.0 kPa). In contrast, at the higher pressures (several kPa), the sample molecules would undergo adduct ionization by attachment with protons from the water clusters.

Supporting Information

This material is available free of charge on the Web at http://www.jsac.or.jp/analsci/.

Acknowledgements

This work was supported by the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University (Proposal No. 18K0002).

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