H2O-Amplified Ionization

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A VUV-excited and CHCl/HO-amplified ionization-coupled mass spectrometry for oxygenated organics analysis Bo Yang, Haixu Zhang, Jinian Shu, Pengkun Ma, Peng Zhang, Jingyun Huang, Zhen Li, and Ce Xu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04122 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Analytical Chemistry

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A VUV-excited and CH2Cl2/H2O-amplified ionization-coupled mass

2

spectrometry for oxygenated organics analysis

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3 4

Bo Yang†,‡, Haixu Zhang†,‡, Jinian Shu*†,‡, Pengkun Ma†,‡, Peng Zhang†,‡, Jingyun

5

Huang†,‡, Zhen Li†, and Ce Xu†

Comment [y1]: Author revision.

6 7

†

8

Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing

9

100085, China

State Key Laboratory of Environment Simulation and Pollution Control, Research

10

‡

11

Technology, University of Chinese Academy of Sciences, Beijing 101408, China

National Engineering Laboratory for VOCs Pollution Control Material &

12 13

Abstract

14

The mass spectrometry analysis of oxygenated volatile organic compounds

15

(OVOCs) remains challenging due to their limited ionization efficiencies. In this study,

16

we surprisingly found that, under VUV excitation, a gaseous mixture of

17

CH2Cl2/H2O/analyte (OVOCs) in N2 buffer generated large amounts of H3O+ and

18

protonated analyte even when the photon energy was lower than the ionization energy

19

of the neutral species involved. In contrast to those obtained with VUV

20

photoionization alone, the signal intensities of oxygenated organics can be amplified

21

by more than three orders of magnitude. The isotope tracing experiment revealed that

22

the proton donor is water, and the dependence of the signal intensities on the VUV

23

photon intensities verified that the reaction was a single-photon process. The observed

24

ionization process is assigned as an un-documented chemi-ionization reaction in

25

which a complex formed from the ion-pair state CH2Cl2*, H2O, and analyte and then

26

autoionized to produce the protonated analyte with the aid of the reorganization

27

energy released from the formation of CH2O and HCl. Essentially, here we present an

Comment [y3]: Reviewer 1, Q8.

28

efficient chemi-ionization method for the direct protonation of oxygenated organics.


29

By the method, the mass spectrometric sensitivities towards acetic acid, ethanol, 1



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30

aldehyde, diethyl ether, and acetone were determined to be 224 ± 17, 245 ± 5, 477 ±

31

14, 679 ± 11, and 684 ± 6 counts pptv-1, respectively, in 10 s acquisition time. In

32

addition, the present ionization process provides a new method for the generation of a

33

high-intensity H3O+ source (~1011 ions s-1, measured by ion current) by which general

34

organics can be indirectly protonated via a conventional proton transfer reaction.

35

These results open new aspects of chemi-ionization reactions and offer new

36

technological applications that have the potentials to greatly improve mass

37

spectrometry sensitivity for detecting trace gaseous organics.



38

39 40

TOC

41 42 43 44 45 46 47 48 49 50 51 2




52

Volatile organic compounds (VOCs) are ubiquitous in various anthropogenic

53

activities and natural processes. They have received extensive attentions for their

54

important roles in tropospheric chemistry, especially the ozone and aerosol formation.

55

In these photochemical processes, oxygenated volatile organic compounds (OVOCs)

56

are not only the degradation products of VOCs but also the direct contributors to the

57

aerosol. Monitoring the dynamics of OVOC in ambient air is crucial to understand the

58

process of aerosol formation. In addition, some OVOCs can serve as biomarkers in

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breath analysis for metabolic profiling and disease diagnosis.1,2 Hence, the

60

development of new and efficient methods for analyzing gaseous OVOCs is

61

beneficial to future advances in environmental and medical sciences.

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Mass spectrometry (MS) is one of the most extensively used techniques in

63

detecting VOCs owing to its high sensitivity and rapid response time.3 The analytical

64

capacity of a mass spectrometer highly depends on the ionization mode and efficiency

65

of the ion source. Electron ionization (EI) is a conventional ionization technique, by

66

which the gaseous neutrals are ionized via collision with high-energy electrons (70

67

eV),4,5 while the production of extensive fragment ions has limited its applications in

68

direct analyzing mixture samples. Chemical ionization (CI) is a softer ionization

69

technique,6 which charges the neutral analyte through molecule-ion reactions with the

70

reagent ions.7-12 Nowadays, the mass spectrometer coupled to a variety of CI and

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CI-like ion sources has been widely used in the online analysis of environmental

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VOCs samples,13-15 and the detection sensitivity of some typical high-sensitive

73

instruments, e.g., proton-transfer-reaction mass spectrometry (PTR-MS) and gas

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discharge-based MS, have reached ~pptv level.16-20 Vacuum ultraviolet single-photon

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ionization (VUV-SPI) is a unique soft ionization technique featured by little

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fragmentation and producing no adducts/cluster ions in contrast to CI ion sources.21,22


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The mass spectrometer with a VUV-SPI source has good performance in the


78

composition analysis of complex organic mixtures. However, confined by the limit of

79

detections, which are generally at ~ppbv level,23,24 traditional VUV-SPI-MS has rarely

80

been applied to the field measurement of VOCs. Though the detection sensitivity of

81

VUV-SPI-MS toward special aromatics has been improved to ~pptv level via the 3



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low-pressure photoionization technique,25 it is difficult for the instrument to monitor

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environmental OVOCs due to the much smaller photon ionization cross sections of

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OVOCs compared with those of aromatics.26,27

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Chemi-ionization reactions are traditionally regarded as an effective channel in

86

plasma physics for the production of molecular ions.28 The reactions involve an

87

ionization process that results from the collision of two neutral species at low

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collision energies.28 In some literatures, chemi-ionization was classified into two

89

elementary reaction types.29,30

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A (excited or not) + B → AB+ + e-

(1)

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A (excited) + B → A + B+ + e-

(2)

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Reaction (1) is associative ionization, and reaction (2) is penning ionization. Penning

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ionization appears as an energy transfer ionization without a new bond formation,31,32

94

while associative ionization allows chemical energy (bond energy) to be converted

95

into the ionization energy.32 Some types of associative ionization reactions are very


96

common in nature, such as CH + O → CHO+ + e- and CH + O2 → CHO2+ + e-,


97

which are considered the primary initial reactions in flames.33,34 Based on these

98

reactions, the flame ionization detector (FID) was developed for the quantitative

99

measurement of gaseous hydrocarbons.35 The existing studies concerning other types


100

of associative ionization reactions mainly focus on excited-state noble gases with


101

ground-state

102

halogens.31,32,36,37 In mass spectrometry ion sources, penning ionization is frequently

103

involved in the generation of highly intense reagent ions to indirectly charge the target

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analytes.38-40 Associative ionization reaction detected with the mass spectrometer has

105

been used to investigate the reaction system of O(3P) and hydrocarbon.41 To the best

106

of our knowledge, associative ionization serving as an independent ionization method

107

for mass spectrometry to monitor analyte has never been reported.

species

and

excited-state

metallic

atoms

with

ground-state

108

Recently, a new phenomenon that doping CH2Cl2 in the low-pressure VUV-SPI

109

process dramatically improved the detection efficiency of OVOCs has been

110

reported.42,43 In addition, an early indication of the CH2Cl2-induced protonation was

111

found in a study investigating the influence of solvents on analyte protonation during 4




112

the photoionization process.44 However, the mechanism of the ionization phenomenon

113

has not been revealed unequivocally. In this study, more significant enhancement

114

effects of CH2Cl2 on the signal intensities of OVOCs were observed. The intrinsic

115

mechanism of the ionization phenomenon was interpreted by the chemi-ionization

116

theory. The features and ionization efficiency of the new ionization process were

117

investigated in detail and the potential application of the ionization process as a new

118

ionization method for the VOC detection was further unfolded.

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Comment [y17]: Reviewer 1, Q1 and Q2.

119 120

METHODS

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The schematic diagram of the ion source is shown in Scheme 1. The setup was

122

mainly composed of three parts: a light source system, a light attenuation cell, and a

123

photochemical ionizer. The vacuum ultraviolet (VUV) light was generated via

124

excitation of a rare gas (Kr or Xe) under ~330 Pa or 180 Pa, respectively, using a

125

13.56 MHz RF power supply running at 60 W. The maximum photon flux of the VUV

126

lamp was ~2 × 1014 photons cm-2 s-1. When the signal dependence on the photon

127

intensity was investigated, the light attenuation cell was used to tune the light power

128

by absorbing the VUV light with methane gas at different pressures. Two

129

plano-convex MgF2 lenses were located at both ends of the light attenuation cell and

130

served as the windows for the VUV light that was transmitted through the

131

photochemical ionizer. Gas phase CH2Cl2 and the analyte were mixed in a 120 L

132

balance cylinder using high-purity nitrogen (>99.999%) as the equilibrium gas before

133

the photochemical ionizer inlet. The pressure of the photochemical ionizer was ~1300

134

Pa when the sample rate was 3.3 cm3 s-1 under atmospheric pressure. The equivalent

135

gas flow rate within the photochemical ionizer was 257 cm3 s-1 under the pressure of

136

~1300 Pa. The partial flow rate of the gas phase analyte was defined as the number of

137

molecules flowing through the ion source per second (number s-1) and was used to

138

describe the quantity of the investigated gaseous species.

139

To obtain the ionization efficiency of the analyte, the quantity of cations

140

generated from the ion source was calculated from the ion currents measured by a

141

picoammeter. In the experiment, a gold-electroplated copper plate serving as the ion 5





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142

collector electrode was placed behind the outlet of the photochemical ionizer. The

143

collector electrode was connected to the ground through a picoammeter from Keithley

144

Instruments (Model 6485) to measure the ion current. The photochemical ionizer was

145

biased with a 4.0 V positive DC source to extract cations. The quantity of the cations

146

generated from the ion source was calculated by the ion currents divided by

147

elementary charge (1.6×10-19 C). The collector electrode was removed when the

148

subsequent time-of-flight (TOF) mass spectrometer was applied to detect these ions.

149

The acquisition time for each mass spectrum was 10 s. The construction of the VUV

150

photoionization mass spectrometer (VUV-PIMS) has been described in detail

151

elsewhere.27 To protect the detector from waste, a slit (0.2 mm × 2 mm) was placed at

152

the entry orifice of the TOF cavity to physically reduced the signal intensity to ~10%

153

for all the measuring events. In addition, the signal intensity was further reduced by

154

adjusting the voltages of the ion-migration lens assembly of the mass spectrometer

155

when the measured signal exceeded 5 × 105 counts. Each method was performed three

156

times to ensure the reproducibility of the signal reduction. The signal intensities of the

157

mass peaks shown in the manuscript and the supporting information have been

158

corrected using the normal ion detection efficiency of the mass spectrometer.


159 160

RESULTS AND DISCUSSION

161

Figure 1 (A & B) shows the mass spectra of the cations generated from the

162

residual H2O in the N2 buffer gas before and after doping with CH2Cl2 under Kr lamp

163

illumination. The partial flow rate of the gaseous H2O passing through the ion source

164

was estimated to be 4.1 × 1014 - 2.6 × 1016 molecules s-1 based on the water impurity

165

in the high-purity N2 (5 ppmv) and the concentration calibrated by H218O, as shown in

166

Figure S1 in the Supporting Information (SI). The partial flow rate of the CH2Cl2

167

dopant in N2 was 1.2 × 1018 molecules s-1. In the presence of CH2Cl2, the signal

168

intensities of H3O+ (m/z 19) and (H2O)2H+ (m/z 37) were enhanced by more than four

169

orders of magnitude. Figure 1 (C) shows the mass spectrum of the corresponding

170

anions, and Cl- was identified as the sole anionic product. The mass peaks of Cl- and

171

its adducts with other neutral molecules (HCl·Cl- and CH2Cl2·Cl-) dominate the 6




172

spectrum. In addition, the amounts of the protonated cations and anions were almost

173

the same. Figure 1 (D-F) show the mass spectra observed for the same gaseous

174

system under illumination by a Xe lamp. The signal intensity of H3O+ increased by

175

more than four orders of magnitude after the CH2Cl2 doping, but the absolute

176

intensities of the cations and anions were approximately one order of magnitude lower

177

than those obtained under Kr lamp illumination.

178

The photon energies of the Kr lamp are 10.0 eV (80%) and 10.6 eV (20%), and

179

those of the Xe lamp are 8.4 eV (98%) and 9.6 (2%) eV45. These energies are lower

180

than the IEs of CH2Cl2 (11.3 eV) and H2O (12.6 eV), and neither CH2Cl2 nor H2O can

181

be ionized directly using the VUV light. The trace amounts of H3O+ and (H2O)2H+

182

before the CH2Cl2 doping were derived from the photoelectron-induced EI ionization.

183

Interestingly, both cations and anions were simultaneously produced from neutral

184

molecules using an insufficient photon energy, which is different from

185

photoionization and chemical ionization processes. In addition, the ionization process

186

is different from the dopant-assisted atmospheric pressure and low-pressure

187

photoionization pathways, which dope with a photoionizable compound with a high

188

ionization efficiency to facilitate the formation of reagent ions and the subsequent

189

ion-molecule reaction to produce analyte ions.46-48

190

Figure 2 (A & B) and (E & F) show the spectra of the cations generated from

191

acetone and methanol in N2 before and after doping with CH2Cl2 under Kr lamp

192

illumination. The partial flow rates for acetone and methanol were 4.1 × 1013 and 8.3

193

× 1013 molecules s-1, respectively, and those of the CH2Cl2 dopant in acetone and

194

methanol were 2.6 × 1018 and 2.1 × 1018 molecules s-1, respectively. The same

195

experiments using Xe lamp illumination are shown in Figure S2. In the presence of

196

CH2Cl2, the signal intensities of protonated acetone and methanol increased thousands

197

of times compared to those obtained with VUV photoionization alone. These results

198

suggest that the observed ionization process can serve as an efficient ionization

199

method for acetone and methanol even though the IE of acetone (9.7 eV) is higher

200

than the photon energy of the Xe lamp and the IE of methanol (10.8 eV) is higher than

201

the energies of the Kr and Xe lamps. In addition, the approximate one-to-one 7


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202

formation of anions (Cl-) and protonated cations from acetone and methanol with a

203

CH2Cl2 dopant was also observed, as shown in Figure 2 and S2.

204

To explore the ionization mechanism, isotope experiments were conducted using

205

deuterated dichloromethane (CD2Cl2) and water (D2O) as dopants to trace the proton

206

source. Figure 3 (A) shows the mass spectrum of the cations generated from acetone

207

in N2 doped with CD2Cl2. Deuterium-protonated cations were not observed for either

208

H2O or acetone, which revealed that dichloromethane was not the proton donor.

209

Figure 3 (B) shows the spectrum obtained with CH2Cl2 and D2O doping, and

210

deuterium-protonated cations were present. These results prove that the proton donor

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is water.

212

To verify whether the ion formation results from a direct, photoinduced

213

excited-state reaction or a separate neutral dissociation followed by an ionization

214

pathway, the possibility of multiphoton absorption was investigated. The dependence

215

of the ion signals on the photon intensities was examined, as shown in Figure 4 (A &

216

B). The slopes obtained from the linear fits of the data points were ~1, which proved

217

that the ion formation proceeded via a single-photon process. This result excludes the

218

involvement of neutral dissociation pathways that require a secondary photon to form

219

ions. Although trace amounts of free-ion pairs (CH2Cl+ and Cl-) may have been

220

generated through the VUV excitation of CH2Cl2, which has a low formation cross

221

section (6.6 × 10-20 cm2),49 H3O+ cannot be solely produced from the CH2Cl+-induced

222

molecular-ion reactions because the IE of CH2Cl (8.75 eV) is lower than the

223

appearance energy of H3O+ (11.73 eV, from the photoionization of (H2O)2).50

224

Therefore, the high number of ion pairs observed in this study was generated via a

225

direct photochemical reaction.

226

Considering that the ionization process was triggered by CH2Cl2 and VUV light

227

and large amounts of Cl- were produced, the ion-pair excited state of

228

halohydrocarbons may play a role in this photochemical reaction. Therefore, the

229

doping effects of four chlorohydrocarbons (CH3Cl, CH2Cl2, CHCl3, and CCl4) on the

230

ion currents were measured and compared, as shown in Figure 4 (C & D) and Figure

231

S6 & 7. The results revealed that CH2Cl2 had a more significant effect on the ion 8



Page 10 of 22

232

formation than the other three chlorohydrocarbons, which may be partially due to

233

CH2Cl2 having the largest formation cross-section of ion pairs (CH2Cl+ and Cl-, which

234

come from the dissociation of the ion-pair state [CH2Cl+-Cl-]*) among the four

235

chlorohydrocarbons under a photon energy of ~10 eV.49 Thus, the observed

236

photochemical reaction that efficiently proceeded in the gas phase most likely

237

involved the ion-pair state [CH2Cl+-Cl-]*.

238 239

Based on the experimental results, a photo-induced chemi-ionization reaction was proposed to interpret the ionization mechanism:

240

CH2Cl2 + hv → [CH2Cl+-Cl-]*

(3)

241

[CH2Cl+-Cl-]* + H2O → [H2O-CH2Cl+-Cl-]

(4)

242

[H2O-CH2Cl+-Cl-] + M → [M-H2O-CH2Cl+-Cl-]

(5)

243

[M-H2O-CH2Cl+-Cl-] → MH+ + CH2O + HCl + Cl-

(6)

244

where M represents H2O and oxygenated organic molecules.

245

The reaction is initiated by the photoexcitation of CH2Cl2 and forms an ion-pair

246

state [CH2Cl+-Cl-]*, which then interacts with H2O to produce a primary complex,

247

[H2O-CH2Cl+-Cl-]. Then, the analytes that can form hydrogen bonds act as hydrogen

248

accepters (H2O molecules or oxygenated organics) bond with the hydrogen atoms of

249

H2O in the primary complex, leading to the formation of a secondary complex,

250

[H2O-H2O-CH2Cl+-Cl-]. Through proton and electron transfers, H3O+ and Cl- are

251

generated, and the OH and CH2Cl radicals reorganize into formaldehyde (CH2O) and

252

hydrogen chloride (HCl) by-products. These two by-products were observed in the

253

mass spectra, which supports the suggested mechanism, as shown in Figure 1 (C & F)

254

and Figure S9. The weak (CH2O)H+ signal was attributed to the low probability of the

255

secondary ionization reaction process for the byproduct CH2O, while the stronger

256

HCl⋅Cl- signal might derive from the direct attachment of HCl with Cl-. Notably, the

257

ions would not be produced without the bond energy released by the reorganization

258

products of CH2O and HCl. The related energy calculation is shown in Table S1.

259

Therefore, the VUV-excited and CH2Cl2/H2O-amplified ionization process can be

260

attributed to a photoinduced chemi-ionization reaction. 9





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261

Since H2O was a major reactant for the present ionization reaction, the

262

concentrations of H2O and H3O+ in the photoionizer were estimated. Based on the

263

partial flow rate of the gaseous H2O passing through the ion source (4.1 × 1014-2.6 ×

264

1016 molecules s-1) and the gas flow rate inside the photoionizer (257 cm3 s-1 under the

265

pressure of ~1300 Pa), the concentration of H2O in the photoionizer was calculated to

266

be 1.6 × 1011-1.0 × 1014 molecules cm-3. According to the maximal ion flow rate (1.40

267

× 1011 ions/s) generated from H2O/CH2Cl2 under Kr lamp illumination (as shown in

268

Figure 4 (C)), the yield of the primary H3O+ was estimated to be ~1011 ions/s. Thus,

269

the equivalent concentration of H3O+ was ~108 molecules cm-3. The effect of the H2O

270

concentration on the ion formation was also examined as shown in the Figures

271

S10&11. The experimental results showed that the increase of gaseous H2O by adding

272

H2O vapor in the balance cylinder had little impact on the ions formation, which

273

indicated that the residual H2O in N2 was excessive for the reaction with the ion-pair

274

excited state of CH2Cl2. Whereas, the extensive decrease of gaseous H2O by the

275

chemisorption with heated magnesium flakes would result in the decrease of ion

276

formation, which was consistent with the reaction process described by the proposed

277

mechanism.

278

To investigate the role of the conventional proton transfer reaction induced by

279

H3O+ in the ion formation, the mass spectrometric signal intensities of methanol,

280

acetone, benzene, and p-xylene in the presence of 8.1 × 1016 molecules s-1 CH2Cl2

281

were compared as shown in Figure 5. The results showed that the observed [M+1]+

282

signal intensities of benzene and p-xylene were 6 and 4 times higher than those

283

contributed from the isotope (13C). We speculate that the extra [M+1]+ signals of

284

benzene and p-xylene resulted from the conventional proton transfer reaction with

285

H3O+. Meanwhile, the [M+1]+ signal intensity of methanol was 4 and 3 times higher

286

than those of benzene and p-xylene, respectively, and that of acetone was 23 and 18

287

times higher than those of benzene and p-xylene, respectively. Considering that the

288

four organics have comparable proton transfer reaction rates (with H3O+),51 this result

289

revealed that the new ionization process, i.e., chemi-ionization, dominated the

290

protonation of oxygenated organics in these experiments. In addition, the ratio of 10



291

H3O+/(H2O)2H+ may affect the protonation efficiency of some compounds (e.g.,

292

formaldehyde) in the conventional proton transfer reaction.52 However, in this study

293

the ratio of H3O+/(H2O)2H+ calculated by the mass spectra was 0.5-1.2, which cannot

294

result in a large difference of signal enhancements between oxygenated and

295

non-oxygenated organics. It should be noted that the direct VUV photoionization

296

followed by thermal collisions of molecules and ions under relative high pressure

297

(~1300 Pa) would contribute a part of protonated ions for polar compounds, e.g.,

298

acetone, but the contribution of this process for the protonated ions of benzene and

299

p-xylene was negligible.

Page 12 of 22


300

To better reveal the feasibility of the ion source for the analysis of more general

301

OVOCs, a mixture sample containing aldehyde, ethanol, diethyl ether, and acetone

302

evaporated from a CH2Cl2 solution and a single sample of acetic acid evaporated from

303

an aqueous solution were measured by the VUV-excited and CH2Cl2/H2O-amplified

304

ionization-coupled mass spectrometry. The linear calibration curves of the five

305

OVOCs were obtained as shown in Figure 6. According to the slope of the linear

306

regression equation, the mass spectrometric sensitivities towards acetic acid, ethanol,

307

aldehyde, diethyl ether, and acetone were determined to be 224 ± 17, 245 ± 5, 477 ±

308

14, 679 ± 11, and 684 ± 6 counts pptv-1, respectively, in 10 s acquisition time. The

309

experimental results reveal that the new ionization method is feasible for sensitive

310

detection of OVOCs with different types of oxygenated functional groups.


311

Furthermore, the present ionization process provides a new method for generating


312

high-intensity H3O+. The intensity of H3O+ reagent ions is a key factor that determines


313

the detection sensitivity of the PTR-MS. After years of development, the H3O+

314

intensity of PTR-MS has reached 106~107 counts s-1,53,54 which is usually obtained via

315

hollow cathode discharge. Surprisingly, the intensity of primary H3O+ generated via

316

the present method is ~1011 ions s-1. Even considering the instrumental efficiency, the

317

present reaction can serve as a much more powerful H3O+ source, which will

318

significantly improve the detection sensitivity of PTR-MS.

319 320

CONCLUSIONS 11


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321

A VUV-excited and CH2Cl2/H2O-amplified ionization process was reported for

322

the first time. Through VUV excitation of gaseous CH2Cl2/H2O/analyte mixtures, a

323

chemi-ionization reaction occurred to produce substantial H3O+ ions and the

324

protonated analyte, and an equal amount of Cl- can be produced with the aid of the

325

reorganization energy released from the formation of CH2O and HCl. The new

326

ionization pathway can serve as a direct ionization method for mass spectrometry to

327

detect OVOCs with a high sensitivity or provide a high-intensity H3O+ source for

328

PTR-MS to detect general VOCs. Both of these mass spectrometry applications are

329

expected to greatly improve the current sensitivity of on-line VOC detection. This

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study not only provides new insight into chemi-ionization reactions but also signifies

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a revolutionary improvement in the sensitive detection of trace gaseous organics.

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ASSOCIATED CONTENT

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Supporting Information

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The Supporting Information is available free of charge on the ACS Publications

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website.

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Materials and Methods, supplementary Text, Figures, and Tables (PDF).

338 339

AUTHOR INFORMATION

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Corresponding Author

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*[email protected]

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ORCID

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Jinian Shu: 0000-0002-6360-6953

344 345

Notes

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The authors declare no competing financial interests.

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ACKNOWLEDGEMENTS 12




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This study was supported by the National Natural Science Foundation of China (Grant

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No. 21527901 and 21777170) and the Strategic Priority Research Program of the

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Chinese Academy of Sciences (XDB05040501). The authors are grateful to Prof.

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Stephen R. Leone and Prof. Hailin Wang for their helpful comments on the

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manuscript.

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Scheme 1. Schematic diagram of the applied ion source.

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451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472

Figure 1. Time-of-flight mass spectra of the cations generated from the residual H2O in the N2 buffer gas before (A) and after (B) doping with CH2Cl2 and the anions after doping with CH2Cl2 (C) under Kr lamp illumination. Time-of-flight mass spectra of the cations generated from the residual H2O in the N2 buffer gas before (D) and after (E) doping with CH2Cl2 and the anions after doping with CH2Cl2 (F) under Xe lamp illumination.

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Figure 2. Time-of-flight mass spectra of the cations generated from acetone in N2 before (A) and after (B) doping with CH2Cl2 and the anions after doping with CH2Cl2 (C) under Kr lamp illumination. The mass peak at m/z 31 shown in Figure 2 (A) is assigned as the daughter ion of the acetone, and the peak in Figure 2 (B) is assigned as both the daughter ion of acetone and the protonated formaldehyde by-product. Time-of-flight mass spectra of the cations generated from methanol in N2 before (D) and after (E) doping with CH2Cl2 and the anions after doping with CH2Cl2 (F) under Kr lamp illumination. The tiny amount of protonated methanol (m/z 33) before doping with CH2Cl2 may originate from EI ionization by photoelectrons or photoionization of methanol dimers (IE, 9.7 eV) followed by an intramolecular ion-molecule reaction.55 The mass peaks at m/z 29, 45, and 47 in Figure 3 (E) are assigned to ethanol and other impurities.

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493 494 495 496 497 498 499 500 501 502 503

Figure 3. Time-of-flight mass spectra of the cations generated from acetone (8.3 × 1013 molecules s-1) doped with CD2Cl2 (8.3 × 1015 molecules s-1) (A) and CH2Cl2 (8.3 × 1015 molecules s-1) (B-blue) followed by doping with D2O (8.3 × 1015 molecules s-1) (B-red) under Kr lamp illumination. The mass peaks at m/z 49 (CH235Cl+) and 51 (CH237Cl+) shown in Figure 3 (B) are mainly due to the photoelectron impacts of the excited-state of CH2Cl2 in the ion immigration region. By increasing the CH2Cl2 or H2O concentration, the signals of the mass peaks at m/z 49 and 51 decrease because the VUV light reaches a saturated absorption in the ionizer due to the high concentrations of CH2Cl2. Once saturated, the VUV light is prevented from leaking and interacting with the H2O and excited-state CH2Cl2 and may depress the dissociation of the excited-state CH2Cl2.

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Figure 4. Dependence of the logarithmic ion signal intensities on the logarithmic photon intensities (A and B). The data points shown in Figure 4 (A) were calculated from the ion currents generated from the N2 buffer gas after doping with CH2Cl2 and measured using a picoammeter. The data points in Figure 4 (B) were calculated from the mass spectrometric signal intensities of protonated water (m/z 19) and acetone (m/z 59) generated from the N2 buffer gas and acetone in N2 after doping with CH2Cl2. The partial flow rate of the acetone was 8.2 × 1013 molecules s-1 and that of the CH2Cl2 dopant was 1.2 × 1018 molecules s-1. The solid lines represent the results of the linear fitting. Figure 4 (C and D) show the doping effects of the four chlorohydrocarbons on the intensities of the cation current generated from the N2 buffer gas (C) and acetone in N2 (D), which were measured with the picoammeter. The partial flow rate of acetone was 4.1 × 1013 molecules s-1. A Kr lamp VUV light source was applied in these experiments. The error bars of the data points represent the standard deviation of three parallel experiments.

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Figure 5. Contributions of different ionization processes to the mass spectrometric signal

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intensities of methanol, acetone, benzene, and p-xylene (8.1 × 1013 molecules s-1, respectively) in

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the presence of 8.1 × 1016 molecules s-1 CH2Cl2 under Kr lamp illumination. PTR represents the

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conventional proton transfer reaction induced by H3O+.

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Figure 6. Linear calibration curves of signal intensities corresponding to the concentrations of

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aldehyde (R2=0.9955), acetic acid (R2=0.9724), ethanol (R2=0.9978), diethyl ether (R2=0.9996),

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and acetone (R2=0.9986) in the presence of 2.4 × 1018 molecules s-1 CH2Cl2.

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H2O-Amplified Ionization

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