CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 46, Issue 4, April 2018 Online English edition of the Chinese language journal

Cite this article as: Chinese J. Anal. Chem., 2018, 46(4): 471–478

RESEARCH PAPER

Development of Dipolar Proton Transfer Reaction Mass Spectrometer for Real-time Monitoring of Volatile Organic Compounds in Ambient Air ZHANG Qiang-Ling1,2, ZOU Xue1, LIANG Qu1,2, ZHANG Ya-Ting1,2, YI Ming-Jian4, WANG Hong-Mei3, HUANG Chao-Qun1, SHEN Cheng-Yin1,*, CHU Yan-Nan1 1 Anhui Province Key Laboratory of Medical Physics and Technology, Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China

2 University of Science and Technology of China, Hefei 230026, China 3 Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China 4 Institute of Atmospheric Environment, Research Academy of Environmental Sciences of Anhui Province, Hefei 230026, China

Abstract: Volatile organic compounds (VOCs) in ambient air can participate in photochemical reactions, which lead to the generation of secondary pollutants such as ozone and aerosol. So real-time and accurate monitoring of atmospheric VOCs plays an important role in the study of the causes of air pollution. On the basis of proton transfer reaction mass spectrometry (PTR-MS) research, a novel dipolar proton transfer reaction mass spectrometer (DP-PTR-MS) for real-time and on-line monitoring of atmospheric VOCs was developed. Compared with conventional PTR-MS with one kind of reagent ion H3O+, DP-PTR-MS had three kinds of reagent ions H3O+, OH‒, (CH3)2COH+, which could be switched according to the actual detection need. So DP-PTR-MS can improve the qualitative ability and expand the detection range effectively. The reagent ion H3O+ can be used for detecting VOCs whose proton affinities are greater than that of H2O. The reagent ion OH‒ can be used to identify VOCs cooperating with the reagent ion H3O+, and can also be used for detecting some inorganic substances such as CO2. The reagent ion (CH3)2COH+ can be used for accurately detecting NH3 under interference elimination circumstances. The limit of detection (LOD) and sensitivity of DP-PTR-MS were measured by using six kinds of standard gases. The results showed that the LOD for detecting toluene was 7 × 10‒12 (V/V) and the sensitivity for detecting ammonia reached 126 cps/10‒9 (V/V). The ambient air in Hefei city was on-line and real-time monitored for continuous 78 h with DP-PTR-MS. The results showed that the newly developed DP-PTR-MS could be used for long-term and real-time monitoring atmospheric VOCs at the concentration of 10‒12 (V/V) level. DP-PTR-MS is an important tool to the study of the causes of atmospheric pollution and the monitoring of trace VOCs emissions. Key Words: Dipolar proton transfer reaction mass spectrometry; Volatile organic compounds; Real-time; On-line monitoring

1 Introduction

Volatile organic compounds (VOCs) are important air pollutants. Furthermore, they can participate in photochemical reactions, which can produce secondary pollutants, such as ozone and aerosol[1]. Therefore, real-time and accurate

monitoring of VOCs is of great significance to the study of the cause of air pollution and the control of VOCs emission.

Proton transfer reaction mass spectrometry (PTR-MS), developed in the late 1990s, is a kind of real-time and on-line monitoring technology for atmospheric VOCs. Its principle is that the reagent ions H3O+ are generated in the ion source by

________________________ Received 6 September 2017; Accepted 28 December 2017 *Corresponding author. E-mail: chyshen@aiofm.ac.cn This work was supported by the National Key R&D Program of China (No. 2016YFC0200200), the National Natural Science Foundation of China (Nos. 21777163, 21477132, 21577145) and the National Science and Technology Support Program of China (No.2014BAC22B06). Copyright © 2018, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(17)61078-8

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glow discharge with water vapor and then are injected into the drift tube under the effect of positive electric field. When the proton affinity (PA) of analyte M is higher than that of H2O, M will undergo a proton transfer reaction with H3O+ to produce ions MH+. The information on molecular weight and concentration of M can be obtained by detecting the mass charge ratio (m/z) and signal intensity of MH+ using mass spectrometer. PTR-MS has been widely applied in the fields of environment[3‒6], food[7‒9], health[10‒12] and public safety[13] due to its advantages such as short response time (~s), low LOD (~10-12 (V/V)) and absolute concentration measurement[2].

The proton transfer reaction between H3O+ and VOCs is a soft ionization reaction, which mainly produces protonated VOCs for most VOCs. However, the proton transfer reaction for partial VOCs could also produce a small amount of fragment ions or cluster ions[14]. These fragment ions and cluster ions could bring interference for distinguishing molecular weight of VOCs by traditional PTR-MS. Multiple reagent ions could assist to distinguish the molecular weight of VOCs using the characteristics of different reagent ions. Therefore, the detection using multiple reagent ions is a kind of developing trend of PTR-MS.

Recently, our team has successfully proposed a new technology of dipolar proton transfer reaction mass spectrometry (DP-PTR-MS). It only used water vapor discharge to obtain two kinds of reagent ions, H3O+ and OH-, under the action of positive and negative electric fields. The gas samples of ketones were detected respectively by switching reagent ions between H3O+ and OH-. Two kinds of reagent ions, H3O+ and OH-, not only realize the detection of organic and inorganic compounds, but also can help to estimate the molecular weight of analytes[15]. However, only the preparation and switching of H3O+ and OH‒ were realized in above work. And it was limited to the detection of high concentration ketones prepared in the laboratory.

Based on the previous validation study of technology principle, this work developed a dipolar proton transfer reaction mass spectrometer based on DP-PTR-MS technology starting from the demand of very low concentration VOCs monitoring in the real atmosphere. In addition to the positive and negative reagent ions of H3O+ and OH‒, it had a specific reagent ion (CH3)2COH+ for the detection of ammonia gas in this DP-PTR-MS. The basic composition of the DP-PTR-MS and different work modes combined with the monitoring software were introduced firstly. Then the mass spectra of three kinds of reagent ions were presented. And the limit of detection (LOD) and sensitivity of DP-PTR-MS were evaluated using standard gases. Finally, the ambient air in Hefei city was real-time and on-line monitored by the DP-PTR-MS.

2 Experimental 2.1 Instrument and reagents

The schematic diagram of the developed DP-PTR-MS for real-time and on-line monitoring atmospheric VOCs is shown in Fig.1A and the instrument picture of DP-PTR-MS is shown in Fig.1B. DP-PTR-MS mainly consists of gas sampling system, ion source, drift tube, and ion detection system. In addition to the sampling pipeline, the gas sampling system also integrates the catalytic converter, which can catalyze the VOCs in the air to CO2 and H2O under high temperature condition. The catalytic converter is used to obtain signal background intensity of the instrument. In addition, the sampling system also includes pressure controller and sampling pump. They are used to adapt to the change of atmospheric pressure and maintain the stability of the pressure in sampling pipeline. The ion source is mainly composed of discharge electrodes and ion guiding electrodes. The discharge of water vapor in the ion source can produce reagent ions H3O+ and OH-, one of which is extracted into the drift tube according to sample detection. As for reagent ion (CH3)2COH+, the acetone vapor is introduced into the ion source from ion guiding electrode under the positive ion mode and then undergoes a proton transfer reaction with H3O+ to generate protonated acetone ions (CH3)2COH+, which are introduced into the drift tube under the effect of positive electric field. In the drift tube, the analytes undergo reactions with selected reagent ions as shown in equations (1)–(3), and thus are ionized. The electric field applied to the drift tube can regulate the kinetic energy of ion-molecular reaction to reduce the formation of fragment ions and cluster ions, and guide the ions into the ion detection system. The ion detection system mainly made up of quadrupole mass filter detects the m/z and number of ions, and then gets the mass spectrum.

H3O+ + VOC [VOC + H]+ + H2O (1) OH‒ + VOC [VOC ‒ H]‒ + H2O (2)

(CH3)2COH+ + NH3 NH4+ (CH3)2CO (3) All the parameters of the DP-PTR-MS are monitored and

controlled by monitoring system software. The monitoring system software can control the water vapor flow, the acetone vapor flow, the pressure of the drift tube, the catalytic temperature, and the multichannel DC voltages, etc. DP-PTR-MS has six work modes, including warm-up, VOCs detection, VOCs background, ammonia detection, ammonia background and shutdown. The work modes can be selected through the shortcut button on the software main interface. Through power polarity and mass spectrometer polarity switching, we can choose positive ions or negative ions to detect analytes under four work modes of VOCs detection, VOCs background, ammonia detection and ammonia background. For example, we can choose H3O+ or OH- to detect VOCs in VOCs detection mode.

In the experiment, the voltage applied to the drift tube was respectively set to 600 V and 500 V under the positive ion mode and negative ion mode. Catalytic temperature was set to 310 ºC[16] and the pressure of drift tube was 110 Pa. Sampling

ZHANG Qiang-Ling et al. / Chinese Journal of Analytical Chemistry, 2018, 46(4): 471–478

Pump Pressure controller

Quadrupole mass filter

EM

Acetone Turbo pump 1H2O

Catalytic converter

Ion source Drift tube Ion detection system

AirA B

H3O++M H2O+[M+H]+

OH-+M H2O+[M-H]-

(CH3)2COH+

+NH3 (CH3)2CO+NH4+

Multichannel DC power

Three-Way solenoid valve

Turbo pump 2

Fig.1 (A) Schematic diagram of DP-PTR-MS for real-time and on-line monitoring of atmospheric VOCs, EM: electron multiplier; (B)

DP-PTR-MS instrument

flow was 480 mL min–1 and the heating temperature for sampling pipeline and drift tube was 55 ºC. The vacuum of mass spectrometer was maintained by a vacuum pump group composed of two molecular pumps and a backing pump made by German Pfeiffer Vacuum. The typical working pressure in the ion detection system was less than 3 × 10‒4 Pa. The monitoring software monitored the above pressure in real time, adjusted the working state of the ion detection system according to this pressure, and even controlled power supply of the vacuum pump group to protect the ion detection system and the vacuum pump group.

Six kinds of standard gases including benzene, toluene, formaldehyde, acetaldehyde, acetone and ammonia, were selected to examine LOD and sensitivity of DP-PTR-MS. Ammonia and benzene were purchased from Shanghai Haizhou Special Gas Co., Ltd (Ammonia in nitrogen, 3.0 × 10‒6

(V/V); Benzene in nitrogen, 3.0 × 10‒6 (V/V)). Formaldehyde was purchased from Wuhan Newradargas Co., Ltd. (Formaldehyde in nitrogen, 1.0 × 10‒5 (V/V)). Acetaldehyde, acetone and toluene standard gases were prepared by dynamically diluting the saturated vapor of pure liquid acetaldehyde, acetone and toluene, respectively. The basic steps of preparation were as follows. Saturated vapor of pure liquid reagents were extracted by micro injectors. The saturated vapor pressure at a certain temperature can be

calculated according to Antoine equation[17,18]. The micro injector with acetaldehyde, acetone or toluene was placed on an injection pump, respectively. According to the flow of 1, 1 and 10 μL min–1, the saturated vapor in the micro injectors was injected into the sampling pipeline. The standard gases (acetaldehyde, 1.85 × 10‒6 (V/V); acetone, 5.01 × 10‒7 (V/V); toluene, 6.43 × 10‒7 (V/V)) were obtained through dilution of sampling flow. High purity nitrogen (99.999%) was purchased from Nanjing special gases factory Co., Ltd. 2.2 Experimental method 2.2.1 Measurement method of LOD and sensitivity

In this study, the LOD of DP-PTR-MS was calculated with

a signal-to-noise ratio of 2:1. In the experiment, the work mode was first set to the VOCs background or ammonia background by the monitoring system software. In these modes, sample gas passed through the catalytic converter and VOCs in the sample gas was removed. The target product ion signal was monitored by mass spectrometer (Data sampling rate 1 Hz, averaging 120 times). The stationary monitoring signal was background signal, whose average value was used as background signal intensity D, and the standard deviation of the background signal was counted as noise signal intensity

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N. Then the work mode was adjusted to VOCs detection or ammonia detection mode. The standard gas, whose concentration was Cvoc, was directly monitored (Data sampling rate 1 Hz), and the average value of stationary monitoring signal was counted as signal intensity M. The LOD and sensitivity of DP-PTR-MS were 2NCvoc/(M ‒ D) and (M ‒ D)/Cvoc, respectively. 2.2.2 Monitoring method of atmospheric VOCs

Atmospheric VOCs monitoring site with DP-PTR-MS was

located on the fifth floor of the experimental building of Research Academy of Environmental Sciences of Anhui Province, China. The sampling pipe extended out of the wall through a sampling hole in the wall and sucked up the outdoor air into the instrument. In the validation monitoring experiment, several low concentration VOCs (Concentration range of 10‒9–10‒12 (V/V)) in the actual air were selected for continuous monitoring to verify the monitoring ability of the instrument for low concentration VOCs such as benzene, toluene, xylene, 2-butanone, acrolein and ethanol. The detection rate of DP-PTR-MS was 1 Hz (Averaging 120 times). The total monitoring time was 78 hours (July 6, 2017–July 9, 2017) with the DP-PTR-MS.

3 Results and discussion

3.1 Three kinds of positive and negative reagent ions Figure 2 shows mass spectra for three kinds of reagent ions:

H3O+, OH‒ and (CH3)2COH+. Figure 2A is the mass spectrum of reagent ions of H3O+. As shown in Fig.2A, the reagent ions are mainly H3O+ and cluster ions H2O (H3O+), at m/z 19 and m/z 37, respectively. In addition, there are extremely small amounts of ions at m/z 30 and 32, which are assigned as NO+ and O2

+. Their intensity is about 0.5% of the sum intensity of H3O+ and H2O (H3O+). So, they could not have a significant impact on the VOCs detection.

Figure 2B is mass spectrum of reagent ions OH- with high purity nitrogen as carrier gas, and the reagent ions are mainly OH- and cluster ions H2O (OH‒), at m/z 17 and 35, respectively. The ions at m/z 46 and 61 were assigned as NO2

‒ and CO2 (OH‒) respectively. NO2

- is the product ion of charge transfer reaction between OH‒ ions and NO2; CO2 (OH‒) is the product ion of addition reaction between OH- ions and CO2. Therefore, the reagent ions OH‒ can be used to detect inorganic compounds NO2 and CO2. Additionally, OH- ions can react with SO2 by a third body collision[19], so it can also be used to detect SO2. The reagent ions OH- mainly has a proton extraction reaction with VOCs, such as ketones[20].

Figure 2C is the mass spectrum of reagent ions (CH3)2COH+. The main ion is protonated acetone ion (CH3)2COH+ at m/z 59. Ions at m/z 30 and 43 are the fragment

Fig.2 Mass spectra of three kinds of reagent ions: (A) H3O+, (B) OH‒, (C) (CH3)2COH+

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ions of (CH3)2COH+ [21]. A small amount of ions at m/z 19 are residual H3O+, which are not completely converted to (CH3)2COH+ by acetone vapor. Ion at m/z 18 is NH4

+, which is the product ion of ammonia. The use of reagent ions (CH3)2COH+ for detecting ammonia is mainly to avoid interference of potential VOCs, and to make the mass spectrum simpler. The PA value of acetone (PA = 812 kJ mol–1)[22] is higher than that of H2O (PA = 691.0 kJ mol–1)[22], so VOCs with PA values between that of water and acetone cannot be detected.

In the above three work modes, the positive ion mode of VOCs detection (H3O+ reagent ions) is the most important mode of VOCs monitoring. The negative ion mode of VOCs detection (OH‒ reagent ions) and the positive ion mode of ammonia detection ((CH3)2COH+ reagent ions) make a supplement to the positive ion mode of VOCs detection on the qualitative ability and detection range. 3.2 LOD and sensitivity measurement of standard gases

VOCs detection positive ion mode (H3O+ reagent ions) is

the most important work mode for VOCs monitoring, so the LOD and sensitivity of DP-PTR-MS in this mode were measured in the experiment. In addition, the LOD and sensitivity for ammonia were measured in ammonia detection positive ion mode ((CH3)2COH+ reagent ions). Figure 3 shows an example of the LOD measurement of formaldehyde. The work mode was first set to the VOCs background mode. The stationary monitoring signal was background signal, from which the values of D and N (1875.2 and 0.960 cps) could be obtained. The work mode was switched to VOCs detection positive ion mode, and then ion intensity of formaldehyde standard gas (1.0 × 10‒5 (V/V)) could be obtained, whose average value M was 34328.1 cps. Finally, the LOD and sensitivity of DP-PTR-MS for formaldehyde could be calculated by the formula described in 2.2.1 (LOD, 5.91 × 10‒10 (V/V); sensitivity, 3.2 cps/10‒9 (V/V)). The LOD and sensitivity of DP-PTR-MS for other five compounds could be obtained by the same method and the results are shown in Table 1. The sensitivities of DP-PTR-MS for six compounds from high to low were ammonia (126.0 cps/10‒9(V/V)), acetone (105.2 cps/10‒9 (V/V)), toluene (93.2 cps/10‒9 (V/V)), benzene (35.2 cps/10‒9 (V/V)), acetaldehyde (35.1 cps/10‒9 (V/V)), and formaldehyde (3.2 cps/10‒9 (V/V)). The LOD of DP-PTR-MS for six compounds from low to high were toluene (7 × 10‒12 (V/V)), benzene (4.7 × 10‒11 (V/V)), acetaldehyde (1.75 × 10‒10 (V/V)), acetone (3.84 × 10‒10 (V/V)), formaldehyde (5.91 × 10‒10 (V/V)), and ammonia (1.144 ×10‒9

(V/V)). It can be seen that DP-PTR-MS had the highest sensitivity for detecting ammonia. However, the LOD for ammonia was poor because of the high background signal of the instrument at m/z 18. The LOD for other five kinds of VOCs all reached the level of 10‒10 (V/V), especially toluene (7 ×10‒12 (V/V)). The concentrations of VOCs in ambient air were usually the level of 1 × 10‒9 (V/V), so DP-PTR-MS could meet the need of real-time and on-line monitoring low concentration VOCs in ambient air.

3.3 Background signal of instrument

Before starting monitoring, the background signal intensity

of the instrument for the target compounds was obtained in the VOCs background positive ion mode, which provided the basis for concentration monitoring of atmospheric VOCs. The monitoring results for six kinds of VOCs are shown in Fig.4, when the work mode was VOCs background positive ion mode. Catalytic converter started work at 42 min, and the catalytic process lasted 20 min. From Fig.4, one can see that the signal intensities of VOCs first decrease rapidly and then stabilize gradually after the catalytic converter starts work, which indicates that the catalytic converter can effectively remove VOCs from the air. Finally, the average value of signal during 55–62 min was selected as the background signal of the instrument. It is worth noting that the signal intensities of most VOCs first rose up a small peak and then droped after the catalytic converter started work. The reason should be the cumulative VOCs released by the three-way valve located in the catalytic converter bypass. Although the intensity would drop and stabilize after a period of catalysis, the VOCs released by the three-way valve located in the downstream of the gas would still improve the background signal intensity of the instrument.

Fig.3 Monitor graph of formaldehyde

Table 1 LOD and sensitivity of DP-PTR-MS for six kinds of compounds

Compounds Benzene Toluene Formaldehyde Acetaldehyde Acetone Ammonia LOD (× 10‒12 (V/V)) 47 7 591 175 384 1144 Sensitivity (cps/10‒9 (V/V)) 35.2 93.2 3.2 35.1 105.2 126.0

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Fig.4 Background signal of DP-PTR-MS for six kinds of VOCs

3.4 Monitoring results of VOCs in ambient air

Figure 5 shows the real-time and on-line monitoring results

of benzene (m/z 79), toluene (m/z 93), xylene (m/z 107),

2-butanone (m/z 73), acrolein (m/z 57) and ethanol (m/z 47) in ambient air during 78 h. Figure 6 shows the results of weather, air quality index (AQI)[23], and temperature and humidity near sampling port during the monitoring process of VOCs. As shown in Fig.5A and Fig.5C, one can see that the concentration variation trends of benzene and xylene are almost identical, indicating the source and transfer conversion mechanisms of both might be substantially the same[16]. Before the second day morning, their concentrations had a significant positive correlation with temperature in Fig.6. In the second and third days, their concentrations decreased from 12: 00 to 15: 00, and the variation trends exhibited a negative correlation with temperature, which might be the result of the combined action of temperature and photochemical reaction. As shown in Fig.5D and Fig.5E, the concentration variation trends of 2-butanone and acrolein were also very similar, but were not regular during monitoring process. It is worth mentioning that five of the six monitored VOCs during 23:00–24:00 of the second day, all rose first and then decreased

Fig.5 Monitoring results of (A) benzene, (B) toluene, (C) xylene, (D) 2-butanone, (E) acrolein and (F) ethanol in ambient air

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Fig.6 Weather parameters during monitoring process of VOCs

except for ethanol. In particular, concentrations of 2-butanone and toluene were 1–3 times higher than the highest concentration of daytime. They are all commonly used organic solvents[24], so the possibility that some factories had short time emissions at night could not be excluded. Figure 5F is the monitoring result of ethanol. Its overall concentration was high and fluctuated significantly during 6:00–12:00 a.m. Because the monitoring site was next to the main road of the city, this might be associated with motor vehicle emissions. The concentration then began to decrease and reached a minimum value during 13:00–15:00, which was the same as the time curve of minimum point for several other compounds. This might be related to the consumption of active photochemical reaction at noon. After that, the concentration increased gradually, reached peak value during 21:00–23:00, then decreased gradually, and finally tended to be stable until 6:00 a.m. of next day. The variation trend was just in line with that traffic volume, which decreased after 21:00–23:00 and increased at 6:00 a.m. of next day. This also indirectly proved that ethanol might be from motor vehicle emissions. All in all, high temperature could lead to the rise of VOCs concentration, the strong light at noon might be the reason for VOCs concentration decline and the moderate rain should also be the reason for overall VOCs concentration decline on the fourth day. Although the reason for concentration change of VOCs during the monitoring period was not entirely clear, the results showed that DP-PTR-MS has the ability of real-time and on-line monitoring atmospheric VOCs at the concentration level of 10‒12 (V/V). 4 Conclusions

A novel mass spectrometer (DP-PTR-MS) for real-time and

on-line monitoring of atmospheric VOCs was developed. DP-PTR-MS had three kinds of reagent ions H3O+, OH- and (CH3) 2COH+, which could be switched according to the actual detection need. DP-PTR-MS could not only detect VOCs, but also detect the inorganic compounds, such as NH3 and CO2. The LOD for detecting toluene reached 7 × 10‒12 (V/V) and the

sensitivity for detecting ammonia reached 126.0 cps/10‒9 (V/V). The ambient air in Hefei city was real-time and on-line monitored for continuous 78 h with DP-PTR-MS. The results show that the novel DP-PTR-MS can be used for long-term and real-time monitoring of atmospheric VOCs at the concentration level of 10‒12 (V/V). DP-PTR-MS is an important tool for the study of the causes of atmospheric pollution and the monitoring of trace VOCs emissions. References [1] Jenkin M E, Derwent R G, Wallington T J. Atmos. Environ.,

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