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Thermal Desorption Low Temperature Plasma Ionization Mass Spectrometry for Rapid and Sensitive Detection of Pesticides in Broomcorn
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 45, Issue 2, February 2017 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2017, 45(2), 175–182.
RESEARCH PAPER
Thermal Desorption Low Temperature Plasma Ionization Mass Spectrometry for Rapid and Sensitive Detection of Pesticides in Broomcorn WANG Shuang1,2, WANG Zhe3, HOU Ke-Yong1,*, LI Hai-Yang1,* 1
Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of sciences, Dalian 116023, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Analysis and Test Center of Jing Brand Co., Ltd., Huangshi 344000, China
Abstract: A thermal desorption low temperature plasma (TD-LTP) ionization source was developed for the direct, rapid and sensitive detection of pesticides by mass spectrometry without complex sample pretreatment. A thermal desorption sampler was added in front of the plasma generator, and the analyte was collected on a polytetrafluoroethylene (PTFE) swab and desorbed into gas molecules, and then gas molecules were transported to the plasma generator by carrier gas to be ionized. The utilization of thermal desorption sampler helped to transform the interaction between gas phase plasma and sample from gas-solid or gas-liquid to gas-gas, which greatly increased the sensitivity and stability especially for non-volatile sample (e.g. pesticides) compared with conventional LTP ionization source. The TD-LTP ionization source was coupled to a homemade rectilinear ion trap mass spectrometry. The operation parameters of the TD-LTP ionization source were optimized and the characteristic ions of 12 different pesticides were successfully obtained and investigated. Then the TD-LTP ionization source was connected with commercial ACQUITY TQD mass spectrometer, and the pesticide residue level in broomcorn was evaluated. Key Words:
Rapid detection of pesticide residues; Thermal desorption sample introduction; Low temperature plasma ion source;
Rectilinear ion trap; Triple quadrupole mass spectrometry
1
Introduction
Numerous pesticides have been widely used to enhance agricultural production. Apart from their agricultural benefits, pesticide residues also cause severe environmental issues[1] and impose acute toxicological effects on human health[2]. It is increasingly important to develop sensitive and efficient methods to detect pesticides residues in environment and foods[3]. Conventional techniques with high precision, such as chromatographic techniques (high performance liquid chromatography (HPLC), gas chromatography (GC)) and immunological analysis method, etc.[4‒7] used for pesticides
detection, often have some limitations like low efficiency[8], time-consuming and laborious process[5,8], requirement of expensive equipments and highly trained technicians etc[9]. So, it is difficult for traditional methods to meet the requirements of on-site and rapid detection of pesticide residues. Massive sample analysis for pesticide residues also requires sensitive and high throughput techniques. Mass spectrometry (MS) is a powerful analytical technique which can be used to separate and detect specific gas phase ion in complex matrix based on mass-to-charge ratio. MS technique has been gradually developed into a "gold standard method" for organic compounds analysis by virtue of fast
________________________ Received 5 September 2016; accepted 23 November 2016 *Corresponding author. Email: [email protected]; [email protected] This work was sponsored by the National Natural Science Foundation of China (No. 21375129), the National Special Fund for the Development of Major Research Equipment and Instrument (No. 2011YQ05006904), and the National Science & Technology Pillar Program of China (No. 2013BAK14B04). Copyright © 2017, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(16)60993-3
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
analysis speed, high specificity and high sensitivity. The role of MS ionization source is to create gas phase atomic and molecular ions. The performance of ionization source determines the catalog of analytes, and the innovation of the ionization technique often brings leap-forward development in the application of MS technique[10]. In 2004, Takats et al[11] firstly reported a new atmospheric environmental ionization source, desorption electrospray ionization (DESI) technology. DESI can realize fast, high throughput and real-time analysis of trace components on the surface of liquid or solid samples without sample preparation[12]. Since its advent, DESI attracted numerous attentions. A variety of new types of atmospheric ionization technologies have been developed. Up to now, more than 30 kinds of atmospheric ionization sources have been designed, such as direct analysis in real time (DART)[13], extractive electrospray ionization (EESI)[14], laser ablation electrospray ionization (LAESI)[15], dielectric barrier discharge ionization (DBDI)[16] and so on. Low temperature plasma (LTP) ionization is a kind of atmospheric ionization source based on dielectric barrier discharge[17]. It has been successfully applied to the detection of explosives[18‒20], drugs[21,22], pesticides[23], etc. The analyte is usually put in the open air, and the plasma of LTP is sprayed on the surface of solid to ionize the analyte, and then the ions are carried into the mass spectrometer by the airflow. Due to the open operating environment, conventional LTP ionization source is susceptible to surrounding environment (such as temperature, humidity and airflow) partly, and its relative standard deviation (RSD) of signal intensity is as high as 30%[23]. LTP is a nonequilibrium plasma ionization source, and the temperature of plasma is close to or slightly higher than the room temperature. The ion generation of sample with LTP ionization source is based on a complicated chemical ionization. The higher the vapor pressure, the more sensitive is obtained in the analysis. So, the sensitivity of conventional LTP is low for non-volatile or semi-volatile organic compounds (e.g. pesticides). Improving the surface
temperature of the analyte can increase the vapor pressure of non-volatile samples. Hence it can greatly improve the efficiency of ionization and increase the sensitivity of the mass spectrometer by introducing a thermal desorption device in front of the LTP ionization source. Our group introduced a novel non-contact halogen lamp heating assisted LTP ionization source for in situ and highly sensitive detection of explosives and pesticide samples[24,25]. The LTP probe was positioned at an angle of 45° and a distance of 3 mm away from MS inlet. The RSDs for three kinds of pesticide detection using this ionization source were at around 11%, and the stability of this ionization source was not very suitable for accurate quantitative analysis. In this work, a thermal desorption low temperature plasma (TD-LTP) ionization source was developed for rapid and sensitive detection of pesticides by direct mass spectrometry. For the TD-LTP, a sample thermal desorption section was introduced to increase vapor pressure of samples. The plasmasolid interaction between plasma and samples in conventional LTP ionization source was transformed into plasma-gas interaction, which effectively improved the ionization process. In addition, the TD-LTP ionization source was fixed to MS coaxially, resulting in less ion loss and higher sensitivity. Then the thermal desorption LTP ionization source was coupled to a commercial ACQUITY TQD mass spectrometer for detection of 12 kinds of pesticides and qualitative evaluation of residue level of these pesticides in broomcorn.
2 2.1
Experimental Instrument design and detection principle
As shown in Fig.1, TD-LTP ionization source consists of two parts, thermal desorption sample injector and LTP plasma generator. The thermal desorption sample injector is constituted of thermal desorption module (including heating and temperature control) and controlling module, desorption
Fig.1 Schematic diagram of thermal desorption low temperature plasma (LTP) ion source (A) and conventional LTP ion source (B)
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
temperature can be regulated in the range from room temperature to 240 ºC. LTP generator is constituted of radio frequency power supply, a three-way holder, airway, a threeway glass tube, a ring metal electrode and a cylindrical metal inner electrode. In comparison with conventional LTP, TD-LTP ionization source has several advantages. Firstly, a thermal desorption sampler is introduced to promote the evaporation of the analytes and improve the sample utilization rate. Secondly, sample is introduced into the discharging area where the plasma concentration is high, thus ensuring thorough interaction of plasma and samples. Thirdly, the ionization source is connected to the injection port of mass spectrometer coaxially, thus improving the ion transmission to mass spectrometer. The improvement in these three aspects increases sensitivity of instrument and stability of ionization. For liquid samples, a certain volume of sample solution is dripped onto the surface of a polytetrafluoroethylene (PTFE) swab by a micropipettor, and the solvent is volatilized at room temperature. For solid sample, the surface is gently wiped by the PTFE swab. Then the PTFE swab is inserted into the thermal desorption sampler. Gas sample molecules produced in the thermal desorption sampler is carried by carrier gas (air) to the three-way glass tube, then collided with plasma, and ionized by chemical ionization such as the proton transfer and charge transfer reaction. The positive and negative ions are produced by TD-LTP ionization simultaneously.
2.3.1
The working solutions of pesticide from 0.1 mg L–1 to 10.0 mg L–1 were prepared by diluting the pesticide standard solution (1000 mg L–1) with step dilution method. One micro liter of the liquid solution was sampled by micropipettor and dripped onto the surface of a PTFE swab. After volatilization of solvent at room temperature, PTFE swab was placed on the sample holder. The sample detection time was less than 2 s. 2.3.2
Instruments and reagents
Mass spectrum analysis of broomcorn samples was carried out on an ACQUITY TQD triple quadrupole mass spectrometer (Waters, USA). The home-made rectilinear ion trap mass spectrometer[24] contains a discontinuous atmosphere sampling interface (DAPI), a rectilinear ion trap mass analyzer (x0 = 5.0 mm, y0 = 4.0 mm, z0 = 43.2 mm), an electron multiplier detector with conversion dynode, and a vacuum system. The vacuum system of this mass spectrometer is constituted of a 15 L min‒1 diaphragm pump and a 80 L s‒1 molecular pump (Pfeiffer Vacuum Inc., Nashua, NH, Pfeiffer HiPace 80). The size of the vacuum chamber is 14 cm × 11.5 cm × 8.5 cm. Helium (99.999%) (Dalian Special Gas Company, China) was used as the discharge gas. Methanol and acetone (both are analytical grade) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (China). Phoxim, ethion, diazinon and other 9 kinds of pesticide standards were purchased from Ministry of Agriculture Environmental Quality Supervision, Inspection and Testing Center (Tianjin, China). Experimental methods
Screening methods of pesticide residues in broomcorn sample
The broomcorn sample solution and spiked sample solution were detected on TD-LTP triple quadrupole mass spectrometer. If the characteristic ion peak of a pesticide appeared in the mass spectrum of broomcorn sample with the signal to noise ratio (S/N) greater than three, and the peak intensities of the spiked sample solution were significantly enhanced than that of the broomcorn sample, the presence of pesticide residue in the broomcorn sample could be ascertained. Otherwise the pesticide residue was believed to be absent.
3
Results and discussion
In this study, the optimization of experimental conditions and performance test of TD-LTP ionization source were carried out on rectilinear ion trap mass spectrometer in Dalian Institute of Chemical Physic, Chinese Academy of Sciences. The practical application of TD-LTP ionization source was carried out in combination with a commercialized triple quadrupole MS. 3.1
2.3
Preparation of broomcorn sample and spiked sample solution
The broomcorn sample was prepared as follows. Approximately 25 g of broomcorn sample was added into 50 mL of methanol directly without treatment, followed by an ultrasonic extraction (35 Hz) for 5 min. The extract was then filtrated, concentrated to 1 mL, and diluted to 5 mL with methanol. The spiked sample solution was prepared as follows. Firstly, 1 mL of pesticide standards (Table 1) at concentration level of 10 mg L‒1 was added to 25 g of broomcorn sample. Then 38 mL of methanol was added. The mixture was then stirred thoroughly, ultrasonicated for 5 min (35 Hz), and separated by filtration. The obtained filtrate was concentrated to 1 mL, and diluted to 5 mL with methanol. 2.3.3
2.2
Preparation and detection of pesticide standard solutions
Optimization and characterization of TD-LTP ionization sources
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
3.1.1
Optimization of TD-LTP ionization sources
The performance of TD-LTP ionization source is mainly determined by flow rate of discharge gas (helium), thermal desorption temperature, and flow rate of carrier gas (air). The experimental conditions were optimized via a single variable method on the homemade rectilinear ion trap mass spectrometer. Pentaerythritol tetranitrate (PETN) was chosen as testing sample at a concentration level of 10 mg L–1. The detection was carried out under negative mode with 1 μL of sampling volume. The optimal experimental results were shown in Fig.2. As shown in Fig.2A, when the thermal desorption temperature and the carrier gas flow was kept constant, the signal intensity of PETN increased with increasing helium flow rate and achieved a maximum value at 150 mL min–1, and then decreased with further increasing helium flow rate. This was due to the limitation of the chemical reaction ionization of the LTP. When the discharging helium flowed slowly, the plasma density was low and the reagent ion used for chemical ionization was not adequate enough to ionize all the analyte molecules. With the increase of flow rate of helium, the amount of reagent ions was increased and thus the signal intensity increased, but the extra high flow of helium would dilute the analyte and reagent ion, thus resulted in decline of the signal intensity. The effect of thermal desorption temperature on the intensity of PETN is shown in Fig.2B. The signal intensity of the samples increased with the increase of thermal decomposition temperature and achieved a maximum value at 180 ºC, and then decreased. When the thermal desorption temperature increased, the desorption rate of the sample
increased, and more gaseous analyte molecules reached ionization zone to react with reagent ions. When the thermal desorption temperature was higher than 180 ºC, the speed of sample desorption was higher than the ion sampling speed used at pulse mode with DAPI inlet, resulting in decline of the signal intensity. Figure 2C shows that, when the thermal desorption temperature and the helium flow was constant, the gaseous analyte molecules could not reach the ionization zone in time at a low carrier gas flow rate, which led to low signal intensity. If the carrier gas flow rate was higher than 100 mL min‒1, the ionized sample ion had been blow out of the ionization zone by the carrier gas before DAPI was turned on. So the sample signal intensity increased with the increase of carrier gas flow at first and then decreased. After optimization, the optimal ionization parameters used in the subsequent experiments were as follows: helium flow rate of 150 mL min‒1, thermal desorption temperature of 180 ºC and air flow of 100 mL min‒1. 3.1.2
Characterization of TD-LTP ionization source
To demonstrate the performance of TD-LTP ionization source, traditional LTP ionization source and TD-LTP ionization source were coupled with the homemade rectilinear ion trap mass spectrometer respectively for analysis of the same samples. Under the optimal conditions, phosmet (10 mg L‒1) and dursban solution (10 mg L‒1) were tested at the sampling volume of 1 μL. The results are shown in Fig.3. The sensitivity of TD-LTP ionization source for detection of phosmet and dursban were increased by 8 times and 8.5 times respectively compared to traditional LTP ionization source.
Fig.2 Optimization of parameters for the thermal desorption LTP: (A) helium flow rate; (B) thermal temperature; (C) air flow rate
Fig.3 Mass spectra of (A) phosmet and (B) dursban obtained with traditional LTP and the TD-LTP respectively
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
The pre-condition of practical application for TD-LTP is that the sensitivity is high enough for the requirements of laws and standards. The limit of detections (LODs) of 12 kinds of pesticide standards were tested under optimized ionization conditions using TD-LTP ionization source triple quadrupole mass spectrometer. The results are shown in Table 1. Compared with the maximum permitted limit of pesticide residues regulated in national standard[26], the detection limit of TD-LTP ionization source triple quadrupole mass spectrometer could satisfy the national standard. This method can be used as a high throughput screening tool for pesticide residues in cereals.
The precision of the TD-LTP ionization source was also investigated, and the relative standard deviations (RSDs) for 6 repeated testing of the two pesticides were 8.6% and 7.2%, respectively, which were increased by about 4 and 3 times respectively compared with traditional LTP ionization source. To characterize the ion species produced by TD-LTP ionization source, TD-LTP was combined with homemade rectilinear ion trap mass spectrometer to detect 12 kinds of pesticide standards. The sampling volume for each pesticide standard was 1 μL. The detailed information of the pesticides and the characteristic ion species are listed in Table 1. The results show that TD-LTP ionization source is a soft ionization source. At positive mode, the characteristic ion is [M + H]+ or [M + H2O + H]+, and at negative mode, the characteristic ion is [M ‒ R]‒ (R is a alkyl group inside M molecule). 3.2
3.2.2
According to the experimental procedure described above, the broomcorn sample and spiked broomcorn sample were detected with the TD-LTP ionization triple quadrupole mass spectrometer at positive mode and negative mode respectively. The results are shown in Fig.4 and Fig.5.
Practical application of TD-LTP ionization source
3.2.1
Fast screening of pesticide residues in broomcorn
LODs of TD-LTP ionization source triple quadrupole mass spectrometry for pesticide analysis
Table 1 Detailed information of 12 kinds of different pesticides tested in this study No.
Sample name
Detection mode
Molecular weight
Characteristic ion ‒
Characteristic ion (m/z)
Limit of
Maximal residue level in GB2763-2014[26]
detection ‒1
(LOD, mg L )
(mg L‒1)
1
Phosmet
Negative mode
317
[M – CH3]
302
1
2.5
2
Ethion
Negative mode
384
[M – C2H5]‒
355
0.2
1.0
3
Dursban
Negative mode
351
[M – C2H5]‒
322
5
2.5
185, 187/206
1
0.5
4
DDVP
Negative mode
221
[M – Cl + O] ‒/ [M – CH3]‒ ‒
5
Oxadiazon
Negative mode
345
[M +
471
1
0.25
6
Phorate
Negative mode
260
[M – C2H5]‒-
231
0.5
0.1
7
Malathion
Negative mode
330
[M – CH3] ‒
315
0.1
40
8
Phoxim
Negative mode
298
[M – C2H5]‒
269
0.1
0.25
9
Aldrin
Positive mode
365
[M + H2O + H]+
384
0.5
0.1
10
Heptachlor
Positive mode
373
[M + H2O + H]+
391
0.4
0.1
11
Diazinon
Positive mode
304
[M + H]+
305
0.01
0.5
Positive mode
305
[M + H]+
306
0.01
25
12
Pirimiphosmethyl
Cl
Cl
]
(B)
Fig.4 Mass spectra (A) and local amplification figure (B) of sorghum sample and spiked sample at positive mode
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
Fig.5 Mass spectra (A) and local amplification figure (B) of sorghum sample and spiked sample at negative mode a, b, c, d, e, f are corresponding amplification zones respectively
At positive mode, diazinon and methylpyrimidine were detected in spiked broomcorn sample solution but not detected in sample solution, which indicated that the sensitivity of TD-LTP ionization source could satisfy the detection requirements of diazinon and methyl pyrimidine, but these two pesticide residues did not exist in broomcorn samples. At negative mode, seven pesticide residues (dichlorvos, phoxim, phosmet, malathion, chlorpyrifos, heptachlor and aldrin) were detected in spiked broomcorn sample. In the mass spectrum of the broomcorn sample, the characteristic ion peaks of dichlorvos, malathion and chlorpyrifos were found. From the point of view of S/N > 3, it could be determined that the residues of dichlorvos were present in broomcorn sample. The results were consistent with those obtained by the recommended national standards method with GC-MS. The experimental results demonstrated that TD-LTP direct mass spectrometry could be used as a high-throughput screening tool for pesticide residues detection.
4
Conclusions
In this study, thermal desorption process of sample was introduced on the basis of traditional LTP ionization source. A thermal desorption low temperature plasma ionization source was designed and built, which could produce both positive and negative ions at the same time. The TD-LTP ionization source was combined with a homemade rectilinear ion trap mass spectrometer to identify characteristic ions of 12 pesticide standard samples. Then the TD-LTP ionization source was applied to a commercialized triple quadrupole mass spectrometer, and LOD and stability for pesticide residue detection could satisfy the requirements of laws and standards. Finally, the TD-LTP ionization source was applied to analyze the pesticide residues in broomcorn sample and the pesticide residues were successfully recognized without complex sample pretreatment. These results demonstrated that TD-LTP mass spectrometry could
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
minimize sample preparation and provide a tough analytical approach for complex matrices.
[14] Chen H W, Venter A, Cooks R G. Chem. Commun., 2006, 19:
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Cite this article as: Chin J Anal Chem, 2017, 45(2), 175–182.
RESEARCH PAPER
Thermal Desorption Low Temperature Plasma Ionization Mass Spectrometry for Rapid and Sensitive Detection of Pesticides in Broomcorn WANG Shuang1,2, WANG Zhe3, HOU Ke-Yong1,*, LI Hai-Yang1,* 1
Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of sciences, Dalian 116023, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Analysis and Test Center of Jing Brand Co., Ltd., Huangshi 344000, China
Abstract: A thermal desorption low temperature plasma (TD-LTP) ionization source was developed for the direct, rapid and sensitive detection of pesticides by mass spectrometry without complex sample pretreatment. A thermal desorption sampler was added in front of the plasma generator, and the analyte was collected on a polytetrafluoroethylene (PTFE) swab and desorbed into gas molecules, and then gas molecules were transported to the plasma generator by carrier gas to be ionized. The utilization of thermal desorption sampler helped to transform the interaction between gas phase plasma and sample from gas-solid or gas-liquid to gas-gas, which greatly increased the sensitivity and stability especially for non-volatile sample (e.g. pesticides) compared with conventional LTP ionization source. The TD-LTP ionization source was coupled to a homemade rectilinear ion trap mass spectrometry. The operation parameters of the TD-LTP ionization source were optimized and the characteristic ions of 12 different pesticides were successfully obtained and investigated. Then the TD-LTP ionization source was connected with commercial ACQUITY TQD mass spectrometer, and the pesticide residue level in broomcorn was evaluated. Key Words:
Rapid detection of pesticide residues; Thermal desorption sample introduction; Low temperature plasma ion source;
Rectilinear ion trap; Triple quadrupole mass spectrometry
1
Introduction
Numerous pesticides have been widely used to enhance agricultural production. Apart from their agricultural benefits, pesticide residues also cause severe environmental issues[1] and impose acute toxicological effects on human health[2]. It is increasingly important to develop sensitive and efficient methods to detect pesticides residues in environment and foods[3]. Conventional techniques with high precision, such as chromatographic techniques (high performance liquid chromatography (HPLC), gas chromatography (GC)) and immunological analysis method, etc.[4‒7] used for pesticides
detection, often have some limitations like low efficiency[8], time-consuming and laborious process[5,8], requirement of expensive equipments and highly trained technicians etc[9]. So, it is difficult for traditional methods to meet the requirements of on-site and rapid detection of pesticide residues. Massive sample analysis for pesticide residues also requires sensitive and high throughput techniques. Mass spectrometry (MS) is a powerful analytical technique which can be used to separate and detect specific gas phase ion in complex matrix based on mass-to-charge ratio. MS technique has been gradually developed into a "gold standard method" for organic compounds analysis by virtue of fast
________________________ Received 5 September 2016; accepted 23 November 2016 *Corresponding author. Email: [email protected]; [email protected] This work was sponsored by the National Natural Science Foundation of China (No. 21375129), the National Special Fund for the Development of Major Research Equipment and Instrument (No. 2011YQ05006904), and the National Science & Technology Pillar Program of China (No. 2013BAK14B04). Copyright © 2017, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(16)60993-3
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
analysis speed, high specificity and high sensitivity. The role of MS ionization source is to create gas phase atomic and molecular ions. The performance of ionization source determines the catalog of analytes, and the innovation of the ionization technique often brings leap-forward development in the application of MS technique[10]. In 2004, Takats et al[11] firstly reported a new atmospheric environmental ionization source, desorption electrospray ionization (DESI) technology. DESI can realize fast, high throughput and real-time analysis of trace components on the surface of liquid or solid samples without sample preparation[12]. Since its advent, DESI attracted numerous attentions. A variety of new types of atmospheric ionization technologies have been developed. Up to now, more than 30 kinds of atmospheric ionization sources have been designed, such as direct analysis in real time (DART)[13], extractive electrospray ionization (EESI)[14], laser ablation electrospray ionization (LAESI)[15], dielectric barrier discharge ionization (DBDI)[16] and so on. Low temperature plasma (LTP) ionization is a kind of atmospheric ionization source based on dielectric barrier discharge[17]. It has been successfully applied to the detection of explosives[18‒20], drugs[21,22], pesticides[23], etc. The analyte is usually put in the open air, and the plasma of LTP is sprayed on the surface of solid to ionize the analyte, and then the ions are carried into the mass spectrometer by the airflow. Due to the open operating environment, conventional LTP ionization source is susceptible to surrounding environment (such as temperature, humidity and airflow) partly, and its relative standard deviation (RSD) of signal intensity is as high as 30%[23]. LTP is a nonequilibrium plasma ionization source, and the temperature of plasma is close to or slightly higher than the room temperature. The ion generation of sample with LTP ionization source is based on a complicated chemical ionization. The higher the vapor pressure, the more sensitive is obtained in the analysis. So, the sensitivity of conventional LTP is low for non-volatile or semi-volatile organic compounds (e.g. pesticides). Improving the surface
temperature of the analyte can increase the vapor pressure of non-volatile samples. Hence it can greatly improve the efficiency of ionization and increase the sensitivity of the mass spectrometer by introducing a thermal desorption device in front of the LTP ionization source. Our group introduced a novel non-contact halogen lamp heating assisted LTP ionization source for in situ and highly sensitive detection of explosives and pesticide samples[24,25]. The LTP probe was positioned at an angle of 45° and a distance of 3 mm away from MS inlet. The RSDs for three kinds of pesticide detection using this ionization source were at around 11%, and the stability of this ionization source was not very suitable for accurate quantitative analysis. In this work, a thermal desorption low temperature plasma (TD-LTP) ionization source was developed for rapid and sensitive detection of pesticides by direct mass spectrometry. For the TD-LTP, a sample thermal desorption section was introduced to increase vapor pressure of samples. The plasmasolid interaction between plasma and samples in conventional LTP ionization source was transformed into plasma-gas interaction, which effectively improved the ionization process. In addition, the TD-LTP ionization source was fixed to MS coaxially, resulting in less ion loss and higher sensitivity. Then the thermal desorption LTP ionization source was coupled to a commercial ACQUITY TQD mass spectrometer for detection of 12 kinds of pesticides and qualitative evaluation of residue level of these pesticides in broomcorn.
2 2.1
Experimental Instrument design and detection principle
As shown in Fig.1, TD-LTP ionization source consists of two parts, thermal desorption sample injector and LTP plasma generator. The thermal desorption sample injector is constituted of thermal desorption module (including heating and temperature control) and controlling module, desorption
Fig.1 Schematic diagram of thermal desorption low temperature plasma (LTP) ion source (A) and conventional LTP ion source (B)
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
temperature can be regulated in the range from room temperature to 240 ºC. LTP generator is constituted of radio frequency power supply, a three-way holder, airway, a threeway glass tube, a ring metal electrode and a cylindrical metal inner electrode. In comparison with conventional LTP, TD-LTP ionization source has several advantages. Firstly, a thermal desorption sampler is introduced to promote the evaporation of the analytes and improve the sample utilization rate. Secondly, sample is introduced into the discharging area where the plasma concentration is high, thus ensuring thorough interaction of plasma and samples. Thirdly, the ionization source is connected to the injection port of mass spectrometer coaxially, thus improving the ion transmission to mass spectrometer. The improvement in these three aspects increases sensitivity of instrument and stability of ionization. For liquid samples, a certain volume of sample solution is dripped onto the surface of a polytetrafluoroethylene (PTFE) swab by a micropipettor, and the solvent is volatilized at room temperature. For solid sample, the surface is gently wiped by the PTFE swab. Then the PTFE swab is inserted into the thermal desorption sampler. Gas sample molecules produced in the thermal desorption sampler is carried by carrier gas (air) to the three-way glass tube, then collided with plasma, and ionized by chemical ionization such as the proton transfer and charge transfer reaction. The positive and negative ions are produced by TD-LTP ionization simultaneously.
2.3.1
The working solutions of pesticide from 0.1 mg L–1 to 10.0 mg L–1 were prepared by diluting the pesticide standard solution (1000 mg L–1) with step dilution method. One micro liter of the liquid solution was sampled by micropipettor and dripped onto the surface of a PTFE swab. After volatilization of solvent at room temperature, PTFE swab was placed on the sample holder. The sample detection time was less than 2 s. 2.3.2
Instruments and reagents
Mass spectrum analysis of broomcorn samples was carried out on an ACQUITY TQD triple quadrupole mass spectrometer (Waters, USA). The home-made rectilinear ion trap mass spectrometer[24] contains a discontinuous atmosphere sampling interface (DAPI), a rectilinear ion trap mass analyzer (x0 = 5.0 mm, y0 = 4.0 mm, z0 = 43.2 mm), an electron multiplier detector with conversion dynode, and a vacuum system. The vacuum system of this mass spectrometer is constituted of a 15 L min‒1 diaphragm pump and a 80 L s‒1 molecular pump (Pfeiffer Vacuum Inc., Nashua, NH, Pfeiffer HiPace 80). The size of the vacuum chamber is 14 cm × 11.5 cm × 8.5 cm. Helium (99.999%) (Dalian Special Gas Company, China) was used as the discharge gas. Methanol and acetone (both are analytical grade) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (China). Phoxim, ethion, diazinon and other 9 kinds of pesticide standards were purchased from Ministry of Agriculture Environmental Quality Supervision, Inspection and Testing Center (Tianjin, China). Experimental methods
Screening methods of pesticide residues in broomcorn sample
The broomcorn sample solution and spiked sample solution were detected on TD-LTP triple quadrupole mass spectrometer. If the characteristic ion peak of a pesticide appeared in the mass spectrum of broomcorn sample with the signal to noise ratio (S/N) greater than three, and the peak intensities of the spiked sample solution were significantly enhanced than that of the broomcorn sample, the presence of pesticide residue in the broomcorn sample could be ascertained. Otherwise the pesticide residue was believed to be absent.
3
Results and discussion
In this study, the optimization of experimental conditions and performance test of TD-LTP ionization source were carried out on rectilinear ion trap mass spectrometer in Dalian Institute of Chemical Physic, Chinese Academy of Sciences. The practical application of TD-LTP ionization source was carried out in combination with a commercialized triple quadrupole MS. 3.1
2.3
Preparation of broomcorn sample and spiked sample solution
The broomcorn sample was prepared as follows. Approximately 25 g of broomcorn sample was added into 50 mL of methanol directly without treatment, followed by an ultrasonic extraction (35 Hz) for 5 min. The extract was then filtrated, concentrated to 1 mL, and diluted to 5 mL with methanol. The spiked sample solution was prepared as follows. Firstly, 1 mL of pesticide standards (Table 1) at concentration level of 10 mg L‒1 was added to 25 g of broomcorn sample. Then 38 mL of methanol was added. The mixture was then stirred thoroughly, ultrasonicated for 5 min (35 Hz), and separated by filtration. The obtained filtrate was concentrated to 1 mL, and diluted to 5 mL with methanol. 2.3.3
2.2
Preparation and detection of pesticide standard solutions
Optimization and characterization of TD-LTP ionization sources
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
3.1.1
Optimization of TD-LTP ionization sources
The performance of TD-LTP ionization source is mainly determined by flow rate of discharge gas (helium), thermal desorption temperature, and flow rate of carrier gas (air). The experimental conditions were optimized via a single variable method on the homemade rectilinear ion trap mass spectrometer. Pentaerythritol tetranitrate (PETN) was chosen as testing sample at a concentration level of 10 mg L–1. The detection was carried out under negative mode with 1 μL of sampling volume. The optimal experimental results were shown in Fig.2. As shown in Fig.2A, when the thermal desorption temperature and the carrier gas flow was kept constant, the signal intensity of PETN increased with increasing helium flow rate and achieved a maximum value at 150 mL min–1, and then decreased with further increasing helium flow rate. This was due to the limitation of the chemical reaction ionization of the LTP. When the discharging helium flowed slowly, the plasma density was low and the reagent ion used for chemical ionization was not adequate enough to ionize all the analyte molecules. With the increase of flow rate of helium, the amount of reagent ions was increased and thus the signal intensity increased, but the extra high flow of helium would dilute the analyte and reagent ion, thus resulted in decline of the signal intensity. The effect of thermal desorption temperature on the intensity of PETN is shown in Fig.2B. The signal intensity of the samples increased with the increase of thermal decomposition temperature and achieved a maximum value at 180 ºC, and then decreased. When the thermal desorption temperature increased, the desorption rate of the sample
increased, and more gaseous analyte molecules reached ionization zone to react with reagent ions. When the thermal desorption temperature was higher than 180 ºC, the speed of sample desorption was higher than the ion sampling speed used at pulse mode with DAPI inlet, resulting in decline of the signal intensity. Figure 2C shows that, when the thermal desorption temperature and the helium flow was constant, the gaseous analyte molecules could not reach the ionization zone in time at a low carrier gas flow rate, which led to low signal intensity. If the carrier gas flow rate was higher than 100 mL min‒1, the ionized sample ion had been blow out of the ionization zone by the carrier gas before DAPI was turned on. So the sample signal intensity increased with the increase of carrier gas flow at first and then decreased. After optimization, the optimal ionization parameters used in the subsequent experiments were as follows: helium flow rate of 150 mL min‒1, thermal desorption temperature of 180 ºC and air flow of 100 mL min‒1. 3.1.2
Characterization of TD-LTP ionization source
To demonstrate the performance of TD-LTP ionization source, traditional LTP ionization source and TD-LTP ionization source were coupled with the homemade rectilinear ion trap mass spectrometer respectively for analysis of the same samples. Under the optimal conditions, phosmet (10 mg L‒1) and dursban solution (10 mg L‒1) were tested at the sampling volume of 1 μL. The results are shown in Fig.3. The sensitivity of TD-LTP ionization source for detection of phosmet and dursban were increased by 8 times and 8.5 times respectively compared to traditional LTP ionization source.
Fig.2 Optimization of parameters for the thermal desorption LTP: (A) helium flow rate; (B) thermal temperature; (C) air flow rate
Fig.3 Mass spectra of (A) phosmet and (B) dursban obtained with traditional LTP and the TD-LTP respectively
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
The pre-condition of practical application for TD-LTP is that the sensitivity is high enough for the requirements of laws and standards. The limit of detections (LODs) of 12 kinds of pesticide standards were tested under optimized ionization conditions using TD-LTP ionization source triple quadrupole mass spectrometer. The results are shown in Table 1. Compared with the maximum permitted limit of pesticide residues regulated in national standard[26], the detection limit of TD-LTP ionization source triple quadrupole mass spectrometer could satisfy the national standard. This method can be used as a high throughput screening tool for pesticide residues in cereals.
The precision of the TD-LTP ionization source was also investigated, and the relative standard deviations (RSDs) for 6 repeated testing of the two pesticides were 8.6% and 7.2%, respectively, which were increased by about 4 and 3 times respectively compared with traditional LTP ionization source. To characterize the ion species produced by TD-LTP ionization source, TD-LTP was combined with homemade rectilinear ion trap mass spectrometer to detect 12 kinds of pesticide standards. The sampling volume for each pesticide standard was 1 μL. The detailed information of the pesticides and the characteristic ion species are listed in Table 1. The results show that TD-LTP ionization source is a soft ionization source. At positive mode, the characteristic ion is [M + H]+ or [M + H2O + H]+, and at negative mode, the characteristic ion is [M ‒ R]‒ (R is a alkyl group inside M molecule). 3.2
3.2.2
According to the experimental procedure described above, the broomcorn sample and spiked broomcorn sample were detected with the TD-LTP ionization triple quadrupole mass spectrometer at positive mode and negative mode respectively. The results are shown in Fig.4 and Fig.5.
Practical application of TD-LTP ionization source
3.2.1
Fast screening of pesticide residues in broomcorn
LODs of TD-LTP ionization source triple quadrupole mass spectrometry for pesticide analysis
Table 1 Detailed information of 12 kinds of different pesticides tested in this study No.
Sample name
Detection mode
Molecular weight
Characteristic ion ‒
Characteristic ion (m/z)
Limit of
Maximal residue level in GB2763-2014[26]
detection ‒1
(LOD, mg L )
(mg L‒1)
1
Phosmet
Negative mode
317
[M – CH3]
302
1
2.5
2
Ethion
Negative mode
384
[M – C2H5]‒
355
0.2
1.0
3
Dursban
Negative mode
351
[M – C2H5]‒
322
5
2.5
185, 187/206
1
0.5
4
DDVP
Negative mode
221
[M – Cl + O] ‒/ [M – CH3]‒ ‒
5
Oxadiazon
Negative mode
345
[M +
471
1
0.25
6
Phorate
Negative mode
260
[M – C2H5]‒-
231
0.5
0.1
7
Malathion
Negative mode
330
[M – CH3] ‒
315
0.1
40
8
Phoxim
Negative mode
298
[M – C2H5]‒
269
0.1
0.25
9
Aldrin
Positive mode
365
[M + H2O + H]+
384
0.5
0.1
10
Heptachlor
Positive mode
373
[M + H2O + H]+
391
0.4
0.1
11
Diazinon
Positive mode
304
[M + H]+
305
0.01
0.5
Positive mode
305
[M + H]+
306
0.01
25
12
Pirimiphosmethyl
Cl
Cl
]
(B)
Fig.4 Mass spectra (A) and local amplification figure (B) of sorghum sample and spiked sample at positive mode
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
Fig.5 Mass spectra (A) and local amplification figure (B) of sorghum sample and spiked sample at negative mode a, b, c, d, e, f are corresponding amplification zones respectively
At positive mode, diazinon and methylpyrimidine were detected in spiked broomcorn sample solution but not detected in sample solution, which indicated that the sensitivity of TD-LTP ionization source could satisfy the detection requirements of diazinon and methyl pyrimidine, but these two pesticide residues did not exist in broomcorn samples. At negative mode, seven pesticide residues (dichlorvos, phoxim, phosmet, malathion, chlorpyrifos, heptachlor and aldrin) were detected in spiked broomcorn sample. In the mass spectrum of the broomcorn sample, the characteristic ion peaks of dichlorvos, malathion and chlorpyrifos were found. From the point of view of S/N > 3, it could be determined that the residues of dichlorvos were present in broomcorn sample. The results were consistent with those obtained by the recommended national standards method with GC-MS. The experimental results demonstrated that TD-LTP direct mass spectrometry could be used as a high-throughput screening tool for pesticide residues detection.
4
Conclusions
In this study, thermal desorption process of sample was introduced on the basis of traditional LTP ionization source. A thermal desorption low temperature plasma ionization source was designed and built, which could produce both positive and negative ions at the same time. The TD-LTP ionization source was combined with a homemade rectilinear ion trap mass spectrometer to identify characteristic ions of 12 pesticide standard samples. Then the TD-LTP ionization source was applied to a commercialized triple quadrupole mass spectrometer, and LOD and stability for pesticide residue detection could satisfy the requirements of laws and standards. Finally, the TD-LTP ionization source was applied to analyze the pesticide residues in broomcorn sample and the pesticide residues were successfully recognized without complex sample pretreatment. These results demonstrated that TD-LTP mass spectrometry could
WANG Shuang et al. / Chinese Journal of Analytical Chemistry, 2017, 45(2): 175–182
minimize sample preparation and provide a tough analytical approach for complex matrices.
[14] Chen H W, Venter A, Cooks R G. Chem. Commun., 2006, 19:
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