Mass spectrometry Optimization.
In the ESI+ and ESI− modes, the precursor and product ions were scanned using eight pesticide standards (0.1 μL/mL) respectively, and the decluttering potential and collision energy were optimized (Table 1).
The chemical structure skeletons of chlorantraniliprole, bromoantraniliprole, flonicamid, and cyantraniliprole are similar and their mass spectrum signals in the ESI+ mode were better than those in the ESI− mode. In the positive ion mode, both m/z 286.0 and 177.1 appeared in the product ions of the four compounds. The amide bonds of chlorantraniliprole and bromoantraniliprole were broken to form product ions m/z 286.0, and then the amide bond and C-Br bond of bromoantraniliprole were broken to form product ions m/z 177.1. The both sides of the C=N bond of cyantraniliprole were broken to form product ions m/z 286.0 and 177.1. The fragmentation of the secondary mass spectrometry of chlorantraniliprole, bromoantraniliprole and cyantraniliprole were speculated as shown in Figure 1.
In the negative ion mode, product ions m/z 146.9 produced by tetrachlorantraniliprole and cyclaniliprole, which may be the 3-bromo-1H-pyrazole produced after fragmentation. The fragmentation of the secondary mass spectrometry of tetrachlorantraniliprole and cyclaniliprole were speculated as shown in Figure 2.
In mass spectrometry analysis, the response of the compound was enhanced and the peak shape was improved when a certain amount of volatile salt and acid was added to the mobile phase [11, 15, 16]. Therefore, the response intensity of 0.1% formic acid-water, 0.1% formic acid-water (containing 1 mmol/L ammonium acetate) and pure water as the mobile phase were compared in the experiment (Figure 3). Generally in the positive ion mode, the ionization efficiency of the compound was improved and the response intensity was enhanced when the acid and ammonium salt are added. Therefore, after adding formic acid and ammonium acetate to the pure water phase, the response intensity of chlorantraniliprole, bromoantraniliprole, cyantraniliprole and tolfenpyrad was significantly improved compared to pure water as the mobile phase. However, the response of flonicamid in formic acid solution is not as strong as in pure water, but the response is also significantly enhanced with the addition of ammonium acetate. For the pesticides tested in the negative ion mode such as flubendiamide, tetrachlorantraniliprole and cyclaniliprole, their response was reduced to a certain extent with the addition of formic acid and ammonium acetate. In the negative ion mode, the [M-H]- ion in the compound was inhibited by excess hydrogen ion and ammonium ion, which reduces its response. The chromatograms are shown in Figure 4.
Matrix effects
As shown in Figure 5, for some fruits and vegetables with complex matrices, most of the eight amide compounds have strong matrix effects in the absence of purification, especially the avocado with higher oil content, leeks and garlic with more sulfur compounds have a strong matrix effect (the range of matrix effect was: -44.1%~748.5%). Pesticides such as flubendiamide, tetrachlorantraniliprole, chlorantraniliprole, flonicamid were present strong matrix effect in various matrices (range of matrix effect: -44.1%~748.5%). However, with the addition of multi-walled carbon nanotubes, the matrix effect can be effectively suppressed, and better results have been obtained.
Modified cleanup process
Optimization of the amount of the MWCNTs
It was found in the experiment that the amount of multi-walled carbon nanotubes added had a greater impact on the recovery effect. To evaluate the effect of this parameter, the experiment was performed using 2 mL of the acetonitrile extract at the spiked level of 0.01 mg/kg that was placed into 2.0 mL centrifuge tubes containing 300 mg MgSO4 and different amounts of MWCNTs (i.e., 10, 20, 30 mg). With the increase in the amount of multi-walled carbon nanotubes, the recoveries rate of the 8 amide pesticides had decreased to a certain extent. When the added amount of MWCNTs was 30 mg, the recoveries of chlorantraniliprole, bromoantraniliprole and cyantraniliprole in onions were 53.3%, 45.9% and 54.5%, flonicamid in leek was 64.0%, and cyantraniliprole in garlic was 62.1%. Consequently, 20 mg of MWCNTs was selected to be added to 2 mL of acetonitrile extract to obtain a better recovery effect.
Comparison of purification effects between MWCNTs and PSA
In order to compare the purification effects of MWCNTs and PSA, 20 mg MWCNTs and 20 mg PSA were added to 2 mL of the acetonitrile extract at the spiked level of 0.01 mg/kg to compare the purification effects, respectively. The results are shown in Figure 6, when PSA were added, except for the low recovery of flonicamid in chives and shallots, the low recovery of tolfenpyrad in avocado, and the high recovery of cyantraniliprole in celery, the recoveries of other pesticides were at the acceptable range. However, the purification effect of flubendiamide, tetrachlorantraniliprole and cyclaniliprole was poor, and the recoveries were higher than 120%. On the other hand, for substrates with higher pigment content such as oranges, shallot, chives, mangoes, and celery, as shown in Figure 7, the samples processed by MWCNT looked transparent, while the PSA-cleanup sample had deeper color. Compared with PSA, The pigment in the sample can be effectively removed by the MWCNT, which can further eliminate the matrix effect caused by pigment.
Method Validation.
Linearity
Linearity was studied in the range 0.15–20 ng/mL for eight amide pesticides by matrix-matched standard calibration in blank extracts of orange, chives, shallot, garlic, mango, onion, avocado, celery, litchi. As shown in Table 3, good linear range was found for all pesticides with R2 values better than 0.999.
Limits of quantification
The described method was tested for simultaneous extraction and determination of eight amide pesticides in nine representative matrices. Table 2 showed the LOQs for the eight pesticides studied in orange, chives, shallot, garlic, mango, onion, avocado, celery, litchi. The LOQs for eight pesticides ranged from 0.03 to 0.8 μg/kg. Tolfenpyrad also had lower LOQs than the other seven pesticides.
Table 2 Linear ranges, correlation coefficients (r), limits of quantitation (LOQs) and Matrix effect of amide pesticides in different matrices
Compound
|
Matrix
|
Linear
ranges/(ng/mL)
|
r
|
LOQ/(μg/kg)
|
ME/%
|
Chlorantraniliprole
|
Orange
|
0.15-20.0
|
0.9997
|
0.15
|
7.2
|
|
Celery
|
0.15-20.0
|
0.9996
|
0.15
|
2.7
|
|
Onion
|
0.15-20.0
|
0.9998
|
0.15
|
-0.3
|
|
Litchi
|
0.15-20.0
|
0.9994
|
0.15
|
6.9
|
|
Mango
|
0.15-20.0
|
0.9996
|
0.15
|
3.0
|
|
Shallot
|
0.15-20.0
|
0.9992
|
0.15
|
-2.7
|
|
Chives
|
0.15-20.0
|
0.9996
|
0.15
|
9.0
|
|
Avocado
|
0.15-20.0
|
0.9994
|
0.15
|
2.7
|
|
Garlic
|
0.15-20.0
|
0.9999
|
0.15
|
1.2
|
|
Bromoantraniliprole
|
Orange
|
0.30-20.0
|
0.9992
|
0.30
|
5.4
|
|
Celery
|
0.30-20.0
|
0.9995
|
0.30
|
6.9
|
|
Onion
|
0.30-20.0
|
0.9993
|
0.30
|
1.0
|
|
Litchi
|
0.30-20.0
|
0.9996
|
0.30
|
-0.5
|
|
Mango
|
0.30-20.0
|
0.9997
|
0.30
|
8.4
|
|
Shallot
|
0.30-20.0
|
0.9996
|
0.30
|
-1.0
|
|
Chives
|
0.30-20.0
|
0.9996
|
0.30
|
8.9
|
|
Avocado
|
0.30-20.0
|
0.9994
|
0.30
|
8.4
|
|
Garlic
|
0.30-20.0
|
0.9996
|
0.30
|
3.4
|
|
Flonicamid
|
Orange
|
0.80-20.0
|
0.9993
|
0.80
|
-8.8
|
|
Celery
|
0.80-20.0
|
0.9997
|
0.80
|
-9.6
|
|
Onion
|
0.80-20.0
|
0.9996
|
0.80
|
-8.1
|
|
Litchi
|
0.80-20.0
|
0.9994
|
0.80
|
-2.2
|
|
Mango
|
0.80-20.0
|
0.9999
|
0.80
|
1.5
|
|
Shallot
|
0.80-20.0
|
0.9991
|
0.80
|
-5.1
|
|
Chives
|
0.80-20.0
|
0.9993
|
0.80
|
-6.6
|
|
Avocado
|
0.80-20.0
|
0.9992
|
0.80
|
8.8
|
|
Garlic
|
0.80-20.0
|
0.9995
|
0.80
|
-8.1
|
|
Cyantraniliprole
|
Orange
|
0.20-20.0
|
0.9996
|
0.20
|
5.5
|
|
Celery
|
0.20-20.0
|
0.9996
|
0.20
|
2.3
|
|
Onion
|
0.20-20.0
|
0.9997
|
0.20
|
2.9
|
|
Litchi
|
0.20-20.0
|
0.9998
|
0.20
|
6.2
|
|
Mango
|
0.20-20.0
|
0.9996
|
0.20
|
7.2
|
|
Shallot
|
0.20-20.0
|
0.9994
|
0.20
|
-2.9
|
|
Chives
|
0.20-20.0
|
0.9997
|
0.20
|
7.8
|
|
Avocado
|
0.20-20.0
|
0.9993
|
0.20
|
5.2
|
|
Garlic
|
0.20-20.0
|
0.9995
|
0.20
|
4.5
|
|
Tolfenpyrad
|
Orange
|
0.03-20.0
|
0.9993
|
0.03
|
4.9
|
|
Celery
|
0.03-20.0
|
0.9996
|
0.03
|
3.8
|
|
Onion
|
0.03-20.0
|
0.9994
|
0.03
|
4.9
|
|
Litchi
|
0.03-20.0
|
0.9997
|
0.03
|
4.9
|
|
Mango
|
0.03-20.0
|
0.9994
|
0.03
|
6.0
|
|
Shallot
|
0.03-20.0
|
0.9997
|
0.03
|
3.8
|
|
Chives
|
0.03-20.0
|
0.9995
|
0.03
|
-1.6
|
|
Avocado
|
0.03-20.0
|
0.9995
|
0.03
|
-1.1
|
|
Garlic
|
0.03-20.0
|
0.9998
|
0.03
|
5.5
|
|
Flubendiamide
|
Orange
|
0.20-20.0
|
0.9997
|
0.20
|
-1.8
|
|
Celery
|
0.20-20.0
|
0.9996
|
0.20
|
9.5
|
|
Onion
|
0.20-20.0
|
0.9993
|
0.20
|
9.0
|
|
Litchi
|
0.20-20.0
|
0.9993
|
0.20
|
8.6
|
|
Mango
|
0.20-20.0
|
0.9995
|
0.20
|
6.8
|
|
Shallot
|
0.20-20.0
|
0.9996
|
0.20
|
5.2
|
|
Chives
|
0.20-20.0
|
0.9997
|
0.20
|
8.5
|
|
Avocado
|
0.20-20.0
|
0.9999
|
0.20
|
8.2
|
|
Garlic
|
0.20-20.0
|
0.9998
|
0.20
|
8.6
|
|
Tetrachlorantraniliprole
|
Orange
|
0.10-20.0
|
0.9998
|
0.10
|
3.2
|
|
Celery
|
0.10-20.0
|
0.9999
|
0.10
|
7.6
|
|
Onion
|
0.10-20.0
|
0.9999
|
0.10
|
2.2
|
|
Litchi
|
0.10-20.0
|
0.9994
|
0.10
|
4.3
|
|
Mango
|
0.10-20.0
|
0.9994
|
0.10
|
8.6
|
|
Shallot
|
0.10-20.0
|
0.9995
|
0.10
|
1.1
|
|
Chives
|
0.10-20.0
|
0.9997
|
0.10
|
5.4
|
|
Avocado
|
0.10-20.0
|
0.9994
|
0.10
|
2.2
|
|
Garlic
|
0.10-20.0
|
0.9999
|
0.10
|
2.2
|
|
Cyclaniliprole
|
Orange
|
0.50-20.0
|
0.9994
|
0.50
|
-6.0
|
|
Celery
|
0.50-20.0
|
0.9994
|
0.50
|
6.5
|
|
Onion
|
0.50-20.0
|
0.9996
|
0.50
|
6.5
|
|
Litchi
|
0.50-20.0
|
0.9997
|
0.50
|
2.5
|
|
Mango
|
0.50-20.0
|
0.9998
|
0.50
|
1.5
|
|
Shallot
|
0.50-20.0
|
0.9995
|
0.50
|
-8.0
|
|
Chives
|
0.50-20.0
|
0.9997
|
0.50
|
1.0
|
|
Avocado
|
0.50-20.0
|
0.9998
|
0.50
|
8.0
|
|
Garlic
|
0.50-20.0
|
0.9999
|
0.50
|
6.0
|
|
Recovery and reproducibility
All the recoveries were determined from the analyses of eight amide pesticides in the matrices, orange, chives, shallot, garlic, mango, onion, avocado, celery, litchi by carrying out six consecutive extractions (n = 6) of spiked matrices at three concentration levels (LOQs, 5 × LOQs, 10 × LOQs). The values were calculated using matrix-matched calibration standards, as Table 3 shows detailed recovery and relative standard deviation data for eight pesticides analyzed in the nine matrices. The recoveries of eight pesticides were in the range 71.2–120.0% with the relative standard deviations (RSDs) were in the range 3.8–9.4% for all cases.
Table 3 Recoveries and relative standard deviations of amide pesticides
Compound
|
Matrix
|
added
(μg/kg)
|
Recovery/%(RSD/%)
|
added
(μg/kg)
|
Recovery/%(RSD/%)
|
added
(μg/kg)
|
Recovery/%(RSD/%)
|
Chlorantraniliprole
|
Orange
|
0.15
|
99.9(6.4)
|
0.75
|
94.3(5.6)
|
1.50
|
96.7(5.9)
|
Celery
|
0.15
|
106(7.2)
|
0.75
|
101(7.7)
|
1.50
|
103(6.7)
|
Onion
|
0.15
|
73.7(6.9)
|
0.75
|
77.7(5.3)
|
1.50
|
83.5(6.2)
|
Litchi
|
0.15
|
93.8(5.9)
|
0.75
|
97.3(4.6)
|
1.50
|
95.6(4.8)
|
Mango
|
0.15
|
92.8(4.8)
|
0.75
|
95.3(5.1)
|
1.50
|
98.2(4.5)
|
Shallot
|
0.15
|
94.0(9.1)
|
0.75
|
97.3(6.8)
|
1.50
|
98.5(7.5)
|
Chives
|
0.15
|
113(8.4)
|
0.75
|
111(5.8)
|
1.50
|
108(6.0)
|
Avocado
|
0.15
|
106(5.3)
|
0.75
|
110(5.9)
|
1.50
|
108(6.8)
|
Garlic
|
0.15
|
73.5(5.9)
|
0.75
|
78.6(5.3)
|
1.50
|
86.8(4.5)
|
Bromoantraniliprole
|
Orange
|
0.30
|
98.5(8.7)
|
1.50
|
96.7(7.8)
|
3.00
|
99.4(5.6)
|
Celery
|
0.30
|
103(7.7)
|
1.50
|
100(5.4)
|
3.00
|
105(4.5)
|
Onion
|
0.30
|
99.0(9.1)
|
1.50
|
95.7(8.4)
|
3.00
|
99.7(6.3)
|
Litchi
|
0.30
|
91.8(9.4)
|
1.50
|
96.4(7.5)
|
3.00
|
95.3(8.8)
|
Mango
|
0.30
|
95.0(8.4)
|
1.50
|
94.2(4.7)
|
3.00
|
96.7(5.2)
|
Shallot
|
0.30
|
100(8.8)
|
1.50
|
98.3(6.1)
|
3.00
|
103(6.9)
|
Chives
|
0.30
|
117(9.2)
|
1.50
|
114(7.5)
|
3.00
|
118(6.7)
|
Avocado
|
0.30
|
111(6.5)
|
1.50
|
115(6.9)
|
3.00
|
113(5.5)
|
Garlic
|
0.30
|
75.1(5.8)
|
1.50
|
78.3(5.6)
|
3.00
|
76.7(4.5)
|
Flonicamid
|
Orange
|
0.80
|
72.0(6.4)
|
4.00
|
75.4(7.5)
|
8.00
|
77.9(4.6)
|
Celery
|
0.80
|
78.0(6.3)
|
4.00
|
79.1(6.9)
|
8.00
|
88.3(4.8)
|
Onion
|
0.80
|
78.1(7.9)
|
4.00
|
88.7(6.7)
|
8.00
|
85.3(7.5)
|
Litchi
|
0.80
|
78.5(6.7)
|
4.00
|
77.8(7.3)
|
8.00
|
83.1(5.4)
|
Mango
|
0.80
|
99.0(6.3)
|
4.00
|
94.5(7.2)
|
8.00
|
95.6(6.6)
|
Shallot
|
0.80
|
74.1(4.8)
|
4.00
|
77.4(6.3)
|
8.00
|
79.6(4.0)
|
Chives
|
0.80
|
77.5(5.3)
|
4.00
|
75.2(5.9)
|
8.00
|
79.3(4.7)
|
Avocado
|
0.80
|
73.6(5.1)
|
4.00
|
78.9(6.3)
|
8.00
|
79.4(4.5)
|
Garlic
|
0.80
|
71.9(6.8)
|
4.00
|
83.4(7.4)
|
8.00
|
84.5(6.0)
|
Cyantraniliprole
|
Orange
|
0.20
|
101(8.2)
|
1.00
|
91.3(6.4)
|
2.00
|
95.4(6.9)
|
Celery
|
0.20
|
120(8.3)
|
1.00
|
110(6.7)
|
2.00
|
107(7.5)
|
Onion
|
0.20
|
76.4(7.6)
|
1.00
|
86.5(5.6)
|
2.00
|
84.7(5.1)
|
Litchi
|
0.20
|
87.6(7.4)
|
1.00
|
89.5(4.9)
|
2.00
|
90.4(5.0)
|
Mango
|
0.20
|
101(7.9)
|
1.00
|
91.9(7.7)
|
2.00
|
96.3(6.6)
|
Shallot
|
0.20
|
91.3(6.4)
|
1.00
|
94.6(5.5)
|
2.00
|
98.4(5.3)
|
Chives
|
0.20
|
108(6.0)
|
1.00
|
105(6.7)
|
2.00
|
109(5.5)
|
Avocado
|
0.20
|
99.5(5.9)
|
1.00
|
96.8(5.6)
|
2.00
|
97.7(4.6)
|
Garlic
|
0.20
|
84.9(5.8)
|
1.00
|
89.3(3.9)
|
2.00
|
90.5(4.9)
|
Tolfenpyrad
|
Orange
|
0.03
|
98.9(6.4)
|
0.15
|
94.6(5.7)
|
0.30
|
96.7(6.0)
|
Celery
|
0.03
|
87.7(5.7)
|
0.15
|
89.5(4.9)
|
0.30
|
94.5(5.2)
|
Onion
|
0.03
|
82.1(5.3)
|
0.15
|
87.3(5.9)
|
0.30
|
89.9(4.9)
|
Litchi
|
0.03
|
92.7(4.8)
|
0.15
|
90.8(4.3)
|
0.30
|
95.9(4.0)
|
Mango
|
0.03
|
96.1(4.3)
|
0.15
|
95.8(6.5)
|
0.30
|
97.4(4.8)
|
Shallot
|
0.03
|
93.0(4.9)
|
0.15
|
96.2(5.3)
|
0.30
|
95.6(6.7)
|
Chives
|
0.03
|
82.9(5.8)
|
0.15
|
88.6(4.7)
|
0.30
|
89.7(5.0)
|
Avocado
|
0.03
|
76.5(6.3)
|
0.15
|
79.7(5.7)
|
0.30
|
87.9(4.8)
|
Garlic
|
0.03
|
71.2(6.1)
|
0.15
|
73.6(5.6)
|
0.30
|
76.5(4.9)
|
Flubendiamide
|
Orange
|
0.20
|
99.4(5.0)
|
1.00
|
101(5.4)
|
2.00
|
99.7(4.2)
|
Celery
|
0.20
|
92.5(5.8)
|
1.00
|
97.3(6.4)
|
2.00
|
95.8(4.6)
|
Onion
|
0.20
|
81.9(7.2)
|
1.00
|
83.6(6.8)
|
2.00
|
88.6(5.4)
|
Litchi
|
0.20
|
101(7.4)
|
1.00
|
100(5.7)
|
2.00
|
97.8(8.3)
|
Mango
|
0.20
|
100(6.4)
|
1.00
|
108(5.8)
|
2.00
|
105(5.6)
|
Shallot
|
0.20
|
91.2(5.9)
|
1.00
|
96.0(5.5)
|
2.00
|
95.6(4.5)
|
Chives
|
0.20
|
100(9.0)
|
1.00
|
98.0(5.6)
|
2.00
|
95.9(5.7)
|
Avocado
|
0.20
|
103(8.9)
|
1.00
|
109(5.4)
|
2.00
|
105(6.5)
|
Garlic
|
0.20
|
83.1(8.6)
|
1.00
|
88.8(6.8)
|
2.00
|
89.6(5.0)
|
Tetrachlorantraniliprole
|
Orange
|
0.10
|
111(5.8)
|
0.50
|
106(6.9)
|
1.00
|
117(4.6)
|
Celery
|
0.10
|
110(6.3)
|
0.50
|
112(3.9)
|
1.00
|
107(4.7)
|
Onion
|
0.10
|
90.2(6.8)
|
0.50
|
98.8(5.0)
|
1.00
|
96.9(5.4)
|
Litchi
|
0.10
|
116(6.9)
|
0.50
|
112(4.6)
|
1.00
|
117(5.2)
|
Mango
|
0.10
|
111(7.5)
|
0.50
|
115(6.3)
|
1.00
|
112(5.9)
|
Shallot
|
0.10
|
114(7.2)
|
0.50
|
115(5.6)
|
1.00
|
118(4.8)
|
Chives
|
0.10
|
113(7.4)
|
0.50
|
116(5.0)
|
1.00
|
119(5.5)
|
Avocado
|
0.10
|
112(6.9)
|
0.50
|
105(4.2)
|
1.00
|
107(5.9)
|
Garlic
|
0.10
|
95.7(5.7)
|
0.50
|
97.2(4.5)
|
1.00
|
98.8(4.3)
|
Cyclaniliprole
|
Orange
|
0.50
|
98.3(5.2)
|
2.50
|
95.7(6.1)
|
5.00
|
97.9(5.6)
|
Celery
|
0.50
|
94.2(6.3)
|
2.50
|
99.1(5.7)
|
5.00
|
96.8(4.8)
|
Onion
|
0.50
|
78.8(6.9)
|
2.50
|
83.5(5.4)
|
5.00
|
87.9(5.8)
|
Litchi
|
0.50
|
97.8(7.4)
|
2.50
|
96.6(5.9)
|
5.00
|
98.1(6.1)
|
Mango
|
0.50
|
97.4(7.3)
|
2.50
|
95.6(5.6)
|
5.00
|
98.9(6.0)
|
Shallot
|
0.50
|
89.0(8.5)
|
2.50
|
93.4(5.8)
|
5.00
|
96.5(6.3)
|
Chives
|
0.50
|
87.5(3.8)
|
2.50
|
89.3(4.3)
|
5.00
|
93.2(4.5)
|
Avocado
|
0.50
|
97.7(4.4)
|
2.50
|
95.4(4.7)
|
5.00
|
98.5(5.3)
|
Garlic
|
0.50
|
81.6(5.3)
|
2.50
|
87.4(4.2)
|
5.00
|
92.8(5.8)
|