Evaluation of Pyridaben Residues on Fruit Surfaces and Their Stability

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Evaluation of Pyridaben Residues on Fruits Surface and its Stability by A Novel On-line Dual-frequency Ultrasonic Device and Chemiluminescence Detection Wangbing Zhang, Mei Wei, Wei Song, Yi-Xuan Gong, and Xin-An Yang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03357 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 18, 2017

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Journal of Agricultural and Food Chemistry

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Evaluation of Pyridaben Residues on Fruits Surface and

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its Stability by A Novel On-line Dual-frequency

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Ultrasonic Device and Chemiluminescence Detection Wang-Bing Zhang a*, Mei Wei a, Wei Song b, Yi-Xuan Gong a and Xin-An Yang a*

4 5

a

Department of Applied Chemistry, Anhui University of Technology, Maanshan, Anhui 243002, P. R.

6 7

China b

Anhui Entry Exit Inspection and Quarantine Bureau, Hefei, Anhui 230022, P. R. China

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ABSTRACT: In this paper, we firstly report the development of a highly sensitive and economical

9

method for accurate analysis of pyridaben residues on fruits based on dual-frequency ultrasonic

10

treatment (DFUT) and flow injection chemiluminescence (CL) detection. The DFUT device is

11

constituted

12

with different structures have been designed and applied to evaluate the function of DFUT in the

13

detection process. Recorded data showed that DFUT is an effective method for improving the pyridaben

14

CL signal. The signal of pyridaben in response to DFUT is 2.0- 3.3 times stronger than the response to

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only ultrasonic probe at 20kHz or ultrasonic bath at 40kHz, respectively. In addition, the response

16

obtained from the concentric circles QGC is 2.1 times stronger than the response to the spiral tube QGC.

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Under the optimized condition, the proposed method has advantages, such as wide linear range

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(0.8-100.0 µg L-1), high sensitivity (limit of detection is 0.085 µg L-1) and good stability (RSDs ≤ 4.7%

19

in the linear range) for pyridaben determination. We apply this method to monitor the residue pyridaben

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on some fruits. The data shows that the maximum amounts of residue on fruits surface after soaking in

21

water (50 mg L-1, 5 min) are 0.583 mg kg-1 (apple), 0.794 mg kg-1 (orange) and 0.351 mg kg-1 (pear).

by integrating an ultrasonic bath with an ultrasonic probe. Two quartz glass coils (QGC)

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However, the concentration of pyridaben in presence of sunlight decreases rapidly, showing its poor

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light stability.

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Keyword: Dual-frequency ultrasonic irradiation; on-line CL determination; Pyridaben; Stability;

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Residue on fruits surface

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INTRODUCTION

27

As

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4-chloropyridazin-3(2H)-one (Figure S1, Supporting Information), is widely used for controlling

29

mites in fruit, tobacco, vegetable and other crops 1. Despite its low toxicity, overuse still can cause

30

harm to humans through food chain. For instance, Boyd et al. have reported that pyridaben can

31

affect the activity of mitochondrial complex I in the electron transport chain 2. Therefore, the

32

maximum residue limits of pyridaben in some countries have been established, such as 0.5 mg kg-1

33

in Spain (peppers, 7 day pre-harvest interval) 3, and 5 mg kg-1 in China (tea) 4.

one

of

the

nonsystemic

acaricide,

Pyridaben,

2-tert-butyl-5-(4-tert-butylbenzylehio)-

34

Generally, chromatography with mass spectrometry or other selective and sensitive detectors is a

35

standard method for the determination of pyridaben. For example, Yang et al. reported a magnetic

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mixed hemimicelles dispersive solid-phase extraction (MMHDSPE) with high performance liquid

37

chromatography (HPLC)-diode array method for determination of pyridaben in fruit juice 5. Sun et al.

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developed a selective method for determination of pyridaben in citrus fruits by ultra-performance liquid

39

chromatography coupled with tandem mass spectrometry (UPLC–MS/MS), which limit of detection is l

40

µg kg-1 6. Douglas et al. established a sensitive procedure based on gas

41

triple-Quadrupole MS for pyridaben determination in several teas 7. Xiong et al. analyzed pyridaben in

42

water and beverage samples by GC coupled with a flame photometric detector within a range of 8.0-600

43

µg L-1 8. Zhang et al. developed a protocol for the collection of LC electrospray ionization quadruple

44

linear ion trap MS for identification of pyridaben in peach 9. Frenich et al. proposed a method, in which

45

pressurized liquid extraction coupled with GS-MS/MS is used for identification of pyridaben in food

46

commodities with limit of quantitation of 0.01 mg kg-1

10

chromatography

(GC)

. Liu et al. presented a quantification method


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Journal of Agricultural and Food Chemistry 11

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based on HPLC-tandem MS for studying the dissipation of pyridaben in cabbage

. Many of these

48

above methods have excellent separation performance, which can analyze a large number of pesticide

49

residues in complex samples at the same time. However,

50

such separation and quantitative analysis,

comprehensive instruments are required in

thus high costs and other issues are particularly inevitable.

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Because of its easy operation, low cost and high sensitivity, chemiluminescence (CL) technique have

52

been more and more used for pesticide residue detection in recent years. The analytes of this method

53

including organophosphorus, organochlorines, and carbamate pesticide, and the limit of determination

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for each compound is lower than 0.01 mg L-1

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picoxystrobin, azoxystrobin, and fenpyroximate) determination, our studies indicate that direct detecting

56

by CL method is impossible, because these residues have no apparent effect on the traditional oxidant

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luminol CL systems (including luminol-H2O2, luminol-KMnO4, and luminol-K2S2O8, etc.). Subsequent

58

experiments show that the CL intensity of these residues in luminol-oxidant system could be promoted

59

in varying degrees after 10-30 min off-line ultrasonic bath treatment. And this enhancement is linearly

60

related to the concentration of residues. The sensitivity and precision of the proposed method in our

61

early works were superior or at least at the same level as those reported methods such as HPLC-MS and

62

GC-MS

63

detection of pesticide residues is promising.

17-19

12-16

. For acaricide and fungicide residues (such as

, which indicated that the application prospect of ultrasonic assisted CL technology in the

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Compared with the previous work, the novelty of this study is to design a new dual-frequency

65

ultrasonic (DFUT) device, and build an on-line detection system with chemiluminescence detector, then

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solve the problems, such as loss of analyte, pollution and time-consuming during the off-line ultrasonic

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processing. Specifically, the ultrasonic device incorporating ultrasonic bath and ultrasonic probe, on one

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hand, does not contact the sample solution directly during the process of ultrasonic irradiation, thus

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avoiding potential pollution problems. On the other hand, on-line processing and detection technology

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not only provides more rapid and real-time data acquisition, but also avoids the loss of the analyte. Also,

71

the high ultrasonic densities of ultrasonic probe

20

coupled with the temperature control function of


Journal of Agricultural and Food Chemistry 21

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ultrasonic bath

may have higher efficiency for enhancing the response of chemiluminescence signal.

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The proposed on-line DFUT assisted CL method is used to evaluate pyridaben residues on fruits surface

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and its stability in sunlight. Its accuracy is confirmed by recovery experiments and HPLC-MS. As far as

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we know, no other study about the application of CL in the pyridaben monitoring had been reported.

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Figure 1. Design of dual-frequency ultrasonic extraction scheme (a), a photo of QGC (b), and

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scheme of the FIA-CL system (c). QGC quartz glass coil; UBG ultrasonic bath generator; UPG

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ultrasonic probe generator; S sample; C acetonitrile solution (0.1% v/v); P1 P2 pump; V eight-port

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value; RC reaction cell; W waste; HV high voltage; PMT photomultiplier tube.

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EXPERIMENTAL PROCEDURES

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Apparatus and Materials. A homemade ultrasonic device (Figure 1A) is used to evaluate the

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efficiency of ultrasonic treatment. The device consists of an ultrasonic bath instrument

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(240mm×140mm×100mm) and an ultrasonic probe generator (20 kHz, Shanghai Guante Ultrasonic

85

Instrument Co., Shanghai, China). Two quartz glass coils (QGC, 2mm i.d., 300mm length, ~ 920µL,

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Figure 1B) with different structures fixed on iron support as the reaction tubes are tested. Both ends

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of the QGC are made into elongated shape, to be easily connected to emulsion tube. The distance

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between QGC and the bottom of ultrasonic instrument (H1, cm) can be adjusted by changing the iron

89

support height. The ultrasonic probe (8mm diameter, 150mm length) is positioned at a certain 4 Environment ACS Paragon Plus

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distance (H2, cm) above the QGC. The ultrasonic frequency of ultrasonic bath can be switched in

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three modes, 40, 50, 59 kHz. Besides, the temperature of water bath can be controlled by adjusting

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the flow rate of homemade external circulating water. Information about the CL detector, chemical

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reagents and sources used in the experiment are listed in Supporting Information.

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Sample Preparation. Fruit samples (apples, oranges, and pears) used in the experiment were

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purchased from local market. Before preparation, these samples are washed with double deionized

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water (DDW) and then air dried at 20oC. According to the literature

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follows. Firstly, nine samples including three apples (202.5g, 164.8g and 175.6g), three oranges

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(124.7g, 133.2g and 102.8g) and three pears (284.7g, 353.8g and 332.7g) are respectively dipped

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into beakers containing solution A (50.0 mg L-1 pyridaben, 300 mL) for 5 min and then the samples

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are taken out and air dried. Secondly, the dried samples are dipped into solution B (DDW, 300 mL)

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for 10 min and then the samples are taken out and air dried again. Thirdly, the dried samples are

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dipped into solution C (DDW, 200 mL) for 10 min.

13

, the samples are prepared as

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In order to know the change of pyridaben concentration in the above eluted solutions, 2.50 mL

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solution B and 10.00 mL C are quantitatively transferred into a 25mL colorimeter tube, respectively.

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And 2.50 mL acetonitrile-water solution (1% v/v) is added then dilute it to volume with DDW for

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CL determination.

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Analytical Procedures. The on-line DFUT and flow injection CL determination of samples are

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described in Figure 1C. Two peristaltic pumps named P1 and P2 are used to transport luminol and

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KMnO4 (P1), NaOH and pyridaben/carrier [acetonitrile-water (0.1% v/v)] solutions (P2) at the

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selected flow rate, respectively. Follow the experimental procedures listed in Table 1, the sample

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solution is firstly pumped from the colorimeter tubes to storage coil by P2 at a flow rate of 4.0 mL

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min-1. Next, the sample solution is driven into the QGC by the carrier at the same flow rate, where

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the sample is irradiated by selected ultrasonic conditions. The above solutions are mixed and reacted

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with luminol and KMnO4 in the reaction cell lastly, and the optical signal is recorded with the 5 Environment ACS Paragon Plus


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photomultiplier tube and the luminometer. Quantitative analysis is achieved by comparing the

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changes before and after the implementation of pyridaben. The calculation formula is ∆I=I-I0. I is

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the signal of pyridaben-luminol-KMnO4 system, and I0 is obtained under the same condition with

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acetonitrile-water (0.1% v/v) solution instead of pyridaben. Instrument settings and operational

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parameters of the experimental DFUT-CL system are summarized in Table S1 (Supporting

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Information).

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Table 1. Working program of the on-line ultrasonic treatment coupled with CL determination Flow rate/ mL min-1 Step

Time/s

Events

P1

P2

1

2.8

4.0

20

Draw sample solution through sampling tube

2

0.0

0.0

5

Move sampling tube into acetonitrile solution

3

2.8

4.0

20

Sample is carried into the QGC with acetonitrile solution

4

0.0

0.0

480 Sample is irradiated by ultrasound in the QGC

5

2.8

4.0

40

Reacting with luminol and KMnO4 , then record the CL signal

6

0.0

0.0

5

Return to step 1

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HPLC equipped with a ZORBAX XDB-C18 analytic column （2.1×150mm） and an MS detector

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(Agilent series 6430, USA), is used for recovery test to validate the accuracy and feasibility of the

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proposed method. Instrument settings and operational parameters used for the HPLC-MS system are

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summarized in Table S2 (Supporting Information).

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RESULTS AND DISCUSSION

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DFUT-CL Determination of Pyridaben. In general, the quantitative basis of the CL method is that

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the analyte improves or inhibits the optical signal of different luminescence system 6 Environment ACS Paragon Plus

11-13

. However,

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preliminary experimental data shows that the addition of pyridaben doesn't change the CL signals of

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luminol with H2O2, KMnO4 or K2S2O8. When the on-line DFUT procedure is carried out before the

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pyridaben mixing with oxidant and luminol, fortunately, the CL signal of the luminol-oxidant

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system is markedly enhanced. Figure 2 shows the effect of different concentration of pyridaben on

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the CL signal in the luminol-KMnO4 system after DFUT. A good linear relationship (R2=0.9985)

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between the relative CL signal (∆I) and pyridaben concentration (inset of Figure 2) range from 0.8

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to 100.0 µg L-1 displays the wide linear range of the proposed methods in the field of monitoring

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pyridaben.

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Figure 2. CL intensities of different concentration of pyridaben after on-line DFUT (For the

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convenience of description, the time required for ultrasonic treatment in each measurement process

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is subtracted). Conditions of DFUT: 20kHz ultrasonic probe + 59kHz ultrasonic bath (frequency),

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20W + 80 W (power), 50oC (temperature), 6min (time).

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The mechanism of the enhancing effect on the CL signal is considered as a kind of energy transfer

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process based on the following experimental facts: i) there is no difference in HPLC-MS

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chromagraphy between before and after ultrasonic treatment (Figure S2, Supporting Information),

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which indicates that ultrasound does not promote degradation of pyridaben. ii) Except for peak

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intensity change,there is no new peak is found in the fluorescence and UV spectra of the 7 Environment ACS Paragon Plus


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luminol-KMnO4-pyridaben with/without DFUT (Figure S3, Supporting Information), which confirm

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that the emission spectrum belongs to 3-aminophthalate ion (3-AP). iii) the enhancement of CL

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signal is linearly related to the concentration of pyridaben, showing that pyridaben is involved in the

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luminescence process, not just because of hydroxyl radical produced in ultrasound.

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Figure 3. Diagram of the mechanism. We present the reaction process depicted in Figure 3. Briefly, under the irradiation of ultrasound, 22, 23

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water molecules produce hydroxyl radical (·OH) with high activity

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pyridaben gaining higher energy (excited). When the excited state of aminophthalate (3-AP*,

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produced by reaction between luminol and KMnO4) releases energy from the excited state back to

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the ground state, it can obtain energy from excited pyridaben molecules and re-excited to 3-AP*.

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Obviously, CL intensity is enhanced in the above process 24. More detailed explanations are listed in

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the Supporting Information.

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Optimum of Ultrasonic Conditions. Effect of ultrasonic mode on the signal of pyridaben. Based on

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the frequency and number of ultrasonic waves applied, we tested the effect of pyridaben on the CL

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signals of the luminol-KMnO4 system in seven ultrasonic modes after 10 min irradiation. The results

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are shown in Figure 4, the highest CL signal is obtained in the 20kHz ultrasonic probe + 59kHz 8 Environment ACS Paragon Plus

, then these radicals cause

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ultrasonic bath mode. Compared with this mode, the CL signals of pyridaben irradiated by 20kHz

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ultrasonic probe and 59kHz ultrasonic bath separately are about 50% and 30%, respectively.

166 167

Figure 4. Effect of ultrasonic mode on the CL intensities. Conditions of ultrasonic treatment are listed in

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Table 1. UB ultrasonic bath; UP ultrasonic probe.

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Gogate pointed out that the cavitation bubbles produced by high frequency ultrasonic wave can be 25

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used as the nuclei of lower frequency cavitation

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dual-frequency ultrasound is quite different from that of single frequency 26, 27. And these cavitation

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bubbles lead to more homogeneous and intense cavitation effect as compared to a single frequency

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sounding

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formation of hydroxyl radicals (·OH). Besides, an interesting experimental phenomenon either

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dual-frequency mode or single-frequency mode is found, that the increase of the CL signal is related

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to the applied ultrasonic frequency. The CL signals of pyridaben in Figure 4 are increased about

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30% (dual-frequency mode) and 22% (single-frequency mode) with the UB frequency from 40kHz

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to 59kHz. As we know, the cavitation effect is related to the size of the resonant bubble radius (r)

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and bubble collapse time (τ). Although small bubbles produce weak cavitation 30, a decrease of more

28, 29

. Therefore, the size of cavitation bubbles in

. This high intensity cavitation effect under dual-frequency mode promotes the



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than 40% in collapse time (Table S3, Supporting Information) give rise to more acoustic cycles and

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results in the formation of large amounts of smaller sizes cavitations bubbles at higher frequencies,

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and thus more cavitations per unit time

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production, thus promote pyridaben to form excited products.

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Effect of QGC shape and position.Two shapes of QGC are applied to the ultrasonic irradiation of

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pyridaben (Figure 1B). One of the QGC is concentric circles shape, and the other one is a spiral

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tube. The position of QGC in the ultrasonic device is also important in the process of ultrasonic

187

irradiation

188

considered together in this paper.

32

31

. Obviously, these factors are beneficial to radical

. Therefore, the effects of QGC shape and position on the signal intensity of CL are

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Figure 5 displays the signal change tendency after changing the position of QGC in the device. This

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experiment is carried out by fixing one of distance (H1 or H2) at 1.0 cm. Figure 5A indicates that the CL

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signal intensity decreases remarkably both with the distance of H1 and H2 from 0.5 to 4.0cm for 20 µg

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L-1 pyridaben solution when the threaded tubes QGC is used. Figure 5B indicates that the CL signal

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intensity increases both with the distance of H1 and H2 from 0.5 to 1.0cm, and then rapidly decreases

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when the concentric circle QGC is used. Compared with the distance between QGC and bottom of

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ultrasonic bath (H1), the change of ultrasonic probe position (H2) caused more severe signal changes,

196

which indicated that the distance has more influence on ultrasonic probe.

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Figure 5 also shows that the shape of QGC is an important factor that impacts signal intensity. Under

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the same conditions, the response obtained from the concentric circles QGC is 2.1 time of that from the

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spiral tube QGC. The reason may be that the size (2.0 cm) of the spiral tube caused the solution in the

200

tube to be farther away from the probe than the solution in the concentric circles QGC. Therefore, the

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concentric circles QGC placed at the middle of the two ultrasonic devices (H1= H2 = 1.0 cm) is used as

202

the optimum conditions in the sample analysis process with the best stability. Meanwhile, RSDs of 11

203

times measurements are less than 4.7% within linear range (e.g. 1.3% for 20µg L-1 and 2.5% for 50µg

204

L-1, Figure S4, Supporting Information). Also, good stability of the proposed methods is documented. 10 Environment ACS Paragon Plus

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205 206

Figure 5. Effect of QGC shape and position on the CL intensity of 20 µg L-1 pyridaben. (a) for spiral

207

tube shape, and (b) for concentric circles shape. H1 (H2) is optimized by fixing the distance of H2 (H1) at

208

1cm.

209 210

Figure 6. Effect of time on the CL intensities. Conditions of ultrasonic treatment were listed in Table S1.

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UB ultrasonic bath ; UP ultrasonic probe.

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Effect of Ultrasonic Time. Figure 6 lists the change of the CL signal of pyridaben-luminol-KMnO4

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with different ultrasonic time. The relative signal of the luminol-KMnO4 system increases with the

214

increased residence time for pyridaben sample solution stay in the QGC, and reaches the maximum 11 Environment ACS Paragon Plus


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signal at 8min (20kHz UP + 59kHz UB), 22min (20kHz UP) and 40min (59kHz UB), respectively.

216

Then the signal decreases with the prolonging of time. In addition, the maximum CL signals

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obtained under the three modes are also different. As can be seen in Figure 6, the relative CL signals

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of the pyridaben-luminol-KMnO4 system are only 73% (20kHz UP) and 61% (59kHz UB) of that

219

irradiated by dual-frequency mode with the same concentration.

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Moreover, in a dual-frequency ultrasonic mode, the higher ultrasonic frequency, the shorter the

221

time required to reach maximum signal. For instance, when the 20kHz UP + 59kHz UB

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dual-frequency mode is applied, it’s need 8 min to reach the maximum. When the 20kHz UP +

223

40kHz UB mode is used, the time is increased to 12 min (data not shown here). Meanwhile, the

224

signals of pyridaben are decreased after ultrasonic treatment for a long time. It’s shows that

225

prolonging the ultrasonic time does not yield stronger signal. It should be noted that the appropriate

226

response signal can also be obtained by shortening the ultrasonic processing time to 3-4min.

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Effect of Ultrasonic Power and Temperature. The effect of ultrasonic power on the CL signal in the

228

pyridaben-luminol-KMnO4 system is studied by fixed the ultrasonic probe power at 80 W. Figure

229

S5A-C (Supporting Information) shows the changes in the relative CL signals of the

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pyridaben-luminol-KMnO4 system produced by varying the power of the ultrasonic bath under different

231

dual-frequency modes. Due to violent cavitation at high ultrasonic power, the acoustic reaction of H2O

232

is accelerated 32, resulting in linear enhancement of the relative CL signal of pyridaben-luminol-KMnO4

233

with ultrasonic power (Figure S5D, Supporting Information). As a result, 200W (the maximum power

234

of the ultrasonic bath) is selected.

235

The influence of bath temperature is investigated by the temperature control function of ultrasonic

236

bath. The results are shown in Figure S6 (Supporting Information). When the temperature reaches 50oC,

237

the DUFT-CL signal of pyridaben is the highest. The influence of temperature on the CL signal is

238

basically independent of the applied ultrasonic frequency (Figure S6D, Supporting Information).

239

Although more cavitation bubbles formed in high temperatures

30-32


, the effect of bubble rupture is

Page 13 of 30


240

weakened when the temperature is too high (e.g. up to 70 oC in our experiment). Therefore, 50 oC is a

241

better temperature.

242

Optimization of CL Conditions. In general, the higher luminol concentration, the larger CL signal will

243

be obtained

244

intensity increases with the increase of the luminol concentration in the range of 1.0×10-5mol L-1 to

245

1.0×10-4 mol L-1. As the concentration exceeds 7.0×10-5 mol L-1, the upward tendency of the signal

246

becomes slighter. These data can be found in Figure S7A (Supporting Information). CL reaction usually

247

is not complete at low KMnO4 concentration, and the self-absorption effect of KMnO4 at high

248

concentration will also affect the sensitivity of the method

249

result (Figure S7B, Supporting Information). Therefore, 5.0×10-5 mol L-1 KMnO4 based on the

250

experimental data is selected as the operating condition. The luminol-KMnO4 reaction usually takes

251

place in an alkaline medium. The biggest CL signal of the pyriden-luminol-KMnO4 system in our

252

experiment is obtained at NaOH concentration range from 0.6 to 1.0 mol L-1 (Figure S7C, Supporting

253

Information). Two peristaltic pumps are used to transport the reactants in this experiment, thus the flow

254

rate not only affects the degree of CL reaction but also causes the waste of reagents 19. When the flow

255

rate of two peristaltic pumps are set at 2.8 mL min-1 (P1, luminol, and KMnO4) and 4.0 ml min-1 (P2,

256

NaOH, and analytes), respectively, the relative CL signal are suitable (Figure S8, Supporting

257

Information).

258

Interference Study. The effect of the presence of foreign substances in solution, on pyridaben

259

determination by CL with ultrasonic assistance, is tested. The tolerable concentration ratios with respect

260

to 2.0 µg L-1 pyridaben for interference at 5% level are over 10,000 for Na+, K+, Ca2+, Mn2+, Cl-, SO42-,

261

CO32-, NO3- and H2PO4-; 2000 for Ba2+, Mg2+, Al3+ and Zn2+; 500 for methanol, ethanol, oxalate, tartrate,

262

urea; 100 for ascorbic acid, L-cysteine, Cu2+, Fe3+, Cr3+, Co2+ and Ni2+. The detailed results are listed in

263

Table 2.

264

Table 2. Effect of coexisting substances on the determination of 2.0 µg L-1pyridaben.

15-18

. Our experimental data also confirms this conclusion. Although, briefly, the CL

17

. The experimental data show a similar



Species Cu2+

Co2+

Fe3+

Ni2+

Cr3+ Na+ K+ Ca2+ Ba2+ Mg2+ Al3+

C, mg L-1 Recovery, % 0.1 101 0.2 105 1.0 121 0.1 97 0.2 95 1.0 118 0.1 99 0.2 104 1.0 112 0.1 97 0.2 102 1.0 106 0.1 105 0.2 104 1.0 107 20 96 20 97 20 97 4 98 4 102 4 104

Species methanol oxalate ethanol tartrate urea ascorbic acid L-cysteine

ClNO3H2PO4CO32SO42HPO42-

C, mg L-1 1.0 5.0 1.0 5.0 1.0 5.0 1.0 5.0 1.0 5.0 0.2 0.5 0.1 0.2 1.0 20 20 20 20 20 20

Page 14 of 30

Recovery, % 98 92 103 94 97 90 95 91 102 94 96 87 102 95 81 101 99 104 98 97 105

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The mechanism of the enhancing effect on the CL signal is considered as a kind of energy transfer

266

process. Hence, some agrochemicals may also enhance the luminescence signal of lumilo-oxidant after

267

ultrasonic treatment. Experimental data show that the presence of 10.0 µg L-1 substances in 2.0 µg L-1

268

pyridaben sample solution produce 35% (picoxystrobin), 43% (azoxystrobin) and 27% (fenpyroximate)

269

positive interference; 20.0 µg L-1 substances in 2.0 µg L-1 pyridaben can lead to 52% (picoxystrobin),

270

69% (azoxystrobin) and 58% (fenpyroximate) changes. Besides, the CL detector itself does not have

271

selective analysis function. These factors will lead to problems, that is the presented method can not be

272

used directly for complex sample analysis. We are seeking for suitable ways (e.g. combination with

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chromatographic separation or selective extraction for multiple-residues samples analysis) to resolve

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this problem.

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Performance and Validation of the Procedure. Under the selective conditions, linear revision

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diagram is available in the scale of 0.8-100.0µg L-1 with the linear regression equations: IF=57.67 +


Page 15 of 30


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214.03 C. C in the equation is the pyridaben concentration (µg L-1). The regression coefficient (r2) is

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0.9974. The limit of detection (LOD) of pyridaben for this method is 0.085 µg L-1, calculated by taking

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3 times the standard deviation of the blank solution divided by the slope from 11 measurements. The

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limit of quantitation is 0.284 µg L-1. An analytic frequency of 6 h-1 (or 15 h-1 with 3min DFUT and the

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LOQ is 1.025 µg L-1) and an RSD value of 1.3% (at the 20.0 µg L-1 level for eleven replicate

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determinations) are also obtained. Table 3 covers the merits of analytical data achieved by using on-line

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DFUT coupled with CL for sample detection and the comparison of analytical performance with several

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other methods are also listed. The sensitivity and precision are superior or at least at the same level as

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reported methodologies.

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Table 3. Comparison of analytical parameters with other methods. Analytical Method

Linear range, µg L-1

LOD, µg L-1

RSD, %

Ref.

DFUT-CL a

0.8-100.0

0.085

Evaluation of Pyridaben Residues on Fruit Surfaces and Their Stability

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