preparation and application of a molecularly imprinted polymer for the determination of trace...

Research Article

Preparation and application of a molecularlyimprinted polymer for the determination oftrace metolcarb in food matrices by highperformance liquid chromatography

In this article, for the first time, a molecularly imprinted polymer (MIP) for the metolcarb

was prepared by bulk polymerization using metolcarb as the template, methacrylic acid as

the functional monomer and ethylene glycol dimethacrylate as the cross-linker. The

prepared polymer was characterized by FT-IR, static and kinetic adsorption experiments,

and the results showed that it has been successfully synthesized and had good selective

ability for metolcarb. The MIP was applied as a sorbent in molecularly imprinted SPE

coupled with HPLC-UV for separation and determination of trace metolcarb in three

kinds of food matrices at three concentration levels. Under the optimal conditions, the

LODs (S/N 5 3) of cabbage, cucumber and pear were 7.622, 6.455 and 13.52 mg/kg,

respectively, and recoveries were in the range of 68.80–101.31% with RSD (n 5 3) below

3.78% in all cases. To demonstrate further the selectivity of the MIP obtained, a

comparison with commercially available C18 SPE was performed. The results indicated

that molecularly imprinted SPE showed better chromatography, better selectivity and

higher recoveries for metolcarb than commercially available C18 SPE.

Keywords: Food matrices / Metolcarb / Molecularly imprinted polymer / SPEDOI 10.1002/jssc.200900877

1 Introduction

Carbamate pesticides, which comprise the major proportion

of common agricultural pesticides, are extensively used as

broad-spectrum insecticides in today’s agricultural industry

to improve the productivity for their wide range of biological

activity [1–5]. But these compounds inhibit acetylcholine

esterase and the N-nitrosocarbamates formed are potent

mutagens [6]. Particularly, metolcarb (m-tolyl methylcarba-

mate) is one of the major ingredients of many commercially

available carbamates insecticides. To safeguard the human

health, it is necessary to develop sensitive and selective

analytical methods to monitor the pesticide residue and

control the bioaccumulation process.

Currently, metolcarb is commonly detected by GC-MS/

HPLC-MS [7, 8], CZE [9] and ELISA [10, 11]. Many of the

above-mentioned determination methods are accurate and

selective, but require relatively expensive instrumentation

or high cost of antibodies. The HPLC-UV system is gener-

ally used in food sample analysis and the instrument is

more popular in practice, but the relatively higher LOD of

the UV detector makes it difficult to determine trace

metolcarb in complicated matrices and needs effective

extraction–purification sorbents. Sample preconcentration

based on SPE has been shown to be a good alternative to

liquid–liquid extraction, since the use of solid sorbent offers

the advantages of convenience, time and cost saving and

minimal consumption of organic solvents. The conventional

SPE materials show indiscriminate adsorption effects, so

that a large amount of matrix interferences are extracted

simultaneously with the target analyte. Developing novel

sorbents with good selectivity applied to cleanup methods is

of great significance, and the SPE methods based on

molecularly imprinted polymer (MIP) seem to represent

natural candidates to circumvent the drawbacks typically of

more traditional SPE techniques [12–19]. The imprinting

process uses a template molecule and functional monomer

for copolymerization in the presence of a cross-linking

agent, leading to polymer formation of the three-dimen-

sional structure. The template molecules are extracted after

the polymerization, leaving complementary recognition

sites in the polymer network. Hence, using MIP as adsor-

bent in SPE is very desirable for the development of selective

and sensitive methods for trace analysis.

Kun QianGuozhen FangJinxing HeMingfei PanShuo Wang

Key Laboratory of Food Nutritionand Safety, Ministry ofEducation, Tianjin University ofScience & Technology, Tianjin,P. R. China

Received December 29, 2009Revised April 27, 2010Accepted May 2, 2010

Abbreviations: AIBN, 2,20-azobisisobutyronitrile; DDW,

doubly deionized water; MAA, methacrylic acid; MIP,

molecularly imprinted polymer; MISPE, molecularlyimprinted SPE; NIP, non-imprinted polymer; NISPE,

nonimprinted SPE

Correspondence: Dr. Shuo Wang, Key Laboratory of FoodNutrition and Safety, Ministry of Education, Tianjin Universityof Science & Technology, Tianjin 300457, P. R. ChinaE-mail: [email protected], [email protected]: 186-22-6060-1332

& 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

J. Sep. Sci. 2010, 33, 2079–2085 2079

To the best of our knowledge, no study has been

reported about the preparation and application of an MIP

for metolcarb. In this article, a metolcarb MIP for SPE was

prepared and characterized, and the conditions of molecu-

larly imprinted SPE (MISPE) were optimized in detail.

Under the optimal condition, MISPE coupled with HPLC-

UV was successfully applied to the extraction and determi-

nation of trace metolcarb from food matrices for the first

time.

2 Materials and methods

2.1 Chemicals and materials

Metolcarb, propoxur, carbaryl, isoprocarb, fenobucarb

(Fig. 1), methacrylic acid (MAA) and ethylene glycol

dimethacrylate were all purchased from Sigma-Aldrich

(Madrid, Spain). 2,20-Azobisisobutyronitrile (AIBN), THF,

toluene, ethanol and acetonitrile were obtained from

Tianjin No. 1 Chemical Reagent Factory (Tianjin, China).

MAA and AIBN were purified before use. HPLC

solvents were supplied by Concord (Tianjin, China). The

reagents used were at least of analytical grade. Doubly

deionized water (DDW, 18.2 MO/cm) obtained from a

Water Pro water system (Labconco, Kansas City, MO,

USA) was used throughout the experiments. The commer-

cially available C18 cartridges (200 mg/3 mL) were obtained

from UCT.

The stock standard solutions of 1000 mg/L were

prepared in methanol. Working solutions were prepared

from the standard stock solutions by stepwise dilution in

DDW or methanol to a desired concentration just before

use. These solutions were stored at 41C in the dark.

2.2 Instrumentation

HPLC system consisted of two LC-10ATVP pumps and a

Shimadzu SPD-10AVP UV detector (Shimadzu, Kyoto,

Japan). All separations were achieved on an analytical

reversed phase C18 column (Alltima-C18, 5 mm,

4.6 mm� 250 mm, Alltech, USA) at a mobile flow rate of

0.8 mL/min under isocratic conditions and a mixture of

methanol/water (55:45, v/v) was used as the mobile phase.

Class-vp software was used to acquire and process spectral

and chromatographic data. The sample amount injected was

20 mL and the UV detection wavelength was 210 nm.

FT-IR spectra (4 000 400 cm�1) in KBr were recorded

using a Vector 22 spectrometer (Bruker). For the determi-

nation of the adsorption capacity of the polymers, a Cary 50-

Bio UV spectrometer (Victoria, Australia) was used.

2.3 Synthesis of the MIP

The synthesis of the MIP was carried out as follows: 0.165 g

of metolcarb (1.0 mmol) was dissolved in 2.0 mL of THF and

2.0 mL of toluene, and mixed with 0.340 mL (4.0 mmol) of

MAA. The mixture was stirred for 60 min before 1.88 mL

(10.0 mmol) of cross-linker ethylene glycol dimethacrylate

and 0.02 g of initiator AIBN was added. After stirring for

another 30 min, the solution was purged with nitrogen for

10 min. The mixture was sealed and thermally initiated in a

water bath at 551C for 20 h. After the polymerization, the

rigid polymer was crushed and the template was removed by

Soxhlet extraction with 300 mL of methanol/acetic acid (9:1,

v/v) until no analyte was detected using UV spectrometry at

210 nm. The dried polymer was passed through a 200-mesh

steel sieve. For comparison, nonimprinted polymer (NIP) was

also prepared in the same way in the absence of template.

2.4 Adsorption test

2.4.1 Static adsorption test

To measure adsorption capacity, 50.0 mg of MIP or NIP was

mixed with 10.0 mL of working methanol solution of

metolcarb at different concentrations (10, 20, 30, 40, 60,

80, 100 and 120 mg/L). The mixture was shaken (200 times/

min) for 120 min at a room temperature with a horizontal

shaker and separated centrifugally (4000 rpm) for 15 min.

The supernatant was measured for the unbound metolcarb

by UV spectrometry at 210 nm. The same procedure was

applied to test the static adsorption of the NIP. The

adsorption capacity (Q) was calculated according to the

following equation:

Q ¼ ðCi � Cf Þ � V=W ð1Þ

where Ci is the initial and Cf the final concentration of the

analytes in the solution. V and W are the volume of solution

and the mass of polymer, respectively.

CH3

O

O

NH

H3C

metolcarb

O

O

HN

H3C

O CH3

CH3

propoxur

ONH3C

O

carbaryl

O

O

NH

H3CCH3

CH3

isoprocarb

O

O

NH

H3CCH3

CH3

fenobucarb

H

Figure 1. The structures of metolcarb, propoxur, carbaryl,isoprocarb and fenobucarb.

J. Sep. Sci. 2010, 33, 2079–20852080 K. Qian et al.


2.4.2 Kinetic adsorption test

A 50.0 mg of the MIP was mixed with 10.0 mL of 80 mg/L

working methanol solution of metolcarb. The mixture was

shaken (200 times/min) for different times (10, 30, 60, 120

and 180 min) at a room temperature and separated

centrifugally (4000 rpm) for 15 min. The unbound metol-

carb in the supernatant was measured by UV spectrometry.

2.4.3 Competitive adsorption test

The competitive adsorption test was performed with

metolcarb and four kinds of carbamates as the competitors

with similar structures. In total, 50.0 mg of MIP or NIP was

added to a flask containing 10 mL of 10 mg/L metolcarb,

propoxur, carbaryl, isoprocarb and fenobucarb mixed work-

ing methanol solution, shaken at a room temperature for

120 min and separated centrifugally. HPLC-UV was used to

measure the five unextracted target molecules. The distribu-

tion coefficient (Kd) and selectivity coefficient (K) were

calculated from the following equations:

Kd ¼ fðCi � Cf Þ=Cfg� fVolume of solution ðmLÞg=fMass of gel ðgÞg

ð2Þ

K ¼ Kd ðMetolcarbÞ=Kd ðCompetitorsÞ ð3Þ

2.5 Procedures for separation and determination of

metolcarb using MISPE-HPLC

To evaluate the applicability of the MIP for separation of

trace metolcarb in food, 100.0 mg of MIP or C18 was packed

into an empty SPE cartridge. The MISPE cartridge was

consecutively conditioned with 3 mL of methanol and DDW

prior to extraction, followed by loading with 100.0 mL of

metolcarb aqueous solution at a flow rate of 1.67 mL/min.

The analytes adsorbed on the cartridge were eluted by

10.0 mL of methanol. The effluents were collected, dried

using a rotary evaporator, dissolved with 1.0 mL of methanol

and detected by HPLC-UV at 210 nm after filtration. When

completed, the MISPE cartridge was rinsed with 5.0 mL of

methanol and DDW for the next sample preconcentration.

The extraction procedures were carried out on NIP column

under the same experimental conditions.

The extraction of metolcarb was employed with the C18

cartridges at their suitable conditions. The C18 SPE condi-

tions were optimized as follows: conditioning was performed

with 5.0 mL of methanol and 5.0 mL of water in turn, loading

with 100.0 mL of metolcarb aqueous solution, washing with

3.0 mL H2O and elution with 10.0 mL methanol.

2.6 Preparation of food samples

Cabbage, cucumber and pear samples, determined to be

free of the analyte, were bought from local market. To 10.0 g

of each sample which was crushed up, 50, 100 and 200 mL of

the working aqueous solution (10.0 mg/L) of metolcarb

were added separately. The mixture was homogenized, left

for 30 min and added 30 mL of H2O, respectively, shaken by

sonication for 30 min and separately centrifuged. The

extraction procedure was repeated three times, and the

supernatant was collected, diluted to 100.0 mL with H2O

which was applied to the MISPE, nonimprinted SPE

(NISPE) or C18 SPE.

3 Results and discussion

3.1 Characteristics of metolcarb and MIP FT-IR

spectra

The band around 3339 cm�1 was assigned to the character-

istic peaks for the –NH– of metolcarb. Characteristic bands

around 768 and 946 cm�1 were attributed to vibrations of

Ar–CH3 bond in metolcarb which did not participate in the

reaction. As shown in Fig. 2, both (a) and (b) have the

characteristic peaks at around 3339 cm�1. But compared with

the spectrum of metolcarb, the –NH– vibration of (b)

becomes wider than the template. The reason might be that

–NH– groups on the metolcarb have reacted with C=O

groups attached with the functional monomer MAA to form

hydrogen bonds. In the spectrum of (b), the observed feature

around 1715 cm�1 becomes wider than the spectrum of

metolcarb at the same position which might also indicate that

the C=O groups on MAA have bonded to the template. These

results showed that the metolcarb-MIP has been successfully

synthesized. There are nearly no peaks observed in (c) as

compared with (b) were shown at the bands around 768 and

946 cm�1, indicating that the template was removed from the

MIP after extraction.

3.2 Evaluation of adsorption test

3.2.1 Evaluation of static adsorption

The adsorption capacity was an important factor to evaluate the

combined capability of the MIP toward template molecules.

The adsorption isotherm curve is shown in Fig. 3a. It was

shown that the adsorption capacity of MIP or NIP increased

with the increase in metolcarb initial concentration. The

adsorption capacity of the MIP (up to 2.77 mg/g) was about 1.8

times of NIP (up to 1.57 mg/g) at a 120 mg/L solution. The

results showed that the MIP had a significantly higher

adsorption capacity for metolcarb compared with the NIP.

Affinity of the MIP and NIP was assessed by Scatchard

analysis (Fig. 3b). The equation of the Scatchard model being:

Q=C ¼ �Q=Kd 1 Qmax=Kd ð4Þ

where C is the initial concentration of the analytes in the

solution, Q is the adsorption capacity at adsorption equili-

brium and Qmax is the saturated adsorption capacity.

J. Sep. Sci. 2010, 33, 2079–2085 Liquid Chromatography 2081


Results from Scatchard analysis indicated that the

adsorption isotherm of MIP toward the template molecules

was in good accordance with linearity and a Scatchard

model R2 5 0.99 in the experimental conditions. The satu-

rated adsorption capacity (Qmax) of MIP toward metolcarb

was 6.62 mg/g obtained by the linear slope (Kd�1). For NIP,

the nonlinearity indicated that there were no selective

adsorption sites for metolcarb on it. From the Scatchard

analysis, it could be assumed that the rebinding sites in the

MIP are mainly dependent on hydrogen bonding, whereas

the interaction between NIP and metolcarb was mainly

from nonspecific adsorption. Here, we obtained a similar

conclusion as Song et al. [20].

3.2.2 Evaluation of kinetic adsorption

To evaluate the adsorption rate of the MIP, the kinetics of

metolcarb were also examined. A 50 mg amount of MIP was

equilibrated for different times with 10 mL of 80 mg/L

metolcarb dissolved in methanol. The results indicate that

nearly 50% of binding was obtained within a short shaking

period of 10 min, and binding equilibrium was obtained

within 2 h. It should be noted that if the concentration of

metolcarb was lower, the time to saturation would be

correspondingly shorter.

3.2.3 Evaluation of competitive adsorption

Metolcarb, propoxur, carbaryl, isoprocarb and fenobucarb

were selected to validate the selectivity of the MIP to

template molecules. The results (Table 1) summarize the

data (Kd and K) which were obtained in competitive binding

experiments. The large Kd value of the MIP was an

indication of its high selectivity for metolcarb over the

competitors due to the tailor-made cavities in the MIP. The

distribution coefficients for the five carbamates on the MIP

were 64.09, 0.8148, 0.8768, 3.141 and 4.856, respectively.

The distribution coefficients for metolcarb and other

carbamates on the NIP were 13.80 and 0, respectively. The

selectivity coefficients for the four competitors were 78.66,

73.10, 20.40 and 13.20 for the MIP, whereas 0.9, 5.2 and 9.2

for the NIP. Among the four kinds of cabamate competitors,

adsorptions were more for isoprocarb and fenobucarb,

whereas less for carbaryl and propoxur. As shown in

Fig. 1, the structures of isoprocarb and fenobucarb were

3339

2944

1715

1678

1609

1531

148

81

420

126

712

3711

5911

0110

0294

69

2287

780

276

872

568

564

0

5001000150020002500300035004000

Wavenumber cm-1

0

20

40

60

80

100

Tra

nsm

itta

nce

[%

]

(a)

(b)

(c)

Figure 2. FT-IR spectra of metolcarb (a), MIP(the template not extracted) (b) and MIP (thetemplate extracted) (c).

0

0.5

1

1.5

2

2.5

3

0

0 0.5 1 1.5 2 2.5 3

20 40 60 80 100 120 140

Initial concentration of metolcarb (mg L-1)

Ads

orpt

ion

capa

city

(m

g g-

1 )

MIP

NIP

y = -0.0058x + 0.0384

R = 0.99

00.0050.01

0.0150.02

0.0250.03

0.0350.04

Q (mg g-1)

Q/C

(m

g m

L-1

)

MIP

NIP

A

B

Figure 3. (A) Binding isotherm of MIP and NIP for metolcarb inmethanol. (B) Scatchard plot of MIP and NIP.



more analogous to metolcarb compared with other compe-

titors with the only difference occurring in the size of the R-

group. Carbaryl suffers more steric hindrance due to its

naphthalene ring structure and propoxur has a more

complex R-group which make the adsorptions of the two

kinds of carbamates least.

3.3 Optimization of the MISPE process

To evaluate the applicability of the MIP for enrichment of

trace metolcarb, preliminary experiments, such as loading

flow rate, elution solution composition and volumes, and

sample acidity were optimized in metolcarb working

aqueous solution which is similar to the aqueous mediums

of the vegetable and fruit.

The effect of loading flow rate on the extraction of

100.0 mL, 10 mg/L metolcarb working aqueous solution was

studied. It was found that the recoveries of metolcarb

increased as the sample loading flow rate decreased from 3.34

to 1.67 mL/min. There is no significant variation occurred as

the sample loading flow rate decreased from 1.67 to 0.83 mL/

min. Based on the above results, 1.67 mL/min was chosen as

the loading flow rate in subsequent experiments.

The influence of sample pH on the extraction of

100.0 mL, 10 mg/L of metolcarb working aqueous solution

was studied in the pH range of 4–8 at a sample flow rate of

1.67 mL/min. Figure 4 shows the effect of the sample pH

on metolcarb retention, showing that the recoveries

obtained increased as the sample pH from 4 to 5. There is

no significant difference observed among the recoveries of

analyte at pH 5–7, whereas the recoveries decreased from

pH 7 to 8. The loading solution at pH 5 6.5 was used in

subsequent experiments.

Optimizing the eluting step is necessary to get the

satisfactory MISPE extraction results. About 100.0 mL,

10 mg/L of metolcarb working aqueous solution was loaded

onto the MIP cartridge. In the eluting step, different eluents

at the same volume (12.0 mL) were investigated to identify

their influence on desorption of metolcarb. Results showed

that 12.0 mL of methanol was sufficient to completely strip

the metolcarb retained on the cartridge, and the recovery

obtained was 99.98%. When ethanol and acetonitrile were

applied to the elution step at the same volume (12.0 mL), the

recoveries of the analyte were 75.65 and 90.92%, respec-

tively. Acetic acid can increase the eluting strength, weaken

the binding of template to the imprinted polymer and

release the template from the imprinted cavity of the MIP

more quickly [21]. However, as acetic acid disrupts the peak

position of metolcarb at 210 nm, it was not used as the

eluent in this study.

For selection of the eluent volume, varying volumes

(5.0–12.0 mL) of methanol were applied for the MISPE

process. Results showed that the recoveries of metolcarb

increased as the eluent volume increased from 5.0 to

10.0 mL, and there is no significant difference observed in

the range of 10.0–12.0 mL. Accordingly, the eluent volume

of 10.0 mL was selected to ensure complete stripping of the

adsorbed metolcarb from the MIP cartridge.

Linearity was achieved at the concentration range of

3–30 mg/L in metolcarb aqueous solution through the

MISPE procedure with good correlation coefficient

(R2 5 0.999).

3.4 Determination of metolcarb in real samples

To evaluate the application of the MISPE-HPLC method, the

cabbage, cucumber and pear samples at three spiked levels

were analyzed, and the corresponding chromatograms are

shown in Fig. 5. Under the optimum conditions, LODs (S/N 5 3) of MISPE for the three samples achieved 7.622, 6.455

and 13.52 mg/kg, respectively, and LODs, (S/N 5 3) of C18

SPE for the three samples achieved 5.370, 10.26 and

15.27 mg/kg, separately. Table 2 summarizes the recoveries

of the analytes from the MISPE, NISPE and C18 SPE. It is

clear that the metolcarb recoveries are slightly different for

the three samples. As listed in Table 2, the recovery of

NISPE was lower than for MISPE under the same

Table 1. Competitive loading of metolcarb, propoxur, carbaryl,

isoprocarb and fenobucarb by MIP and NIP

Sorbents MIP NIP

Metolcarb 0.6023 0.1361

Propoxur 0.008141 0

Loading capacity (mg/g) Carbaryl 0.008760 0

Isoprocarb 0.03131 0

Fenobucarb 0.04841 0

Metolcarb 64.09 13.80

Propoxur 0.8148 0

Kd Carbaryl 0.8768 0

Isoprocarb 3.141 0

Fenobucarb 4.856 0

Propoxur 78.66 0

K Carbaryl 73.10 0

Isoprocarb 20.40 0

Fenobucarb 13.20 0

0

20

40

60

80

100

120

4 5 6 7 8 9

pH

Rec

over

y (%

)

Figure 4. Recoveries of metolcarb in MISPE with differentsample-loading pHs.



experimental conditions, which also proved that specific

interaction between the MIP sorbent and the analyte exactly

existed during the binding procedure. The recoveries of

analyte from MISPE ranged from 62.40 to 101.32% and the

RSDs (n 5 3) were from 1.13 to 3.78%, whereas the

recoveries from C18 SPE ranged from 60.30 to 90.91% and

the RSDs (n 5 3) were from 1.07 to 3.26%. Compared with

MISPE, more peaks of impurities appeared in the chroma-

tograms through C18 SPE, which might be attributed to the

complex sample matrices. This confirmed that the MIP

exhibited higher selectivity for its template than C18.

4 Concluding remarks

In this study, a metolcarb-MIP was prepared and showed

good selectivity, high adsorption capacity for target mole-

cule, which indicated that it was suitable as sorbent for SPE.

Effective analytic method of trace metolcarb was developed

by using this polymer as enrichment sorbent coupled with

HPLC. To demonstrate further the selectivity of the MIP

obtained, a comparison between MISPE and commercially

available C18 SPE was performed. Under the optimal

conditions, the MISPE gave better abilities of purification

and higher recoveries than C18 SPE in spiked food matrices

at three low concentration levels. This is a promising

method for selective adsorption and determination of

metolcarb from food matrices by MISPE-HPLC.

This work was supported by the National Natural ScienceFoundation of China (Project No. 30872126 and 20775054)and the Ministry of Science and Technology of China (ProjectNo. 2008AA10Z420).

The authors have declared no conflict of interest.

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0 2 4 6 8 10 12 14 16 18 20

Vol

ts

-0.01

0.00

0.01

0.02

0.03

0.04

0.05

(a)

metolcarb

(c)

(b)

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Table 2. Recoveries of cabbage, cucumber and pear samples

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Food

sample

Spiked

level (mg/kg)

Recovery of metolcarb (%)

MISPE NISPE C18 SPE

0.05 68.807 2.48 No detection 60.307 1.12

Cabbage 0.1 79.747 3.18 24.757 1.01 71.317 1.07

0.2 101.327 1.13 39.637 2.26 85.787 3.14

0.05 72.547 3.78 No detection 62.887 3.26

Cucumber 0.1 77.437 2.90 22.727 1.28 70.327 1.19

0.2 99.477 2.58 41.117 1.69 90.247 2.06

0.05 62.407 2.52 No detection 60.067 3.08

Pear 0.1 70.387 3.73 24.877 0.88 67.677 1.17

0.2 100.657 1.91 41.257 2.33 90.917 2.27



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