Plant hormones (PHs) are a type of pesticide that can potentially affect human health. Therefore, their quantitative detection is particularly important. In this study, a green and economic method for the simultaneous extraction and determination of four PHs, namely thidiazuron, forchlorfenuron, 1-naphthylacetic acid, and 2-naphthoxyacetic acid.
Trang 1Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/chroma
Yanke Lua, Pengfei Lia, Chunliu Yanga, ∗, Yehong Hanb, Hongyuan Yana, b, ∗
a Key Laboratory of Medicinal Chemistry and Molecular Diagnosis of Ministry of Education, College of Public Health, Hebei University, Baoding 071002,
China
b Key Laboratory of Analytical Science and Technology of Hebei Province, College of pharmacy, Hebei University, Baoding 071002, China
a r t i c l e i n f o
Article history:
Received 16 March 2020
Revised 3 May 2020
Accepted 5 May 2020
Available online 8 May 2020
Keywords:
One pot fabrication
Resin adsorbent
Pipette tip solid-phase extraction
Plant hormones
Watermelon juice
a b s t r a c t
Plant hormones (PHs) are a type of pesticide that can potentially affect human health Therefore, their quantitative detection is particularly important In this study, a green and economic method for the simultaneous extraction and determination of fourPHs, namely thidiazuron, forchlorfenuron, 1-naphthylacetic acid, and 2-naphthoxyacetic acid, in watermelon juice was developed by using m-aminophenol–urea–glyoxalresinastheadsorbentforpipettetipsolidphaseextraction(PT-SPE)coupled withliquidchromatography.Theresin wassynthesizedviaasimple(one pot hydrothermalsynthesis) andgreen(ethanolasthesolventandglyoxalascrosslinkingagent)process.Thesynthesizedresin pos-sessesmultiplefunctionalgroups(hydroxyl,amino,andimino,amongothers),highadsorptioncapacity, largerspecific surfaceareathanthe urea–glyoxalresin and m-aminophenol–glyoxalresin,and can be regeneratedeasily.ThePT-SPEdeviceissimple,cheap,andeasytoobtain,andtheadsorbentdosageis only5.0mg.Theproposedmethodhasawidelineardetectionrange,highrecovery,goodprecision,and highsensitivity,and satisfiesthemeasurementrequirementsfordetecting tracelevelsofPHsinfruits andvegetables
© 2020 Elsevier B.V All rights reserved
1 Introduction
Sample preparation, as a key step in modern analysis, plays an
important role in the entire analysis process Currently, the devel-
opment of new, economical, fast, and high-throughput sample pre-
treatment methods is still an urgent issue in the analytical field
Sample pretreatment technologies include solid-phase extraction
(SPE) [1], solid-phase microextraction [ 2, 3], liquid–liquid extraction
[4], matrix solid-phase dispersion [5], dispersive liquid–liquid mi-
croextraction [6], pipette tip SPE (PT-SPE) [7] Among them, SPE
and methods derived therefrom are the most widely used tech-
nologies, especially the PT-SPE method have received widespread
attention from researchers because of its advantages of low cost,
small dead volume, longer flow path, and low consumption of the
adsorbent and organic solvent [8] However, SPE and related meth-
ods are dependent on the adsorption performance of the adsor-
∗ Corresponding authors at: Key Laboratory of Medicinal Chemistry and Molec-
ular Diagnosis of Ministry of Education, College of Public Health, Hebei University,
Baoding 071002, China
E-mail addresses: yangchunliu@hbu.edu.cn (C Yang), yanhy@hbu.edu.cn (H Yan)
bent; therefore it is necessary to explore new adsorbents with ex- cellent adsorption and purification abilities
Phenolic resin, as a commercial synthetic resin, is widely used
in a variety of fields, including in adsorbent owing to its low cost, easy availability, and simple preparation procedure [ 9, 10] Phenol– formaldehyde resins are conventionally utilized, but formaldehyde, used as the crosslinker, is carcinogenic Therefore, researchers have attempted to find alternative crosslinking agents, such as glu- taraldehyde, glyoxylic acid, hexamethylenetetramine, and glyoxal [11–15] Although the toxic side effects of the aforementioned ad- sorbents to the human body are relatively low, the functional groups of the phenolic resin are still limited as only phenolic hy- droxyl groups are present in these resins, which in turn affects the adsorption ability Therefore, the development of new multifunc- tional phenolic resin adsorbents has become a hot research topic Plant hormones (PHs) are widely used in modern agriculture
to increase crop yields [16–23] Thidiazuron (TDZ), forchlorfenuron (CPPU), 1-naphthylacetic acid (NAA), and 2-naphthoxyacetic acid (BNOA), as PHs, regulate watermelon growth, and have been widely used in different periods [ 24, 25] Currently, watermelon juice has become one of the most popular fruit drinks in the sum- https://doi.org/10.1016/j.chroma.2020.461214
0021-9673/© 2020 Elsevier B.V All rights reserved
Trang 2glyoxal resin (MAPGR) and urea–glyoxal resin (UGR) are also syn-
thesized, and the adsorption capacity of the three adsorbents is
compared A small PT-SPE cartridge is assembled by using a pipette
tip, degreased cotton, and rubber suction bulb Finally, a method
based on MAPUGR −PT-SPE coupled with high performance liquid
chromatography (HPLC) for detecting these four PHs in watermelon
juice is established by utilizing the optimized extraction parame-
ters
2 Experimental section
2.1 Reagents and instruments
The details are displayed in the Supplementary material.
2.2 Synthesis of MAPUGR, MAPGR, and UGR
Synthesis of MAPUGR: Aminophenol (30 mmol), urea
(10 mmol), PEG 60 0 0 (0.015 mmol), and anhydrous ethanol
(50 mL) were added to a flask (100 mL) and stirred until thor-
oughly mixed Glyoxal (30 mmol) was added, and pH of the
solvent was adjusted to 9.0 with NaOH solution, the resulting
solution was stirred at 50 °C for 3 h After heating at 75 °C for
24 h, the solid product was washed with methanol and water, and
then dried at 40 °C to obtain MAPUGR
Synthesis of MAPGR: Aminophenol (40 mmol), PEG 60 0 0
(0.015 mmol), and anhydrous ethanol (50 mL) were added to
a flask (100 mL) and stirred until thoroughly mixed Glyoxal
(30 mmol) was added and the pH of the system was adjusted to
9.0 with NaOH solution; the resulting solution was stirred at 50 °C
for 3 h After heating at 75 °C for 24 h, the resulting solid product
was washed with methanol and water, and then dried at 40 °C to
obtain MAPGR
Synthesis of UGR: Urea (40 mmol), PEG 60 0 0 (0.015 mmol), and
anhydrous ethanol (50 mL) were added to a flask (100 mL) and
stirred until thoroughly mixed Glyoxal (30 mmol) was added, the
pH of the system was adjusted to acidic with HCl solution, the re-
sulting solution was stirred at 50 °C for 3 h After heating at 75 °C
for 24 h, the obtained solid product was washed with methanol
and water, and then dried at 40 °C to obtain UGR
2.3 Preparation of watermelon juice
Watermelon samples were randomly purchased from the local
supermarkets in Baoding China First, the watermelons were ho-
mogenized with a homogenizer after peeling The supernatant was
obtained after centrifugation at 10 0 0 0 rpm for 10 min to remove
the solid residues, and the supernatant was stored frozen in a re-
frigerator Finally, the samples were passed through a 0.45 μm fil-
ter for further work
Fig 1 Assembly and operation process of MAPUGR −PT-SPE
2.4 Procedure for MAPUGR −PT-SPE
The PT-SPE was assembled from two pipette tips, degreased cotton and the adsorbent, as shown in Fig.1 First, degreased cot- ton was inserted into the exit of the 200 μL pipette tip to avoid loss of the adsorbent Thereafter 5.0 mg of MAPUGR was added to the 200 μL pipette tip The top of the adsorbent was then covered with degreased cotton A 1.0 mL pipette tip that was cut off was connected to the 200 μL pipette tip filled with adsorbent The final height of the adsorbent in the PT-SPE was approximately 10 mm After activating MAPUGR with 2.0 mL of methanol and water, wa- termelon juice (1.0 mL) at pH 3.0 was loaded onto the cartridge, washed with water (1.0 mL), and eluted with methanol (1.0 mL) The tip of a rubber suction bulb was tightly inserted into the top
of the pipette tip The pressure above the sample solution was con- trolled by squeezing the rubber suction bulb through a clamp fixed
on an iron stand to control the flow rate The eluate was collected, dried under nitrogen, and then re-dissolved in 1.0 mL of the mo- bile phase for HPLC analysis
2.5 HPLC conditions
Chromatographic separation was performed on an LC–20A high performance liquid chromatograph equipped with an LC–20AT sol- vent delivery unit and an SPD −20A ultraviolet detector (Shimadzu, Kyoto, Japan) A Labsolution workstation was used for data ac- quisition (Shimadzu, Kyoto, Japan) An Eclipse XDB–C 18 column (4.6 × 150 mm, 5 mm) was purchased from Agilent Tech Co., Ltd (California, United States) The wavelength of the detection was set
at 224 nm The mobile phase comprised water-acetonitrile (72:28, v/v, containing 0.1% TFA) at a flow rate of 1.0 mL min −1 The col- umn temperature was maintained at 40 °C, and the injection vol- ume was 20 μL
2.6 Statistical analysis
The confidence interval is represented by an error bar Because comparisons among three or more sample means were to be per- formed, one-way analyses of variance and multiple comparison
Student −Newman −Keuls (SNK- q) were selected to analyze the sig-
Trang 3Fig 2 Schematic illustration of MAPUGR synthesis
nificant differences Mean values were considered to have a signif-
icant difference when the significance test value ( P) < 0.05
3 Results and discussion
3.1 Synthesis of MAPUGR, MAPGR, and UGR
The purpose of the MAPUGR adsorbent synthesized herein was
to extract and isolate four PHs in watermelon juice An attempt
was made to synthesize the resin using m-aminophenol and urea
with glyoxal to meet the requirements for pretreatment of the
complex sample The synthesis process and reaction principle of
the adsorbent are shown in Fig 2 The methylolated intermedi-
ate was first formed from m-aminophenol, urea and glyoxal in
ethanol After increasing the temperature, the intermediate poly-
condensed and solidified, and PEG 60 0 0 was also uniformly fixed
in the solid product After washing the product with methanol
and water, the unreacted monomers, crosslinker, and porogen in
the solid product were removed, and the spatial arrangement of
the MAPUGR structure was maintained by vacuum drying The
hydrophilic groups of m-aminophenol and urea were retained in
the product, which provided numerous adsorption sites and en-
hanced the hydrophilicity of the sorbent Glyoxal containing two
aldehyde groups provides adequate reaction sites, and was used as
a crosslinking agent in the synthesis of MAPUGR PEG 60 0 0, as a
porogen and structure-directing agent, plays an important role in
the formation of the three-dimensional porous structure of MA-
PUGR Anhydrous ethanol, as the reaction solvent in this experi-
ment, does not exert the toxic effects of organic solvents on the
human body and the environment, and thus enables green synthe-
sis of the polymer
3.2 Characteristics of MAPUGR, MAPGR, and UGR
Fig.3A shows the scanning electron microscope (SEM) image of
MAPGR, which revealed that the product formed by the reaction
of m-aminophenol with glyoxal had a non-uniform particle size
with close adhesion of the components The average particle size
of the individual microspheres was 3 −5 μm, and the particle size
of the adhesion structure was 5 −30μm The SEM image of UGR in
Fig.3B shows a sheet-like adhesion structure with a particle size
of 2–30 μm The SEM image of MAPUGR in Fig.3C shows the over-
all three-dimensional porous structure formed by the aggregation
of multiple microspheres The microspheres/microspheres junction
may be producted by the reaction of glyoxal with urea The size of
the individual microspheres is 1–3 μm, but the overall particle size
is 30–100 μm The particle size of MAPUGR satisfies the applica- tion range of SPE, and the solid layer formed by the aggregation of the microspheres in the three-dimensional network structure en- sures that MAPUGR is in sufficient contact with the sample solu- tion when loaded into the PT-SPE device
The functional groups of MAPUGR, MAPGR and UGR were con- firmed by Fourier-transform infrared spectrometry (FT–IR) Broad adsorption peaks were found at 3342 and 3232 cm −1 due to the
N –H and O –H telescopic vibrations, and the peak of the C –H stretching vibration was appeared at 2973 cm −1 Furthermore, the stretching vibration of the C =O group of urea was observed at
1689 cm −1 , and the peaks associated with the C = C group in the m- aminophenol benzene ring skeleton were observed at 1612, 1493, and 1460 cm −1 The peaks at 1304, 1203, 1161, and 1105 cm −1 were attributed to C –O and C –N tensile vibrations Finally, the sig- nals at 809 and 766 cm −1 were attributed to the out-of-plane bending vibration of C –H in the aromatic ring, respectively These results indicate that the amino and phenolic hydroxyl groups in m- aminophenol, the carbonyl group in urea, and the hydroxyl group from the reaction of urea and glyoxal were successfully introduced into MAPUGR
The surface area and pore characteristics are listed in Table S1 The average pore diameters of MAPUGR, MAPGR, and UGR were 96.42, 131.54, and 176.86 ˚A, respectively The surface area of MA- PUGR is 2.02 and 1.67 times that of MAPGR and UGR, respectively The adhesion of MAPGR results in a small specific surface area However, during the synthesis of MAPUGR, the addition of urea effectively prevented the adhesion of MAPGR; therefore, the syn- thesized MAPUGR had a high specific surface area
3.3 Evaluation of adsorption behavior
The adsorption capacity of MAPGR, UGR and MAPUGR for the four PHs was evaluated by adding 2.0 mL of solution (30.0 μg mL −1 of the analytes) to a 10 mL centrifuge tube con- taining 3.0 mg of adsorbent As shown in Fig 4A, MAPUGR had
a higher adsorption capacity for the four PHs than MAPGR and UGR ( P < 0.05) This further confirmed that the addition of urea during the preparation procedure enhanced the adsorptive inter- actions with the analytes and increased the surface area, which improved the adsorption capacity of MAPUGR The experimental steps for evaluating the adsorption kinetics are presented in the
Supplementary material As shown in Fig S1, the adsorption capac- ity of MAPUGR for TDZ, CPPU and BNOA could exceed 45% of the adsorption capacity at adsorption equilibrium, and the adsorption capacity of NAA could exceed 33% of the adsorption capacity at adsorption equilibrium within 5 min Therefore, the mass trans-
Trang 4four PHs
3.4 Optimization of the MAPUGR −PT-SPE process
The molecular forms of the compounds may be affected by the
sample pH, which has an effect on the recovery of the four PHs
When the sample pH is less than the pKa, the analytes are pre-
dominantly in the molecular or protonated form, which facilitates
adsorption of the analytes by the adsorbent The pKa values of TDZ,
CPPU, NAA, and BNOA were 8.86, 8.40, 4.26, and 4.75, respectively,
and pH of the watermelon juice was 5.63; thus, the sample pH was
adjusted in the range from 2.0 to 7.0 with 1 M HCl and 1 M NaOH
Fig.4B shows that pH had a strong influence on the adsorption of
NAA and BNOA ( P < 0.05), but had little effect on TDZ and CPPU
( P > 0.05) Therefore, the sample pH was optimized to 3.0
The loading volumes (1.0, 1.5, 2.0, 2.5, and 3.0 mL) of the sam-
ples were studied Fig 4C shows that the loss rate of the ana-
lytes gradually increased ( P < 0.05) with increasing loading vol-
ume, which indicates that limited adsorption could be achieved
with 5.0 mg of the adsorbent, and dynamic adsorption equilibrium
was not reached Considering the extraction speed and efficiency,
a loading volume of 1.0 mL was used for further work The wash-
ing solvent can effectively remove interfering substances Thus,
1.0 mL of water, methanol–water (1:9, v/v), acetonitrile–water (1:9,
v/v), acetone–water (1:9, v/v), water–hydrochloric acid (pH 3), and
water–acetic acid (pH 3) were studied as the washing agent As
shown in Fig.4D, there was almost no loss of the analytes when
water–hydrochloric acid (pH 3) or water was used as the wash-
ing agent ( P > 0.05) Considering the purification effect and cost,
1.0 mL of water was used for the subsequent work Six eluents
were studied for elution of the analytes from MAPUGR; the results
presented in Fig 4E show that the recovery of four PHs was the
highest when methanol was used as eluent ( P < 0.05) The effect
of different eluent volumes (0.20, 0.40, 0.60, 0.80, 1.0, and 1.2 mL)
on the analyte recovery was also investigated As shown in Fig.4F,
the optimal recovery was achieved when the eluent volume was
1.0 mL ( P > 0.05)
3.5 Comparison with commercial adsorbents
A spiked recovery experiment was used to evaluate the extrac-
tion efficiency of MAPUGR and seven commercial adsorbents (C 18 ,
HLB, MCX, PSA, SCX, silica gel, and MAX) for the four PHs in wa-
termelon samples The procedure for MAPUGR–PT-SPE was per-
formed as described in the Experimental Section, and the extrac-
tion conditions for HLB, MCX, C 18 , PSA, SCX, silica, and MAX are
described in previous studies [32–35]; the results are presented in
Fig.5and Fig S2 MAPUGR had higher recoveries for the four PHs
than the commercial adsorbents ( P < 0.05) MCX provided excel-
lent recovery ( >80%) because it is a cation exchange and reverse-
bents (MAX, PSA) presented low recoveries for the PHs, indicating that the cation exchange and reverse-phase dual-retention play an important role in the adsorption process In addition, the recovery
of SCX, dominated by π–π bonds, was low ( <32%), which proves that π–π bonds are not the main force between the analytes and adsorbent This phenomenon is attributed to the large number of hydroxyl groups, amino groups, and imino groups on the surface
of the adsorbent, which sterically hinder π–π bonding However, the weakly polar analytes were minimally adsorbed on the polar adsorbents (silica, PSA) in aqueous medium, whereas higher re- covery for the analytes was achieved with the strongly nonpolar adsorbent (C 18 ), indicating that the nonpolar interactions with the analytes play an important role Finally, more than 92% of the four PHs was recovered with HLB, which proves that the hydrophilic- ity and hydrogen bonding of the adsorbent contribute significantly
to the adsorption of the PHs Overall, MAPUGR undergoes multi- ple adsorptive interactions with the analytes due to its excellent hydrophilicity
3.6 Adsorbent regeneration
The reusability of MAPUGR was investigated by an adsorption- desorption cycle experiment by following the procedure described
in the Experimental Section The concentration of the spiked solu- tion was 5.00 μg mL −1 When the MAPUGR–PT-SPE process was completed, the cartridge was cleaned with 2.0 mL of methanol and water, respectively, before the next cycle The recovery was still higher than 92% after 6 cycles, which indicates that MAPUGR has good reusability The entire process only uses water and methanol, which extends the life of the adsorbent and makes it easy to re- generate
3.7 Validation of MAPUGR–PT-SPE–HPLC method
The proposed method was validated by determining its linear- ity, limit of detection (LOD), limit of quantitation (LOQ), repeatabil- ity, accuracy and precision; the results are summarized in Table1
A good linearity curve was obtained in the range of 0.020 0/0.050 0 –
5.00 μg mL −1 for the four PHs (seven spiked levels of the analytes) with a correlation coefficient ( 2 ) ≥ 0.9998 The LOD and LOQ cal- culated for the signal-to-noise ratios (S/N) of 3 and 10 were 10.9, 14.4, 3.61, and 4.77 ng mL −1 , and 36.3, 48.1, 12.0, and 15.9 ng mL −1 for TDZ, CPPU, NAA, and BNOA, respectively The precision of the developed method was evaluated by repeatedly analyzing the sam- ple solution (5.00 μg mL −1 ) three times on the same day ( n= 3) and on three consecutive days The intra-day and inter-day pre- cisions are expressed as the relative standard deviations (RSDs) were in the range of 2.5–3.3% and 2.1–7.7%, respectively Finally, the recovery determined at three spiking levels (0.050 0, 0.50 0, and 5.00 μg mL −1 ) was 87.2–102.3% with RSDs of 1.6–7.2% ( Table2)
Trang 5Fig 4 Comparison of adsorbents (A) and optimization of MAPUGR–PT-SPE (B–F)
Table 1
Parameters of the MAPUGR–PT-SPE–HPLC method
Analytes Regression equation r2 Linearity ( μg mL −1 ) LOD (ng mL −1 ) LOQ (ng mL −1 ) RSD (%)
Intra-day Inter-day
Trang 6Fig 5 Comparison of MAPUGR with commercial adsorbents
Fig 6 Chromatograms of watermelon juice sample (A) and spiked sample (B)
3.8 Simultaneous determination of the four PHs in watermelon juice
MAPUGR −PT-SPE−HPLC was used to analyze the four PHs in
seven watermelon juice samples to verify the utility of the method
CPPU was detected in one of these samples at a concentration of
61.4 ng mL −1 , which is lower than the maximum residual limit
(100 ng mL −1 ) for watermelon set by China in 2016 The chro-
matograms of the watermelon juice sample and spiked sample
(5.00 μg mL −1 ) are shown in Fig.6 As indicated, no interference
peaks appeared near the retention times of the four PHs The above
results confirm that the MAPUGR–PT-SPE–HPLC method has good
isolation and purification effects and can be applied for the simul-
A comparison of the proposed method with other methods for detecting PHs in fruits and vegetables is shown in Table3 The LOD
of the developed method is comparable to or lower than that of SPE–GC and SPE–HPLC, where SPE requires at least 30 mg of ad- sorbent; thus, the consumption of adsorbents and organic reagents
in these methods is obviously large Compared with SPE–HPLC– MS/MS, the developed method has a higher LOD, but the expen- sive instrumentation of HPLC–MS limits its applicability in routine analysis Compared with other methods, PT-SPE as a sample pre- treatment technology has the advantages of requiring less adsor- bent and reagent consumption, while affording high recovery In addition, the developed MAPUGR–PT-SPE–HPLC method does not require special or expensive equipment, and thus can be used as
an effective strategy for the routine detection of PHs in fruits and vegetables
4 Conclusions
A green and economical MAPUGR–PT-SPE–HPLC process was developed for the extraction and determination of four PHs in wa- termelon juice MAPUGR, synthesized via simple one pot precip- itation polymerization, combines the advantages of MAPGR and UGR, increases the specific surface area, enriches surface functional groups and interaction, and changes the framework structure to provide more contact between the adsorbent and analytes The PT- SPE device is simple, cheap, and easy to obtain, and the adsorbent dosage is only 5.0 mg The recovery of four PHs at three spiking levels ranged from 87.2% to 102.3% with an RSD ≤ 7.2%, which
is consistent with the requirements for monitoring PHs in water- melon juice
Declaration of Competing Interest
The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper
The authors declare no competing financial interest
CRediT authorship contribution statement Yanke Lu: Methodology, Conceptualization Pengfei Li: Valida-
tion Chunliu Yang: Visualization, Project administration Yehong
Han: Writing review & editing Hongyuan Yan: Conceptualiza- tion, Methodology, Supervision
Acknowledgments
The authors are grateful to the Natural Science Foundationof Hebei Province ( B2018201270, H2019201288), the National Nat-ural Science Foundation of China ( 21575033), the Talent Engi- neering Training Foundation of HebeiProvince( A201802002), and
Trang 7the Post-graduate’s Innovation Fund Project of Hebei University ( hbu2019ss073)
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.chroma.2020.461214
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