Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction

Analytica Chimica Acta xxx (xxxx) xxx

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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction Shuangshou Wang a, b, *, Zhendong He a, Wenzhi Li a, Jiayi Zhao a, Tong Chen c, Shimin Shao a, Hongmei Chen d, ** a

School of Chemistry and Chemical Engineering, Anhui University of Technology, #328 Huolishan Avenue, Yushan District, Maanshan, Anhui, 243032, PR China Engineering Research Institute of AHUT, Anhui University of Technology, PR China c Comprehensive Technical Center, Zhenjiang Customs District PR China, Zhenjing, 212004, PR China d School of Mathematics and Physics of Science and Engineering, Anhui University of Technology, PR China b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Reshaped pipette tips were first used as substrates for the application of sorbent packing-free SPE.
Reshaped pipette tip-based SPE strategy technically renovated the structure design of existing pipette tip-based SPE devices.
Reshaped pipette tip-based SPE is of significantly higher extraction efficiency compared to once-shaping pipette tip-based SPE while keeping the advantages of miniaturized procedure, low sample consumption and practical convenience.
Nucleosides were successfully extracted from human urine sample via boronic acid-functionalized reshaped pipette tip.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 September 2019 Received in revised form 21 November 2019 Accepted 23 November 2019 Available online xxx

Pipette tip-based solid phase extraction (PT-SPE) has been proved to be an effective and user-friendly separation technique due to its miniaturized procedure and practical convenience. However, the vast majority of existing PT-SPE devices consist of a filter-sorbents-filter sandwich structure, which may suffer the unforeseen risk of sorbents leakage caused by the looseness of filters. More importantly, many highcapacity nanosorbents with particle size smaller than pore size of filters are unavailable. Thus, sorbent packing-free and sample low-consumption PT-SPE could be a more robust strategy for separation and detection, but such a possibility has not been explored yet. Herein we report a tubing reshaping strategy for facile fabrication of sorbent packing-free PT-SPE devices. Three types of reshaped PTs, namely stretched tube-like, self-crimping and filter in-built PTs, were fabricated via simple heating and stretching operations. The reshaped PTs exhibited flexible surface chemical post-modification. The SPE

Keywords: Solid phase extraction Pipette tip Reshaping

* Corresponding author. School of Chemistry and Chemical Engineering, Anhui University of Technology, #328 Huolishan Avenue, Yushan District, Maanshan, Anhui, 243032, PR China. ** Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (H. Chen). https://doi.org/10.1016/j.aca.2019.11.060 0003-2670/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: S. Wang et al., Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.060

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Miniaturized extraction Boronate affinity

process was directly performed in reshaped PTs with an obviously enhanced extraction efficiency compared to once-shaping PTs while no need of packing sorbents. Extraction of nucleosides from human urine by boronic acid-functionalized reshaped PTs was demonstrated. Our findings technically renovate the structural composition of PT-SPE devices. As PTs are inexpensive and high-plasticity, the sorbent packing-free SPE scheme presented herein could find more promising applications and provides a new perspective for design and fabrication of novel sorbent packing-free SPE devices. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Miniaturization of procedures and devices is a growing trend in the field of solid phase extraction (SPE) in the recent years due to the enhancing analysis requirements for low consumption of reagent, time and increased throughput. Pipette tip-based SPE (PTSPE) [1e3] has been proved to be an effective and user-friendly technique for the applications of miniaturized isolation, purification and enrichment via repeated aspiration and dispensing procedures of pipette controller. Due to its simple and rapid operations, low sample consumption and flexible sorbent encapsulation, a lot of attention have been attracted from researchers in different fields [4e7] and making PT-SPE a workhorse for miniaturized extraction. Most of the existing PT-SPE devices consist of a pipette tip and sorbents placed between two filters (like porous frit, degrease cotton and glass wool), one located at the lower end of tip allowing the solvent flowing through and the other one located at the upper end of tip avoiding contamination of pipette controller by solvent [8e10]. Such PT-SPE devices not only involve in complex fabrication steps but also have the risk of sorbents leakage caused by the looseness of filters, which is destructive to SPE process and may also give rise to contamination to pipette controller. More importantly, an inevitable issue for PT-SPE devices with such a filtersorbents-filter sandwich structure is that nanosorbents with particle size smaller than pore size of filters are unavailable, which leads to the exclusion of a great number of high-capacity nanomaterials and greatly limits the application scopes of PT-SPE. Although porous polymer materials-embedded monolithic PTs can be directly used for SPE without immobilizing filters and some promising applications such as drug analysis [11e13] and isolation of proteins and peptides [14,15] have been demonstrated, their liquid fluidity is relatively poor compared to hollow tube-like PTs, the extraction capacity is restricted by the limited packing of sorbents, and the risk of polymer detaching from the inner wall of PTs is also unforeseen. In addition, as PT-SPE is a relatively new technique, the limited types and quantities of sorbents commercially available and the high cost compared to the traditional SPE cartridges also limit its applicability in routine analysis [16]. In this context, sorbent packing-free PT-SPE technique is one of the possible pathways to end these issues. It is all known that strong plasticity is a universal hallmark of plastic products, and the reshaping is a facile, controllable and versatile method to produce secondary-molding plastics via simple post-processing steps. A variety of shapes and sizes of PTs can be obtained through reshaping procedure while keeping their inherent physicochemical properties. Comparing with commercially available once-shaping PTs, tailor-made reshaped PTs could be endowed with higher specific surface area and lower sample consumption, which is beneficial to enhance extraction efficiency. In addition, fine liquid fluidity of reshaped hollow PTs can be well maintained in a certain range of reshaped size, making it keep compatible with pipette controller. Obviously, the reshaping of PTs could be effective for

sorbent packing-free SPE. However, such a possibility has not been explored as yet. Herein, we report a sorbent packing-free SPE strategy via the reshaping of PTs. Elongated PTs with different shapes and sizes were reshaped as extraction substrates due to their high specific surface area, low sample consumption and fine liquid fluidity. Principle of sorbent packing-free SPE is illustrated in Fig. 1, PTs were firstly stretched into slender or self-crimping tubes with controllable sizes through heating and softening, then the stretched PTs were functionalized by different groups for sorbent packing-free SPE. What’s more, such an elongate tube-like structure was effective for improving extraction efficiency of PT-SPE. Several kinds of compounds, including cis-diol-containing compounds, organic small molecular dyes and aromatic ring compounds, were respectively extracted by boronic acid-, amino group- and octyl groupfunctionalized reshaped PTs with enhanced extraction efficiency compared to once-shaping PTs. The extraction of nucleosides from human urine sample was further demonstrated in this work. 2. Experimental 2.1. Materials and chemicals Triethoxy(octyl)silane, Rhodamine B, malachite green, amaranth red, alizarin red, methyl violet, methylene blue, noctylbenzene, inosine and guanosine were purchased from Macklin (Shanghai, China). Egg white albumin (OVA), 3aminopropyltriethoxysilane (APTES), bovine serum albumin (BSA) and horseradish peroxidase (HRP) were purchased from SigmaAldrich (St. Louis, MO, USA). Adenosine, uridine, cytidine, deoxyguanosine, 4-formylphenylboronic acid (FPBA), naphthalene and NaBH3CN were purchased from Aladdin Industrial (Shanghai, China). Papain, catechol and resorcinol were purchased from J&K Scientific (Beijing, China). HPLC-grade acetonitrile (ACN), anhydrous ethanol, methanol, tetrahydrofuran (THF), benzene, pxylene, cyclohexane, aniline, NaOH, NaCl, acetic acid (HAc),

Fig. 1. Schematic of sorbent packing-free solid phase extraction based on the reshaping of PTs.

Please cite this article as: S. Wang et al., Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.060

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ammonium hydroxide (28%), H3PO4, NaH2PO4 and Na3PO4 were purchased from Nanjing Reagent Company (Nanjing, China). All the chemicals were of analytical grade or higher. Poly-propylene pipette tips (PTs) with different specifications (10 mL, 200 mL, 1 mL and 5 mL) were from Biosharp Life Sciences (Hefei, China). Fused-silica capillaries of 320 mm inner diameter were purchased from Yongnian Optic Plant (Hebei, China). Water used in all experiments was purified by a Milli-Q system (Millipore, Milford, MA). Unless stated otherwise, phosphate buffer solution used in this work was especially referred to the 0.1 M, pH 8.5 phosphate buffer solution. 2.2. Apparatus Scanning electron microscopy (SEM) characterizations were performed on a Nova Nano SEM430 instrument (FEI, Hillsborough, U.S.A) equipped with energy dispersive X-ray spectroscopy (EDX). UV-absorption experiments were carried out on a UV-1800PC spectrophotometer (Mapada Instruments, Shanghai, China). Inner diameter of reshaped PTs was estimated on a XD-RFL optical microscopic system (Sunny Optical Technology, Yuyao, China) equipped with an objective lens (  5) and a M3LY230T Sony Exmor CMOS Sensor. All high performance liquid chromatography (HPLC) were performed on a LC-100 high pressure LC system (Wufeng Scientific Instruments, Shanghai, China) equipped with a variablewavelength UVevis absorbance detector with a 8 mL flow cell for post-column detection, a P100 binary high pressure pump system which provides a constant flow rate, a 7725i manual injection valve, a CO100 air circulation heating column oven and a WS100 chromatographic work station for system operation, data acquisition and processing. Mass spectrum experiments were implemented on a TSQ Quantum Access mass spectrometry (Thermo Scientific, MA, U.S.A) equipped with a TSQ Series 2.0.5 mass data processing software. Analytes were ionized by electrospray ionization (ESI) and all mass spectra reports were obtained in the positive ion mode.

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stretched 200 mL PT with stretched length of 60 ± 0.5 cm to fill the stretched part (the section between two tangent points of both ends of reshaped tube-like PT). Then the stretched volume was measured via weighing the water filled in stretched section. The statistic histogram of stretched volume was plotted by the software OriginPro (OriginLab Corporation, Northampton, MA). 2.5. Amino group, boronic acid and octyl group modification of PTs The amino group modification was carried out as follows: the once-shaping and freshly reshaped tube-like PTs were respectively washed with 3 mL water and ethanol, followed by drying in a vacuum drying oven at 60  C. Then the PTs were filled with a 10% (v/ v) aqueous APTES solution (pH 7.0) and kept airtight in an oven at 60  C for 4 h. Thereafter, the residual reagents in the PTs were removed and the resulted PTs were kept in the oven again at 60  C for 12 h. Finally, the obtained PTs were respectively rinsed by 3 mL ethanol and water, then stored at 4  C for later experiments. Boronic acid functionalization of PTs was performed according to our previously reported methods [17,18] with slightly modifications, which mainly contained two steps, namely derivation of amino group and graft of boronic acid. The derivation of amino group was just the same as mentioned above. The graft of boronic acid was implemented as below: the amino group-modified PTs were washed with 3 mL anhydrous ethanol and then filled with ethanol solution containing 2 mg/mL FPBA and 2 mg/mL NaBH3CN, followed by reacting in an oven for 10 h at 40  C. Subsequently, the boronic acid-functionalized PTs were respectively rinsed with 5 mL ethanol and phosphate buffer solution, then stored at 4  C for later use. For the decoration of octyl group, the PTs were respectively washed with 3 mL water and ethanol, then dried in a vacuum drying oven at 60  C. Thereafter, a THF solution containing 10% (v/v) triethoxy(octyl)silane was pipetted into PTs and airtightly reacted in a water bath for 10 h at 60  C. The resulted octyl group-derivated PTs were respectively rinsed with 3 mL THF and ethanol, then kept sealed at 4  C for further experiments.

2.3. Reshaping of PTs The reshaping of PTs was facile and straightforward, which mainly contained three types of reshaped PTs: the stretched tubelike PTs, filter in-built PTs and the self-crimping PTs. For preparation of stretched tube-like PTs, a heating apparatus (lighter, alcohol lamp, flame gun, etc.) was firstly used to heat and soften the PTs, then the softened PTs were stretched from both ends of tip to an appropriate length and the finished products were obtained after cooling to room temperature. The reshaping procedures of filter in-built PTs were the same as the stretched tube-like PTs excepted that a filter should be preinserted into the PTs prior to heating. This type of reshaped PTs enabled it to directly aspirate and dispense turbid samples by pipette controller, e.g., muddy water, dyes wastewater, milk, raw fruit juice, and so on. To fabricate the self-crimping PTs, a simple circular cylinder, like penholder and centrifuge tube, was applied to regularly crimp the softened PTs, then the finished products were formed through peeling off the crimped PTs after cooling to room temperature. 2.4. Statistics of stretched volume Reshaped tube-like PTs with 1 mL and 200 mL specifications were used for the statistics. Stretched volume was measured via weighing water filled in stretched part of reshaped PTs. Briefly, water with an appropriate volume was respectively pipetted into 50 stretched 1 mL PT with stretched length of 45 ± 0.5 cm and 35

2.6. Extraction of cis-diol compounds by boronic acidfunctionalized PTs Boronate affinity effect [19e23], a reversible and covalent binding between boronic acid and cis-diol compounds, was adopted to evaluate the recognition class-selectivity of boronic acidfunctionalized PTs. 4 cis-Diol compounds (catechol, adenosine, HRP and OVA) and 4 non-cis-diol compounds (resorcinol, deoxyguanosine, BSA and papain) were used as samples for this confirmatory trial. Briefly, boronic acid-functionalized stretched tubelike 1 mL PT with stretched length of 45 ± 0.5 cm were preconditioned with phosphate buffer solution for 30 min. After removing the buffer solution, the PTs were pipetted with 70 mL sample solution (dissolved in phosphate buffer solution with a final concentration of 1 mg/mL) and incubated for 30 min at room temperature. Then all the PTs were washed with 3 mL phosphate buffer solution, followed by a 70 mL aliquot of HAc solution (0.1 M) was pipetted into PTs and incubated for another 30 min at room temperature. Finally, HAc eluent of each PT was individually collected. The absorbance of solutions at different wavelength was measured on UVevis absorption spectrometer. In this work, the maximum absorption wavelength (lmax) was set at 275 nm, 280 nm, 256 nm, 260 nm, 280 nm, 214 nm, 275 nm and 278 nm for absorbance test of catechol, resorcinol, adenosine, deoxyguanosine, HRP, BSA, OVA and papain, respectively. As controls, the binding assays between cis-diol compounds and boronic acidfunctionalized once-shaping PTs as well as amino group-modified

Please cite this article as: S. Wang et al., Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.060

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stretched tube-like TPs with the same stretched length were simultaneously investigated. The procedures were the same as mentioned above excepted that the volume of sample solutions and eluents was 1 mL for once-shaping PTs. This experiment was repeated 4 times in parallel and the extraction result of each sample was averaged. Standard curve method was applied to measure the extraction amount (Qe) in this work. To explore the dependence of the binding between cis-diol compounds and boronic acid-functionalized PTs on pH, a series of adenosine standard solutions (dissolved in phosphate solution and each with a concentration of 1 mg/mL) with different pH (pH 4.0, 5.5, 7.0, 8.5, 10.0 and 11.5) were adopted as samples. 18 Boronic acid-functionalized reshaped tube-like 1 mL PT with a stretched length of 45 ± 0.5 cm were equally divided into 6 groups (numbered 1e6) and pre-conditioned with phosphate buffer solution for 30 min, then the sample solutions were respectively pipetted into PTs grouped 1e6 (70 mL for each PT) in pH ascending order and incubated for 30 min at room temperature. After that, each PT was washed with 3 mL phosphate buffer solution, followed by filling with 70 mL HAc solution (0.1 M) and incubating for another 30 min at room temperature. Subsequently, the HAc eluent was separately collected and the absorbance of each solution was measured by UVevis absorption spectrometer at lmax, and the final results were averaged over 3 PTs in each group. For the testing of binding dynamics, all procedures were the same as mentioned above excepted that adenosine sample solutions were prepared using phosphate buffer solution as a solvent and the incubation period of samples was respectively set at 1, 3, 5, 10, 20, 30 and 40 min. This experiment was repeated 4 times in parallel and the final results were averaged. 2.7. Extraction of organic small molecular dyes by amino groupmodified PTs 6 Dyes, including alizarin red, amaranth red, rhodamine B, methyl violet, methylene blue, and malachite green, were adopted as samples for this assay. Briefly, amino group-modified onceshaping PTs and stretched tube-like PTs with stretched length of 60 ± 0.5 cm were respectively rinsed with 3 mL ethanol and water, then the 6 aqueous sample solutions (each with a concentration of 1 mg/mL) were respectively pipetted into once-shaping (1 mL) and reshaped (100 mL) PTs, followed by incubating for 2 h at room temperature. Thereafter, all the dye-bonded PTs were washed with 5 mL water. After that, a 10% ACN aqueous solution containing 5 M NaCl was supplemented into PTs with an equal volume to sample solution and kept for 2 h at room temperature. Subsequently, the eluent was individually collected and the absorbance of each solution was measured on UVevis absorption spectrometer. The lmax was respectively set at 260 nm, 300 nm, 600 nm, 553 nm, 520 nm and 665 nm for absorbance test of alizarin red, methyl violet, malachite green, rhodamine B, amaranth red and methylene blue. This experiment was repeated 4 times in parallel and the final absorbance of each sample was averaged. As controls, the binding between dyes and unmodified once-shaping PTs and stretched PTs with the same stretched length were also surveyed. All conditions were the same as mentioned above excepted that the amino groupmodified PTs were substituted with unmodified PTs. 2.8. Extraction of aromatic ring compounds by octyl groupderivated PTs 7 Aromatic ring compounds, containing benzene, catechol, aniline, n-butylbenzene, p-xylene, n-octylbenzene and naphthalene, were serviced as samples for this trial. Octyl group-derivated onceshaping and stretched tube-like (with a stretched length of

60 ± 0.5 cm) 1 mL PT were washed with 3 mL ethanol. Then the sample solution (dissolved in ethanol with a concentration of 1 mg/ mL) was respectively pipetted into once-shaping PTs (1 mL) and stretched tube-like PTs (100 mL) and incubated for 1 h at room temperature. After that, all the PTs were washed with 3 mL ethanol. Then equivalent volume of ACN was supplemented for each PT and kept airtight for another 1 h at room temperature. Finally, the eluent was individually collected and stored at 4  C for later use. The absorbance was measured on UVevis absorption spectrometer at lmax of 254 nm, 237 nm, 264 nm, 274 nm, 274 nm, 260 nm and 260 nm for benzene, aniline, p-xylene, catechol, naphthalene, nbutylbenzene and n-octylbenzene, respectively. The binding between aromatic ring compounds and unmodified stretched tubelike PTs as well as once-shaping PTs were performed as controls. All the procedures were the same as mentioned above excepted that octyl group-derivated PTs were replaced by unmodified PTs. This experiment was repeated 4 times in parallel and the final absorbance of each sample was averaged. 2.9. Extraction efficiency comparison between reshaped tube-like PTs and once-shaping PTs via boronate affinity effect 4 cis-Diol compounds, including adenosine, catechol, HRP and OVA, each with a concentration of 1 mg/mL (dissolved in phosphate buffer solution), were used as samples for this experiment. The stretched length of reshaped tube-like PTs used herein was 45 ± 0.5 cm. Briefly, boronic acid-functionalized stretched tube-like 1 mL PT and once-shaping 1 mL PT were pre-conditioned with phosphate buffer solution for 30 min. Then the sample solutions were respectively pipetted into once-shaping PTs (1 mL) and reshaped PTs (100 mL), then incubated for 30 min at room temperature. After incubation finished, the PTs were rinsed with 3 mL phosphate buffer solution. Thereafter, a HAc solution (0.1 M) with equivalent volume to sample solution was supplemented to each PT and incubated for another 30 min. Finally, the HAc eluent was collected separately. The absorbance was measured on UVevis absorption spectrometer at lmax. This experiment was repeated 4 times in parallel and the final results were averaged. 2.10. The dependence of extraction efficiency on stretched length Boronic acid-functionalized stretched tube-like 1 mL PT with different stretched length were used for this assay. Adenosine, a cisdiol compound, was served as a sample (dissolved in phosphate buffer solution with a concentration of 1 mg/mL). Briefly, boronic acid functionalized PTs with stretched length ranged from 45 to 195 cm (15 cm as an interval and the deviation is within 0.5 cm) were pre-conditioned with phosphate buffer solution for 30 min, then the sample solution was pipetted into PTs and incubated for 30 min at room temperature. After then, each adenosine-bounded PT was washed with 3 mL phosphate buffer solution, followed by supplementing with 70 mL HAc solution (0.1 M) and incubating for another 30 min at room temperature. Subsequently, the HAc desorption solutions were collected and the absorbance of each solution was measured on UVevis absorption spectrometer at lmax of 256 nm. This experiment was repeated 4 times in parallel and the final results were averaged. 2.11. Binding isotherms Binding isotherms were applied to evaluate the extraction performance of commercially available silica capillary tubes, plastic tubes and reshaped tube-like PTs via boronate affinity effect using adenosine as a cis-diol containing model compound. All tubes have a uniform length of 60 ± 0.5 cm and a roughly equivalent ID

Please cite this article as: S. Wang et al., Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.060

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(0.32 mm for silica capillary tube, 0.3 mm for plastic tubes and PTs). A series of adenosine solutions with known concentrations (0.00001, 0.00005, 0.0001, 0.0005, 0.001, 0.0025, 0.005, 0.0075, 0.01, 0.025, 0.04 mg/mL) were prepared with 0.1 M HAc aqueous solution and their absorbance at 256 nm was measured. Standard curve was plotted by virtue of the relationship between absorbance and concentration. Adenosine stock solutions with different concentrations (0.00005, 0.0001, 0.0005, 0.001, 0.0025, 0.005, 0.0075, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75 mg/mL, dissolved in PB) were prepared, then boronic acid-functionalized tubes were respectively filled with these adenosine solutions. After incubation for 1 h at room temperature, solutions in tubes were removed and the resulted tubes were washed with PB for 3 times, and then an equivalent volume of 0.1 M HAc was supplemented, followed by incubating at room temperature for another 1 h. The following describtion was overlooked in this manuscript and it should be supplemented here: This assay was repeated 3 times in parallel and the final results were averaged. Thereafter, the eluent was separately collected and their absorbance at 256 nm was tested. The adenosine amounts captured by the boronic acid-functionalized tubes were plotted against the concentration of the adenosine solutions. The Hill equation as shown below was applied to fit the data for estimation of the maximum specific binding capacity (Qmax) of boronic acid-functionalized tubes toward adenosine. y ¼ Qmax
xn/(xn þ Knd) Herein, n is the Hill slope, Kd is the binding constant. 2.12. Evaluation of extraction reliability Boronic acid-functionalized reshaped tube-like 1 mL PT with 45 ± 0.5 cm stretched length were applied for this trial. In order to comprehensively reveal the feasibility and reliability of this method, in-batch repeatability, batch-to-batch repeatability and reusability of reshaped PTs were tested. In addition, the influence of organic solvents on the shape of reshaped PTs was also investigated. For the in-batch repeatability test, 2 cis-diol compounds (catechol and adenosine) and 2 non-cis-diol compounds (resorcinol and deoxyguanosine) were used as samples (dissolved in phosphate buffer solution with a concentration of 1 mg/mL). Briefly, 16 inbatch fabricated reshaped PTs were pre-conditioned with phosphate buffer solution for 30 min. Then the PTs were equally grouped and were respectively pipetted with different sample solutions (70 mL for each PT). After incubation for 30 min at room temperature, all the PTs were rinsed with 3 mL phosphate buffer solution. Thereafter, a 70 mL aliquot of HAc solution (0.1 M) was pipetted into the PTs and incubated for another 30 min. Finally, the HAc eluent was separately collected and the absorbance of each solution was measured on UVevis absorption spectrometer at lmax. The final absorbance was averaged over 4 PTs in each group. To investigate the batch-to-batch repeatability, a cis-diol compound (catechol) and a non-cis-diol compound (resorcinol) were used as samples (dissolved in phosphate buffer solution with a concentration of 1 mg/mL) for this assay. 40 Reshaped PTs equally fabricated in 5 batches were pre-conditioned with phosphate buffer solution for 30 min, then one half PTs in each batch were filled with catechol sample solution and the other half PTs were filled with resorcinol solution (70 mL for each), followed by incubating for 30 min at room temperature. After washing with 3 mL phosphate buffer solution, a 70 mL aliquot of HAc solution (0.1 M) was pipetted into each PT and incubated for another 30 min at room temperature. Thereafter, the HAc desorption solution was separately collected and the absorbance of each solution was tested by

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UVevis absorption spectrometer at lmax. For reusability measuring, adenosine was serviced as a sample and 4 PTs fabricated in one batch were repeatedly used for this trail. All the other procedures were the same as in-batch repeatability test. This experiment was consecutively repeated for 10 times, the obtained results were averaged and normalized over 4 PTs. For the investigation of influence of organic solvent on the shape of reshaped PTs, freshly stretched tube-like PTs were filled with different organic solvents, including p-xylene, methanol, acetonitrile and cyclohexane, and then incubated for 3 days at room temperature after recording their microphotos using optical microscope (as a control). After that, organic solvents were removed when the incubation finished, the obtained PTs were photographed again and compared with their initial states. 2.13. Extraction of nucleosides from human urine by boronic acidfunctionalized reshaped PTs The extraction of nucleosides from human urine sample involved in two steps, namely urine pretreatment and extraction processes. Pretreatment is necessary for extraction of human urine due to its large amplitude of pH fluctuation which may affect the extraction efficiency of boronic acid-functionalized reshaped PTs. Briefly, the pH of freshly collected human urine sample was adjusted to 8.5 using ammonium water and HAc, and then frozen immediately at 20  C. Prior to use, the urine sample was thawed at room temperature. The extraction of nucleosides in urine sample was performed as follows: boronic acid-functionalized reshaped tube-like 1 mL PT with a stretched length of 45 ± 0.5 cm was pre-conditioned with phosphate buffer solution for 30 min, then 70 mL pretreated urine sample solution was pipetted into the PT and incubated for 30 min at room temperature. After washing with 3 mL ammonium acetate solution (0.1 M, pH 8.5), 70 mL HAc solution (0.1 M) was supplemented and incubated for another 30 min at room temperature. After that, the HAc eluent was collected in a centrifuge tube and stored at 20  C for later analysis. This experiment was repeated 3 times in parallel. 2.14. High performance liquid chromatography (HPLC) analysis The extraction performance of the boronic acid-functionalized reshaped PTs toward urine sample was demonstrated by HPLC. The mobile phase was composed by acetonitrile (A) and water (B) with a gradient elution program as follows: 0e14 min, isocratic 2% A; 14e22 min, linear gradient 2e50% A; 22e30 min, isocratic 50% A. A pre-equilibration period of 40 min was used between two consecutive runs. A Pronaos EP-C18 column (Exformma Technologies, 5 mm, 250 mm  4.6 mm) was used for chromatographic separations. The injection volume was 20 mL, the flow rate was set at 0.6 mL/min for qualitative analysis (chromatographic identification of extractives) and 1.0 mL/min for quantitative analysis (standard curve and recovery rate tests), the detection wavelength was set at 260 nm and the column temperature was set at 25  C in all chromatographic experiments. The standard stock solutions were prepared in water and the concentration was 0.1 mg/mL for all analytes. All the solutions were filtrated with 0.22 mm filter membrane and stored at 4  C before analysis. Standard curve method was used to survey the recoveries of adenosine in human urine by HPLC. Briefly, a series of adenosine solutions (dissolved in 0.1 M HAc) with different concentrations (1.0  105, 2.5  105, 5.0  105, 7.5  105, 1.0  104, 2.5  104, 5.0  104, 7.5  104, 1.0  103, 2.5  103, 5.0  103, 7.5  103, 0.01, 0.025, 0.05, 0.075 and 0.1 mg/mL) were prepared and their

Please cite this article as: S. Wang et al., Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.060

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chromatographic retention was investigated. Standard curve was plotted in terms of the function between peak area and concentration. After extraction with boronic acid-functionalized reshaped PTs (0.3 mm ID and 60 ± 0.5 cm in length), the concentration of adenosine in human urine sample spiked or not could be determinated via HPLC standard curve, and the recoveries could be deduced out according to the concentration difference of adenosine in urine samples and adenosine spiked (2.5  103 and 3.5  103 mg/mL) urine samples. 2.15. Mass spectrometric conditions Mass spectrum experiments were carried out on a triple quadrupole mass spectrometer within the mass range of m/z 50e600 in a positive ion mode. The following operation parameters of MS/MS were adopted: gas flow rate 12 L/min, nebulizer gas pressure 35 psi, the capillary temperature 370  C and the fragmentor voltage 4000 V. 3. Results and discussion 3.1. Reshaping and characterization of PTs In this work, elongate tube-like PTs, self-crimping PTs and filter in-built elongate tube-like PTs were fabricated via facile reshaping postprocessing. The fabrication procedures are respectively illustrated in Fig. 2, S1 and S2. The reshaping procedures are basically the same and mainly involves in several post-processing steps, including heating, softening, manual stretching and shaping. What’s different is that crimping and filter pre-insert steps are additionally needed for the fabrication of self-crimping PTs and filter in-built reshaped PTs. The self-crimping PTs are easy to carry while the filter in-built PTs allow for direct extraction of turbid samples (Fig. S3), like muddy water, dye wastewater, orange juice with granules and raw milk, etc., which may cause blockage in traditional sorbents packed SPE devices. Scanning electron microscope (SEM) characterization suggests that both the stretched tubelike and the self-crimping PTs possessed hollow structure (Fig. 3), the former appeared regular circle shape and the latter displayed oval shape, which ensured good liquid fluidity. The maximum length of single manual stretching varied with specifications, and respectively up to 15.65 m, 9.51 m and 5.63 m for 5 mL-, 1 mL- and 200 mL-specification PT (Fig. S4), which is almost 1000 times the initial length. These results indicate that high plasticity enables PTs to be easily reshaped in tailor-made shapes and sizes, which provides the possibility that SPE could be directly carried out in

Fig. 3. SEM characterization of reshaped tube-like (a, b) and self-crimping (c, d) 1 mL PT under different magnifications ((a and c) 400  ; (b and d) 3000  ). The images inserted in (a) and (c) are the real photos of reshaped tube-like PT and selfcrimping PT.

reshaped PTs without the packing of sorbents. After being stretched to a certain length, the stretched volume of different reshaped PTs with the same specification exhibited a normal distribution (Fig. 4A and B and Table S1), the volumes mainly ranged from 60 to 80 mL and 20e26 mL for 1 mL PT with a stretched length of 45 ± 0.5 cm and 200 mL PT with a stretched length of 60 ± 0.5 cm, which respectively occupied 62.0% and 71.4% of the total reshaped PTs, which indicates the acceptable manual reshaping process. The inner diameter (ID) of 1 mL PT was decreased from ~600 mm to ~150 mm and their outer diameter (OD) was decreased from ~1000 mm to ~350 mm when the stretched length increased from 30 cm to 225 cm (Fig. 4C and S5). The response curve of both ID and OD as stretched length was nonlinear as a whole, while an intuitive linear decrease could be observed when the stretched length varied from 75 cm to 180 cm. In addition, the stretched tube-like PTs showed a good ID uniformity (RSD  7.8%, Fig. S6). These results suggest that the manual reshaping of PTs is of fine controllability. The time required for liquid aspirated and dispensed by pipette controller was important in practical applications. As shown in Fig. 4D, when the stretched length is relatively short (within 75 cm, 150 cm and 210 cm for 200 mL, 1 mL and 5 mL PT, respectively), the time required for water pipetted into reshaped PTs was basically less than 1 min, but it almost exponentially increased for all specifications of PTs as the stretched length further increased. However, the larger the specification is, the later the inflection point of the growth trend appears, indicating better liquid fluidity of PTs with larger specification at the same stretched length. Thus, these results provide a guide for selection of optimized stretched length of reshaped PTs with different specifications in practical application, which should be considered in combination with sample consumption and extraction efficiency.

3.2. Post-modification and solid phase extraction

Fig. 2. The representative reshaping processes of reshaped tube-like 1 mL PT.

Surface post-modification of PTs was then performed after being reshaped. Boronic acid, has been proved to be a robust moiety for class-selective recognition of cis-diol compounds by virtue of

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Fig. 4. ((A) and (B)) The statistic histogram of effective extraction volume of 50 reshaped tube-like 1 mL PT with a stretched length of 45 ± 0.5 cm (A) and 35 reshaped tube-like 200 mL PT with a stretched length of 60 ± 0.5 cm (B); (C) The inner diameter (pink square) and outer diameter (blue circle) of reshaped tube-like 1 mL PT as a function of stretched length; (D) The relationship between stretched length and time required for water pipetted into the stretched part of reshaped tube-like PTs. Blue, pink and black was respectively on behalf of 200 mL, 1 mL and 5 mL PT. Error bar was produced by 5 reshaped PTs in (C) and (D). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

boronate affinity effect,1923 was firstly applied to modify PTs via the hydrolysis and condensation polymerization of APTES and Schiff base reaction between amino group of APTES and formyl group of FPBA. As shown in Fig. S7, the atomic percentage of B element was increased to 41.56% from 18.85% characterized by EDX spectrum, indicating boronic acid functionalization was successful. Besides, boronate affinity was used to evaluate the boronic acid functionalization. The boronic acid-functionalized PTs exhibited apparent boronate affinity toward cis-diol compounds (catechol, adenosine, HRP and OVA) at weak alkaline conditions with fast binding dynamics (Fig. S8 and S9), suggesting the PTs were successfully modified with FPBA. To verify the versatility of reshaped PTs, amino group- and octyl group-modified PTs were also fabricated via straightforward hydrolysis and polycondensation of APTES and triethoxy(octyl)silane. Due to less interference in detecting Si element of silanized PTs, the element content change of Si rather than C or N was analyzed by EDX spectrum. As seen in Fig. S10 and S11, obviously increased content of Si element demonstrated that silanization of reshaped PTs with APTES and triethoxy(octyl)silane was successful. Moreover, amino group- and octyl group-modification were further verified by electrostatic interaction between amino group and acid radical ion and hydrophobic effect between octyl group and alkyl chain or aromatic ring. As shown in Fig. S12, the amino groupdecorated PTs showed distinct adsorption toward acidic (alizarin red, amaranth red) and amphoteric (Rhodamine B) dyes while almost no adsorption to all alkaline dyes (methyl violet, methylene blue and malachite green), and the octyl group-derivated PTs displayed obvious retention to all aromatic ring compounds using ethanol as a solvent and acetonitrile as an eluent. As controls, nonfunctionalized reshaped and once-shaping PTs exhibited poor

extraction in these cases. These results confirm that amino groupand octyl group-modification were successful. The extraction amount (Qe) of reshaped and once-shaping PTs with the same surface post-modification was compared. As shown in Fig. 5 and Tables S2-S4, the Qe value of boronic acidfunctionalized reshaped PTs toward cis-diol compounds is comparable to that of boronic acid-functionalized once-shaping PTs just when the effective extraction length (Le) is 45 cm, but the sample consumption of the former is only less than one-fourteenth that of the latter (Fig. 5A and Table S2). It means that the extraction efficiency of reshaped PTs is 14 times higher than that of once-shaping PTs when Le is just 45 cm. The relationship between Qe and Le of boronic acid-functionalized reshaped PTs was illustrated in Fig. 5B using adenosine as a cis-diol model molecule. As a whole, Qe was non-linearly increased as increasing Le, the Qe value of boronic acidfunctionalized reshaped PTs almost exceeds that of once-shaping PTs when Le is just 45 cm and the multiple of Qe was septuple higher when Le was lengthened to 180 cm. Considering that the sample consumption of reshaped PTs is less than one-fourteenth of the once-shaping PTs, the actual extraction efficiency of reshaped PTs is nearly 100 times higher than that of once-shaping PTs. There are two linear regions (30e90 cm and 135e180 cm) in the Qe-Le response curve, which was possibly attributed to the fact that the ID of reshaped PTs appears an intermittent decrease as increasing the stretched length (Fig. 4C). Due to the dramatically increased extraction time (once extraction required more than 10 min), the enhancement index of reshaped PTs with a Le larger than 210 cm is not explored. In spite of this, these results fully demonstrate the claim that the reshaping of PTs is effective for sorbent packing-free SPE. To enable the enhancement of extraction efficiency more

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Fig. 5. (A) Comparison of extraction amount (Qe) between boronic acid-functionalized reshaped tube-like (wathet-blue) and once-shaping (pink) 1 mL PT; (B) The relationship between Qe and effective extraction length (Le) using adenosine as a cis-diol model molecule extracted by boronic acid-functionalized PTs, in which black arrow represented the Qe of once-shaping PTs; ((C) and (D)) Comparison of Qe between reshaped tube-like (wathet-blue) and once-shaping (pink) 1 mL PT modified by amino group (C) and octyl group (D) using dyes and aromatic ring compounds as analytes, respectively. The effective extraction length was respectively 45 ± 0.5 cm, 60 ± 0.5 cm and 60 ± 0.5 cm for (A), (C) and (D). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

convincing, the Qe between reshaped and once-shaping PTs, both modified with amino group or octyl group, was compared using acidic/amphoteric dyes and aromatic ring compounds as analytes. As seen in Fig. 5C and D, Tables S3 and S4, the Qe value of both amino group- and octyl group-modified reshaped PTs is roughly equivalent to that of once-shaping PTs when Le is just 60 cm, while the reshaped PTs kept the sample consumption is only one-tenth of once-shaping PTs, indicating at least 10 times higher extraction efficiency can be obtained by reshaped PTs compared with onceshaping PTs. These results furtherly confirm the feasibility of sorbent packing-free SPE via the reshaping of PTs. 3.3. Evaluation of extraction performance The extraction performance of reshaped tube-like PTs was furtherly assessed via boronate affinity effect and comparison with commercially available plastic tubes and silica capillary tubes with the same length and roughly equivalent ID (0.32 mm for capillary tubes and 0.30 mm for both reshaped PTs and plastic tubes). As shown in Fig. S13, all of these boronic acid-functionalized tubes display a comparable maximum extraction capacity (Qmax) to adenosine and their binding isotherms show good linearity in the range of 1  105-0.05 mg/mL (R2  0.878). The limit of detection (LOD) respectively is 8.1  108, 1.2  107 and 9.7  108 g/mL (S/ N ¼ 3) for capillary tube, plastic tube and reshaped PT, which covers the concentration of adenosine in human serum [24,25] and thus provides a theoretical foundation for extraction and detection of adenosine in human urine. Although reshaped PTs shown quite similar extraction performance with capillary tube and plastic tube, the reshaped PTs exclusively kept fine compatibility with pipette

controller, which greatly simplified the extraction operations, making extraction procedures simpler and faster by virtue of facile aspiration and dispensing procedures. 3.4. Extraction reliability of reshaped PTs The extraction reliability of reshaped PTs was examined at different levels via boronate affinity effect. As shown in Fig. S14, all the boronic acid-functionalized reshaped PTs fabricated in the same batch exhibited fine recognition selectivity toward cis-diol compounds (Fig. S14A), and the boronate affinity of boronic acidfunctionalized reshaped PTs fabricated in different batches could be well repeated (Fig. S14B), giving a batch-to-batch RSD4.8% (n ¼ 5). In addition, the Qe decreased less than 20% after 10 repetitions of extraction by boronic acid-functionalized reshaped PTs (Fig. S14C), suggesting excellent reusability of the reshaped postmodification PTs. Clearly, these results indicate that the reshaped PTs have a good extraction repeatability. Furthermore, the influence of organic solvents on the shape of reshaped PTs was investigated. As seen in Fig. S15, no obvious changes in shape could be observed even if the PTs incubated with different organic solvents for 3 days, indicating that the reshaped PTs possessed prominent tolerance to organic solvents. 3.5. Analysis of real sample The feasibility of nucleosides extraction by boronic acidfunctionalized reshaped PTs for real sample application was investigated using human urine as a representative sample. As shown in Fig. 6, the extractive exhibits overlapping absorption

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Fig. 6. (A) UVevis absorption spectrum of (a) 160-fold diluted urine and (b) extractive from urine by boronic acid-functionalized reshaped PT; (B) UVevis absorption spectrum of uridine, guanosine, inosine, cytidine and adenosine standard solutions (dissolved in 10% HAc aqueous solution with a concentration of 0.008 mg/mL); (C) Chromatographic retention of (a) urine, (b) flow through of urine through a boronic acid-functionalized reshaped PT, ((c) to (g)) the 0.1 mg/mL standard solutions of inosine, guanosine, uridine, cytidine and adenosine, and (h) urine extractive obtained by boronic acid-functionalized PT; (D) The electrospray ionization mass spectrometry characterization of urine sample (black) and the urine extractives generated by boronic acid-functionalized reshaped PT. Peaks identity: 1) cytidine and uridine; 2) inosine and adenosine; 3) guanosine and/or xanthosine; other peaks, unknown.

bands with urine and nucleosides standard samples on UVevis absorption spectrum (Fig. 6A and B), which suggests that nucleoside(s) might be extracted from urine sample. High performance liquid chromatography (HPLC) analysis confirmed that the extractive contained at least 4 nucleosides (inosine, uridine, cytidine and adenosine) and some other substances unknown (Fig. 6C). These conclusions were further demonstrated by electrospray ionization mass spectrometry (Fig. 6D), in which peaks 1e3 respectively indicates the existence of cytidine and uridine, inosine and adenosine, guanosine and/or xanthosine. These results were consistent well with literature reports [26e29]. Therefore, the extraction for complex real sample by reshaped post-modification PTs was experimentally confirmed feasible. The recoveries of adenosine in the real samples were deduced according to two spiked standard amounts. As shown in Fig. S16 and Table S5, the tested concentration of adenosine in human urine is 0.787 mg/mL (z2.94 mM), which is highly accord with previously reported results [24,25,30]. The recoveries of adenosine ranged from 97.32% to 105.23% with the RSD lower than 6.51%, indicating that the reshaped PTs could satisfy the high requirements in precision and reliability for determination of adenosine in complex real bio-samples like human urine.

analysis requirements. More importantly, such reshaped PTs allowed SPE processes to be accomplished without packing of sorbents, and the extraction efficiency could be easily enhanced over two orders of magnitude compared to that of once-shaping PTs-based SPE, making it an ideal candidate for miniaturized sorbent packing-free SPE. As PTs are inexpensive and high-plasticity, the sorbent packing-free SPE strategy proposed herein could find more promising applications, such as biochemical separation and medical diagnosis. To the best of our knowledge, such a reshaping scheme for sorbent packing-free SPE has not been presented in literature so far. Thus, this study paved a solid ground for the design and fabrication of novel sorbent packing-free SPE devices.

4. Conclusions

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

We have presented a sorbent packing-free SPE strategy based on the reshaping and facile post-modification of PTs and demonstrated its feasibility for extraction of complex real sample, e.g., human urine. The reshaping procedure of PTs was simple and straightforward with tailor-made shapes and sizes in terms of

Author contributions Shuangshou Wang designed the experimental scheme and wrote the paper. Zhendong He, Wenzhi Li, Jiayi Zhao, Tong Chen, Shimin Shao and Shuangshou Wang participated in the experiments. Shuangshou Wang and Hongmei Chen discussed the results and approved the final version of the manuscript. Declaration of competing interest

Acknowledgements This work was supported by the National Natural Science

Please cite this article as: S. Wang et al., Reshaping of pipette tip: A facile and practical strategy for sorbent packing-free solid phase extraction, Analytica Chimica Acta, https://doi.org/10.1016/j.aca.2019.11.060

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Foundation of China (Grant No. 21904003), the Natural Science Foundation of Anhui Province (Grant No. 1908085QB66) and the Open Grant from the State Key Laboratory of Analytical Chemistry for Life Science (Grant No. SKLACLS1801). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.aca.2019.11.060. References [1] Z. Yan, B. Hu, Q. Li, S. Zhang, J. Pang, C. Wu, Facile synthesis of covalent organic framework incorporated electrospun nanofiber and application to pipette tip solid phase extraction of sulfonamides in meat samples, J. Chromatogr. A 1584 (2019) 33e41. [2] L. Wang, H. Yan, C. Yang, Z. Li, F. Qiao, Synthesis of mimic molecularly imprinted ordered mesoporous silica adsorbent by thermally reversible semicovalent approach for pipette-tip solid-phase extraction-liquid chromatography fluorescence determination of estradiol in milk, J. Chromatogr. A 1456 (2016) 58e67. [3] M. Kaykhaii, H. Yahyavi, M. Hashemi, M.R. Khoshroo, A simple graphenebased pipette tip solid-phase extraction of malondialdehyde from human plasma and its determination by spectrofluorometry, Anal. Bioanal. Chem. 408 (2016) 4907e4915. [4] S. Oh, S.H. Jung, H. Seo, M.-K. Min, B. Kim, Y.K. Hahn, J.H. Kang, S. Choi, Magnetic activated cell sorting (MACS) pipette tip for immunomagnetic bacteria separation, Sens. Actuators B Chem. 272 (2018) 324e330. [5] Q. Shen, L. Gong, J.T. Baibado, W. Dong, Y. Wang, Z. Dai, H.-Y. Cheung, Graphene based pipette tip solid phase extraction of marine toxins in shellfish muscle followed by UPLC-MS/MS analysis, Talanta 116 (2013) 770e775. [6] H. Zhang, X. Wu, Y. Yuan, D. Han, F. Qiao, H. Yan, An ionic liquid functionalized graphene adsorbent with multiple adsorption mechanisms for pipette-tip solid-phase extraction of auxins in soybean sprouts, Food Chem. 265 (2018) 290e297. [7] Y. Zhang, Y.-G. Zhao, W.-S. Chen, H.-L. Cheng, X.-Q. Zeng, Y. Zhu, Threedimensional ionic liquid-ferrite functionalized graphene oxide nanocomposite for pipette-tip solid phase extraction of 16 polycyclic aromatic hydrocarbons in human blood sample, J. Chromatogr. A 1552 (2018) 1e9. [8] D.C.M. Bordin, M.N.R. Alves, E.G. de Campos, B.S. De Martinis, Disposable pipette tips extraction: fundamentals, applications and state of the art, J. Sep. Sci. 39 (2016) 1168e1172. [9] H. Yan, Sun Ning, S. Liu, K.H. Row, Y. Song, Miniaturized graphene-based pipette tip extraction coupled with liquid chromatography for the determination of sulfonamide residues in bovine milk, Food Chem. 158 (2014) 239e244. [10] M. Wang, H. Yan, Y. Yuan, Y. Han, Pipette-tip solid-phase extraction by use of a sol-gel hybrid adsorbent: a new pretreatment strategy for rapid screening of cucumbers for cyanazine and atrazine, Anal. Bioanal. Chem. 407 (2015) 1231e1239.
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