Attapulgite Nanoparticles-Modified Monolithic Column for Hydrophilic

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Attapulgite nanoparticles-modified monolithic column for hydrophilic in-tube solid-phase microextraction of cyromazine and melamine Tingting Wang, Yihui Chen, Junfeng Ma, Qian Qian, Zhenfeng Jin, Lihua Zhang, and YuKui Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03478 • Publication Date (Web): 07 Jan 2016 Downloaded from http://pubs.acs.org on January 10, 2016

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Analytical Chemistry

Attapulgite nanoparticles-modified monolithic column for hydrophilic in-tube solid-phase microextraction of cyromazine and melamine †

§

†

†

‡

Tingting Wang, ,* Yihui Chen#, Junfeng Ma, Qian Qian, Zhenfeng Jin, Lihua Zhang, , Yukui ‡

Zhang

†

‡

College of Chemical Engineering, Ningbo University of Technology, Ningbo 315016, China Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, Dalian 116023, China #

Xiangshan Entry-Exit Inspection and Quarantine, Xiangshan 310014, China

§

Department of Biological Chemistry, The Johns Hopkins University School of Medicine,

Baltimore MD 21205, USA

*Correspondence author: Tel.: +86 574 65756733. Fax: +86 574 65756710. E-mail: [email protected]

ACS Paragon Plus Environment


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In current study, a novel monolithic capillary column with embedded attapulgite nanoparticles has been developed and exploited as a stationary phase in hydrophilic in-tube solid phase microextraction (SPME) of cyromazine and melamine. The fibrillar

attapulgite

nanoparticles

poly(1-vinyl-3-(butyl-4-sulfonate) methylenebisacrylamide)

(poly

were

imidazolium

embedded

in

the

-co-acrylamide-co-N,

N’-

(VBSIm-AM-MBA))

monolith

via

in

situ

polymerization. The attapulgite/polymerization ratio of the monolith was finely optimized. Primary factors of in-tube SPME including sample solvent, elution solvent, sample loading volume, elution volume, sample loading flow rate and elution flow rate were thoroughly evaluated. Under optimal conditions, the limits of detection (LODs) were found to be 21.1 and 0.3 ng.mL-1 for cyromazine and melamine in the milk formula sample, respectively. And the recoveries of cyromazine and melamine spiked in the sample ranged from 94.5% to 109.9% with RSDs less than 7.6%.


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■ INTRODUCTION In recent years, polymer monolithic columns have attracted tremendous attention in the field of analytical sciences, due to its advantages of relatively easy preparation, diverse surface chemistry, high porosity, excellently pH stability and fast mass transfer resulting from the large pores.1-3 Besides being traditionally used for the separation of proteins, peptides, and small molecules,4,5 polymer monolithic columns have also gained huge popularity recently in sample preparation, especially for in-tube solid-phase microextraction (SPME).6-9 Despite many advantages of the polymer monolithic columns are mentioned above, it is still feasible to further improve the extraction capacity as adsorbent beds for sample preparation. The recent incorporation of nanoparticles into polymer monolithic columns has emerged as an effective way to afford higher surface area of absorbents and thus an enhanced extraction capacity for analytes.10-14 To date, nanomaterials such as TiO2 nanoparticles,15 silica nanoparticles,16 metal organic frameworks,17,18 graphene nanosheets,19 and carbon nanotubes,20 embedded or attached to the polymer monolithic columns have been exploited for extraction and enrichment of small molecules, metal ions and other analytes. Of note, a number of mechanisms

(including

electrostatic

interaction,15,16,20

π-π

interactions,17-20

hydrophobic interaction,18-20 and hydrogen bonding.18,19) have been revealed in those extraction processes. However, the applicability of nanoparticles incorporated polymer monoliths for hydrophilic extraction of polar compounds has been rarely explored. Attapulgite, a natural nano-structural fibrillar material with the formula of [(OH2)4 (Mg, Al, Fe)5 (OH).2Si8O20]·4H2O,21 is abundantly found in Xuyi County, Jiangsu Province, China.22 As can be observed from its formula, there are three different types of water molecules, including adsorbed water, water bonded to octahedral cations and hydroxyl groups.23 Due to its low cost, large specific surface area, porous structure, reactive-OH groups and appropriate cation exchange capacity, attapulgite and modified attapulgite show great potential in adsorption of metal ions



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and organic molecules.24-27 However, little has been done on the application of modified attapulgite to solid phase extraction.28-30 Although it has recently shown that molecularly imprinted polymer using attapulgite as matrix is efficient for solid phase extraction of sudan dyes and estrogens,31,32 monolithic columns embedded attapulgite have not been applied for hydrophilic in-tube SPME of polar compounds. Cyromazine, a pesticide as well as an insect growth regulator, can be metabolized to melamine during dealkylation reactions in both plants and animals (Structures see Figure S1, Supporting Information). Melamine can form insoluble melamine-cyanurate crystal deposits with cyanuric acid (a metabolite of melamine) in kidneys, inducing renal failure in those who ingest a high dose, especially infants or young children. In 2008, it was revealed that the adulteration of melamine in infant formula milk powder resulted in kidney problems for over 51900 infants throughout China33. Thus, it is imperative to develop effective test methods for the analysis of cyromazine and melamine in a wide variety of food samples. Recently, several new types of solid-phase extraction (SPE) sorbents, such as molecularly imprinted polymers,34 magnetic strong cation exchange resins,35 ampholine-functionalized hybrid silica sorbent,36 have been developed for the extraction of cyromazine or melamine. Compared with SPE, SPME has merits of high sensitivity, small sample volume, simplicity, and easy automation. Of note, Shi et al. has developed hollow fiber sorptive extraction using zirconia hollow fiber for the determination of melamine residues from milk products.37 Moreover, in-tube SPME overcomes the SPME shortcomings of fragility, limited lifetime and sample carry-over. However, there are few reports on the use of in-tube SPME methods for the analysis of both cyromazine and melamine from milk formula samples. To this end, herein we fabricated a monolithic column in a capillary format by entrapping attapulgite nanoparticles. The resulting monolithic columns were characterized by scanning electron microscopy (SEM) and fourier-transformed infrared spectroscopy (FT-IR). The applicability of the developed monolithic attapulgite column was demonstrated by the determination of cyromazine and melamine in milk formula sample as a hydrophilic in-tube SPME matrix. Compared


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with currently used SPME sorbents (e.g. commercially available fibers,38 graphitic carbon nitride fiber,39 highly crosslinked polymer nanoparticles coated fiber,40 ZnO nanorods coated fiber41), one striking advantage of the developed monolithic attapulgite material is that it demonstrates high selectivity toward cyromazine and melamine via mainly hydrophilic interaction mechanism as an in-tube SPME sorbent. Moreover, it exhibits high adsorption capacity and low LODs for both analytes in complex matrices (i.e., milk formula samples).

■ EXPERIMENTAL SECTION Reagents and Chemicals, Instrumentation, in-tube SPME Procedure, Sample Preparation and LODs calculation are detailed in the Supporting Information. Preparation of the Monolithic Column with Attapulgite Nanoparticles. The preparation of monolith columns is similar to our previous work,42 but with modifications. Specifically, the polymerization mixture was composed of 50 mg acrylamide (AM) and 167 mg 1-Vinyl-3-(butyl-4-sulfonate) imidazolium (VBSIm), 100 mg N, N’-methylenebisacrylamide (MBA), 1934 mg formamide, 967 mg dimethyl sulphoxide, 106 mg PEG-8,000 and 192 mg PEG-10,000, as well as 1% azobisisobutyronitrile (w/w, with respect to total monomers) (Scheme see Figure S2, Supporting Information). Natural attapulgite nanoparticles were treated with 4 M HCl at 75 °C for 4 h, and then washed with redistilled water repeatedly until pH 7.0. The activated attapulgite was dried at 110 °C for 8 h. A specific amount of activated attapulgite was dispersed in the polymerization mixture for optimization. This dispersion was then filled into vinylized capillaries and polymerized at 75 °C for 20 h. The monolithic column was rinsed with acetonitrile (ACN), water and 95% ACN. Binding Capacity. The binding capacity was measured using frontal elution.43 Monolithic attapulgite columns (25 cm) were equilibrated 95% ACN; then a melamine solution dissolved in 95% ACN (100 µg mL-1) was pumped through the monolithic column at a constant flow rate of 2.4 µL min-1 and absorbance was measured at 214 nm. Toluene dissolved in 95% ACN (100 µg mL-1) was used to



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estimate the void time of this system at the same flow rate due to its non-retention on the monolithic column.

■ RESULTS AND DISCUSSION Preparation of the Monolithic Attapulgite Column. In our previous work, a zwitterionic poly (VBSIm-AM-MBA) monolithic column was developed and applied for hydrophilic interaction liquid chromatography (HILIC) separation of polar compounds.42 Although excellent separation efficiency was achieved, the binding capacity of the monolith was not ideal for polar compounds enrichment, as HILIC columns could more easily get overloaded.44 To this end, activated attapulgite nanoparticles containing abundant hydroxyl groups were entrapped in the porous matrix of poly (VBSIm-AM-MBA) to improve extraction capacity of cyromazine and melamine. Although attapulgite nanoparticles are supposed to largely increase the surface area of the material, the content of attapulgite must be finely controlled for the successful preparation of a monolithic column. To investigate the effect of the amount of attapulgite nanoparticles on the preparation of the monolithic column, varying amounts of attapulgite nanoparticles were added to the polymerization mixture, with a ratio ranging from 0%-10% (w/w). As shown in Table 1, increasing the amount of attapulgite nanoparticles in the polymerization mixture leads to a decrease in permeability. And the monolithic column could not even be pumped through with a percentage of 8%, indicating that the amount of attapulgite nanoparticles encapsulated should be balanced to afford an optimum monolithic column. The binding capacity of the columns with different compositions of attapulgite nanoparticles in the polymerization mixture was investigated. Breakthrough curves for melamine obtained using frontal analysis allow comparison of binding capacities of the monolithic columns. A solution containing 95% ACN was used as a mobile phase, as described previously.36 As shown in Figure 1, the frontal analysis curve of each monolithic column exhibits a very steep rise, indicating a fast kinetic adsorption of melamine on the monolithic column. The void time, estimated by flushing 100 µg


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mL-1 toluene in the same column, was subtracted from the total time consumed for saturating the monolithic column. And the time for saturation of 0%, 1.6%, 3.0% and 5.0% attapulgite in the polymerization mixture was 1.6, 6.2, 7.8 and 10.1 min, respectively. The binding capacity was calculated from the following equation43: Q = c*t*ν/V. Q: the binding capacity (µg mL-1); c: the analyte concentration (µg mL-1); t: saturation time (min); ν: the flow rate (µL min-1); V: the volume of monolithic column (mL). With a flow rate of 2.4 µL min-1, the binding capacity of the 0%, 1.6%, 3.0% and 5.0% ratio of attapulgite/polymerization was 48, 190, 238 and 308 µg mL-1, respectively. Thus, the binding capacity increases with an increasing amount of attapulgite in the polymerization mixture. Due to the slight difference in permeability of monolithic columns with different amounts of attapulgite (Table 1), the RSD of void time detected by toluene was 1.5% (n=8, four monolithic columns with different amounts of attapulgite were assayed, with each column assayed twice). Moreover, with the increased amounts of attapulgite nanoparticles, the adsorption capability of the monolithic attapulgite columns were increased, leading to the different elution time of the monolithic columns with different amounts of attapulgite. Taken the results

from

permeability

and

binding

capacity

assays

together,

an

attapulgite/polymerization ratio of 5% was selected in the following studies. Characterization of the Monolithic Attapulgite Column. SEM provides direct visual images of the poly (VBSIm-AM-MBA) monolithic columns incorporated with and without attapulgite nanoparticles (Figure 2). Clearly, the length and morphology of the 0.5~1.0 µm attapulgite nanoparticles (Figure 2A) prevent the nanoparticles from perfusion through the tortuous pores of the monoliths, the through-pore sizes of which are in the range of 0.5 to 2.0 µm (Figure 2B). Figure 2C confirms the successful entrapment of attapulgite nanoparticles in the monolithic column. SEM with energy dispersive analysis of X-ray (EDAX) detection was used to estimate the elements (except carbon) at the pore surface of the monolithic columns. Indeed, SEM/EDAX reveals that attapulgite nanoparticles-embedded monolithic material contains a number of elements including nitrogen, oxygen, sulphur, magnesium, aluminum, silicon and iron. In contrast, the monolithic column without attapulgite



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particles only contains nitrogen, oxygen and sulphur. These results further suggest that attapulgite nanoparticles are embedded in the poly (VBSIm-AM-MBA) monolithic column. Figure 3 shows the FT-IR spectra of activated attapulgite nanoparticles, monolithic columns without attapulgite nanoparticles, and monolithic columns with attapulgite nanoparticles at a nanoparticle/polymerization ratio of 5%. The peaks of attapulgite nanoparticles at 474 cm−1 and 800 cm−1 correspond to Si-O-Si bonds and the stretching vibration of Al-O-Si, respectively (Figure 3A).45 And the peaks of monolithic columns without attapulgite nanoparticles at 1680 cm−1 and 1455 cm−1 are due to C=O stretching vibration and vibration of the imidazole ring, respectively (Figure 3B). Collectively, our results confirm the successful incorporation of attapulgite into poly (VBSIm-AM-MBA) monolithic column (Figure 3C). In-tube SPME method development. The above-developed attapulgite nanoparticles embedded poly (VBSIm-AM-MBA) monolithic column was applied for hydrophilic in-tube SPME of polar compounds. Cyromazine (log p = -0.04) and melamine (log p = -1.37) were tested for the enrichment of the monolithic attapulgite column by hydrophilic interaction. Since ACN content is an essential factor affecting the adsorption behavior, we investigated the influence of the ACN content on the adsorption of cyromazine and melamine. Both analytes dissolved in different ACN contents were directly pushed through the monolithic attapulgite column, with the flow-through collected and detected by liquid chromatography (LC). As shown in Figure 4A, the adsorption rates are increased with the increase of ACN content from 70% to 95%, reaching a plateau in the range of 95%-100%. In order to avoid solubility limits of polar analytes,46 95% ACN was employed in the subsequent experiments. The effect of ACN content in the elution solvent on the extraction efficiency was also investigated. Since the extraction efficiencies were barely changed at the ACN content between 0 and 50%, the optimum extraction efficiency was achieved with the elution solvent containing 20%/80% ACN/H2O (v/v). Therefore,

20%/80% ACN/H2O (v/v) was selected as elution solvent for further experiments. Considering the monolithic attapulgite column combines the zwitterionic


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character of the poly (VBSIm-AM-MBA) monolith and the hydrophilic property of hydroxyl groups of the embedded attapulgite, the effect of salt concentration and pH on the extraction efficiency were also investigated (Figure S3, Supporting Information). NaCl was added to the elution solvent of 20% ACN. It turns out that the salt concentration played a marginal role on the extraction efficiency. In addition, the extraction efficiencies of cyromazine and melamine were only slightly changed with different pH values. Therefore, we reason that the hydrophilic interaction should be the major mechanism for the extraction of both analytes with the developed in-tube SPME method. Sample loading and elution volumes are also vital factors to be considered for in-tube SPME processes. The effect of sample loading volume on extraction efficiency was evaluated at first. Six different volumes (i.e., 5, 20, 40, 100, 250 and 500 µL) of analytes were investigated; each one was spiked with 0.1 µg cyromazine and 0.25 µg melamine. Figure 4C shows that the sample loading volume greatly affects the adsorption rate of cyromazine. Cyromazine was adsorbed completely on the monolithic

attapulgite column with a sample loading volume of only 5 µL. On the other hand, although the adsorption rates of melamine are decreased with the increase of the sample loading volume, the maximum sample loading volume is up to 100 µL. The sample loading volume of melamine is higher than that of cyromazine, which can be ascribed to

the higher hydrophilicity of melamine. These results further verify that cyromazine and melamine are retained on the monolithic attapulgite column via hydrophilic interaction.

To ensure high extraction efficiency of both cyromazine and melamine, a sample loading volume of 5 µL was chosen. If extraction efficiency of melamine was only considered, a sample loading volume of 100 µL should be chosen. Besides the sample loading volume, the elution volume was also investigated (Figure 4D), with 40 µL as the optimal elution volume. Under the optimal sample loading and elution conditions,

the enrichment factor of cyromazine and melamine was determined to be 0.125 and 2.5, respectively. The flow rates used for sample loading and elution also influence the performance of the in-tube SPME procedure. A higher flow rate decreases the time of



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sample loading and elution, resulting in a shorter sample analysis time. As shown in Figure 4E, extraction efficiencies were slightly increased with the increase of sample loading flow rate. Since a higher flow rate might lead to increased flow resistance, a flow rate of 4 µL min−1 was adopted for sample loading. Elution flow rate was investigated as well, with the highest extraction efficiency achieved at the flow rate of 2 µL min−1 (Figure 4F). Thus, a flow rate of 2 µL min−1 for elution was employed in the subsequent experiments.

The lifetime of the developed in-tube SPME stationary phase has also been tested. Between each run, the column was eluted by 20% ACN and re-equilibrated by 95% ACN. The RSD of peak area for cyromazine and melamine for 5 runs was 5.7% and 5.1%, respectively. When the monolithic attapulgite column was repeated used for 7 times, the RSD of peak area for cyromazine and melamine was increased to 11.5% and 12.8%, respectively. Thus, the developed monolithic attapulgite column can be reusable about 5 times. Method validation. Under the optimized conditions, the analytical performance of the developed hydrophilic in-tube SPME based on attapulgite embedded poly (VBSIm-AM-MBA) monolithic column was further evaluated. And the applicability of the column was demonstrated by the extraction of cyromazine and melamine in milk formula samples. It is noteworthy that although many polar contents and salts were existed in the milk formula samples, it seemed they did not interfere with the analysis of cyromazine and melamine (Figure S4, Supporting Information). Thus, the interference and competition between target analytes and polar or other components were not further investigated. Excellent linearity with correlation coefficients (r2) was achieved (Table S1, Supporting Information). LODs, calculated at a signal-to-noise ratio of 3, were found to be 21.1 and 0.3 ng.mL−1 for cyromazine and melamine, respectively. When milk formula samples were directly analyzed by LC, the LODs of cyromazine and melamine was 100 and 50 ng mL-1, respectively. Thus, compared with direct analysis by LC, the developed in-tube SPME method improves the sensitivity of cyromazine and melamine in the milk formula samples about 4.7 and 167 times. On the other hand, compared with our previously developed HILIC SPE


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approaches,36 the LOD of newly proposed method for melamine is increased about 33 times. This could be explained that the hydrophilicity of the new developed monolithic attapulgite column is higher than the ampholine-functionalized hybrid organic-inorganic silica sorbent, leading to a higher amount of analytes adsorbed and thus improved sensitivity. In addition, the LODs of the developed hydrophilic in-tube SPME are comparable to that of the molecularly imprinted solid-phase extraction, a method which is often regarded as highly sensitive to enrich the target analytes.47 Extraction recoveries of the hydrophilic in-tube SPME method for cyromazine and melamine spiked in milk formula samples was investigated at three concentration levels. The mean recoveries of both analytes were in the range 94.5-109.9%, with RSDs of 1.4-7.6% (Table 2), indicating a good accuracy and repeatability of the developed hydrophilic in-tube SPME method based on attapulgite embedded poly (VBSIm-AM-MBA) monolithic column for the determination of cyromazine and melamine in milk formula samples.

■ CONCLUSION A poly (VBSIm-AM-MBA) monolithic column embedded with attapulgite nanoparticles was successfully prepared and used as extraction medium of in-tube SPME system. Investigation of extraction mechanism showed that analytes were retained on the monolithic attapulgite column via mainly hydrophilic interaction. Frontal analysis indicates that the binding capacity of melamine increases with an increase of the attapulgite/polymerization ratio, which clearly demonstrates the merit of attapulgite nanoparticles to improve the extraction performance of monolithic columns. Excellent extraction efficiency, low LODs and high reproducibility of the method for the analytes show that the hydrophilic in-tube SPME method can be used to determine the polar compounds in real samples. Taken together, this work opens up a new way for the preparation of nanoparticles embedded monolith of high adsorption capacity for the application of in-tube SMPE.



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■ ACKNOWLEDGMENTS The project is supported by the National Natural Science Foundation of China (21405085), the Public Applied Research Programs of Technology of Zhejiang Province (2015C37015), the Zhejiang Provincial Natural Science Foundation of China (LQ12B05001) and Ningbo Natural Science Foundation of China (2012A610091).

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Figure Legends Figure 1 Breakthrough curves of melamine demonstrating the effect of the amount of attapulgite nanoparticles at a nanoparticle/polymerization mixture (w/w): (A) 5%; (B) 3%; (C) 1.6% and (D) 0%. The void time was estimated by flushing 100 µg mL-1 toluene for an attapulgite /polymerization ratio of 0% (E).

Figure 2. SEM micrographs of the activated attapulgite nanoparticles (A), monolithic columns containing attapulgite nanoparticles at a nanoparticle/ polymerization ratio of 0% (B) and 5% (C), and corresponding energy-dispersive X-ray spectroscopy spectra.

Figure 3. FT-IR spectra of activated attapulgite nanoparticles (A), monolithic columns containing no attapulgite nanoparticles (B), and monolithic columns containing attapulgite nanoparticles at a nanoparticle/ polymerization ratio of 5% (C).

Figure 4. Effects of experimental conditions on the in-tube SPME of cyromazine and melamine by the attapulgite nanoparticles-embedded monolithic column. Effect of the ACN content of extraction solvent (A); Effect of the ACN content of elution solvent (B); Effect of sample loading volume (C); Effect of the elution volume (D); Effect of the flow rate of sample loading (E); Effect of the flow rate of elution (F).



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Table 1 Permeability and binding capacity of the monolithic column with different composition of the attapulgite/polymerization mixture. Attapulgite / Polymerization (%, w/w)

K (×10-13 m2)1

Binding capacity2 (µg mL-1)

Morphology

0 1.6

5.09 4.84

48 190

Homogenous dark brown Homogenous dark brown

3.0

4.66

238

Homogenous dark brown

5.0

4.56

308

Homogenous dark brown

8.0

n/a

n/a

Collapse

10.0

n/a

n/a

Collapse

1 2

The permeability (K) was measured using 74/26 (v/v) ACN/water. mL was the volume of the monolithic column.

Table 2 Recoveries of the cyromazine and melamine from the milk formula sample (n=3). Spiked concentration (ng mL-1)

Recovery (%)

RSD(%)

80 400 1600 10 50 200

94.5 95.8 96.6 109.9 100.5 97.9

7.6 4.2 4.5 1.4 1.6 2.6

Cyromazine1

Melamine2 1

Sample loop of 5 µL.

2

Sample loop of 100 µL.


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Figure 1 Breakthrough curves of melamine demonstrating the effect of the amount of attapulgite nanoparticles at a nanoparticle/polymerization mixture (w/w): (A) 5%; (B) 3%; (C) 1.6% and (D) 0%. The void time was estimated by flushing 0.1 mg/mL toluene for an attapulgite /polymerization ratio of 0% (E). 20x11mm (600 x 600 DPI)



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Figure 2. SEM micrographs of the activated attapulgite nanoparticles (A), monolithic columns containing attapulgite nanoparticles at a nanoparticle/ polymerization ratio of 0% (B) and 5% (C), and corresponding energy-dispersive X-ray spectroscopy spectra. 49x61mm (600 x 600 DPI)


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Figure 3. FT-IR spectra of activated attapulgite nanoparticles (A), monolithic columns containing no attapulgite nanoparticles (B), and monolithic columns containing attapulgite nanoparticles at a nanoparticle/ polymerization ratio of 5% (C). 29x22mm (600 x 600 DPI)



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Figure 4. Effects of experimental conditions on the in-tube SPME of cyromazine and melamine by the attapulgite nanoparticles-embedded monolithic column. Effect of the ACN content of extraction solvent (A); Effect of the ACN content of elution solvent (B); Effect of sample loading volume (C); Effect of the elution volume (D); Effect of the flow rate of sample loading (E); Effect of the flow rate of elution (F). 37x28mm (600 x 600 DPI)


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For TOC only 59x45mm (600 x 600 DPI)


Attapulgite Nanoparticles-Modified Monolithic Column for Hydrophilic

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