Zeolite imidazolate framework-8 as sorbent for on-line solid-phase extraction coupled with high-performance liquid chromatography for the determination of tetracyclines in water and milk samples

Accepted Manuscript Title: Zeolite imidazolate framework-8 as sorbent for on-line solid-phase extraction coupled with high-performance liquid chromatography for the determination of tetracyclines in water and milk samples Author: Xue-Qing Yang Cheng-Xiong Yang Xiu-Ping Yan PII: DOI: Reference:

S0021-9673(13)00989-8 http://dx.doi.org/doi:10.1016/j.chroma.2013.06.064 CHROMA 354463

To appear in:

Journal of Chromatography A

Received date: Revised date: Accepted date:

29-4-2013 21-6-2013 25-6-2013

Please cite this article as: X.-Q. Yang, C.-X. Yang, X.-P. Yan, Zeolite imidazolate framework-8 as sorbent for on-line solid-phase extraction coupled with high-performance liquid chromatography for the determination of tetracyclines in water and milk samples, Journal of Chromatography A (2013), http://dx.doi.org/10.1016/j.chroma.2013.06.064 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Zeolite imidazolate framework-8 as sorbent for on-line solid-phase

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extraction coupled with high-performance liquid chromatography

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for the determination of tetracyclines in water and milk samples

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Xue-Qing Yang, Cheng-Xiong Yang, Xiu-Ping Yan*

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State Key Laboratory of Medicinal Chemical Biology, and Research Center for

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Analytical Sciences, College of Chemistry, Nankai University, Tianjin 300071, China

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*Corresponding author.

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E-mail: [email protected]

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Fax: (86)22-23506075.

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ABSTRACT

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Zeolite Imidazolate Framework-8 (ZIF-8) was used as the novel sorbent for on-line solid-phase

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extraction coupled with high-performance liquid chromatography (HPLC) for the determination of

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oxytetracycline (OTC), tetracycline (TC) and chlorotetracycline (CTC) in water and milk samples.

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390 mg of ZIF-8 was packed into a stainless steel column (3 cm long × 4.6 mm i.d.) which was

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mounted on the HPLC injector valve to replace the sample loop. On-line solid-phase extraction of

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OTC, TC and CTC was achieved by loading sample solution at a flow rate of 3.0 mL min-1 for 10

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min with the aid of a flow-injection system. The extracted analytes were subsequently eluted into

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a C18 analytical column (25 cm long × 4.6 mm i.d.) for HPLC separation under isocratic condition

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with a mobile phase (10% MeOH - 20% ACN - 70% 0.02 mol L-1 oxalic acid solution) at a flow

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rate of 1.0 mL min-1. Under optimized conditions, the developed method gave the enhancement

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factors of 35-61, the linearity range of 5-1000 µg L-1, the detection limits of 1.5-8.0 µg L-1,

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quantification limits of 5.0-26.7 µg L−1, uncertainties of 0.9-1.1 µg L−1, and the sample throughput

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of 4 samples h−1. The recoveries of OTC, TC and CTC at 50 µg L-1 in water and milk samples

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ranged from 70.3% to 107.4%.

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Keywords:

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Solid phase extraction

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Zeolite imidazolate framework-8

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High-performance liquid chromatography

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Tetracyclines

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Water samples

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1. Introduction Tetracyclines, a class of broad-spectrum antibiotics, have been widely used as medicines and

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additives in animal foods to resist and treat diseases [1]. The excessive use of tetracyclines causes

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several negative effects on human beings as the release of animal excrement with excessive

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tetracyclines residues to the environment leads to water pollution, soil contamination, and drug

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resistance of bacteria. Tetracyclines residues in animal-producing food can also produce a lot of

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threats to human, including allergies reactions, toxic effects, the damage of liver, yellowing of

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teeth, and gastrointestinal disturbance [2]. Consequently, monitoring tetracyclines in

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environmental and biological samples is of great importance to protect humans from the

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disturbance of tetracyclines. However, direct determination of tetracyclines in environmental and

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biological samples is usually difficult because of their low concentration (usually below 1 mg L−1)

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[3-5].

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Solid phase extraction (SPE) is an attractive technique for the preconcentration of analytes

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with low concentration before determination. SPE can be carried out either off-line or on-line [6].

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Until now, several sorbents such as molecularly imprinted polymers [7], phenyl silica [8,9] and

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monolithic polymeric material [10] have been successfully used in off-line SPE [11] of

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tetracyclines in various matrixes. However, off-line SPE has inherent disadvantages such as

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time-consuming, easy contamination, process complexity and poor reproducibility. On-line SPE is

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an effective way to overcome above-mentioned disadvantages. Although a few sorbents including

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molecularly imprinted polymers [12], commercial SPE sorbents [13,14] and carbon nanotubes [15]

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have been utilized for on-line SPE of trace tetracyclines, development of novel sorbents is still of

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great significance.

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Metal-organic frameworks (MOFs) are an intriguing class of hybrid crystalline materials

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constructed from metal ions connected with organic bridging ligands [16]. The remarkable

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properties, such as high surface area, permanent nanoscale porosity, good thermostability, and

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tunable cavities, make MOFs promising in diverse applications such as gas storage [17],

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separation [18-21], catalysis [22], and drug delivery [23]. In recent years, the application of MOFs

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in sample pretreatment has received increasing attention [24-34]. While the application of MOFs

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for off-line SPE has been widely explored [30-33], the usage of MOFs for on-line SPE has been

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rarely studied. Zhou et. al. [34] reported the first example of utilizing MOFs for on-line SPE of

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polycyclic aromatic hydrocarbons in environmental materials.To further expand the application of

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MOFs for on-line SPE and to explore new methods for simple, automatic and simultaneous

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determination of tetracyclines residues in water samples, here we report zeolite imidazolate

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framework-8 (ZIF-8) for on-line SPE coupled with high-performance liquid chromatography

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(HPLC) for the determination of tetracyclines in water samples. ZIF-8, which has the formula

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Zn(MIM)2 (MIM = 2-methylimidazolate) with a sodalite (SOD)-related zeolite type structure [35],

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not only exhibits high porosity and large surface area like other MOFs, but also has the

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exceptional thermal and chemical stability in water and organic solvents [35,36]. Therefore, ZIF-8

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has the potential to be the candidate sorbent for SPE application [37-39]. In this work, a SPE

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column was prepared by packing 390 mg of ZIF-8 into a stainless steel column (3 cm long × 4.6

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mm i.d.), and mounted on the HPLC injector valve to replace the sample loop for on-line SPE of

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tetracyclines. Potential factors affecting the on-line SPE were studied in detail. The developed

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method was then applied for the determination of tetracyclines in water samples.

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2. Experimental

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2.1. Chemicals and reagents All reagents used were at least of analytical grade. Ultrapure water (Tianjin Wahaha Foods

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Co. Ltd., Tianjin, China) was used throughout this work. 2-Methylimidazole (H-MeIM) was

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purchased from Alfa Aesar Chemical Reagent Co. Ltd. ( Tianjin, China). N,N-dimethylformamide

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(DMF), methanol (MeOH), ethanol, and acetonitrile (ACN) were obtained from Concord Fine

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Chemical Research Institute (Tianjin, China). Oxalic acid dihydrate was purchased from Guangfu

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Fine Chemical Research Institute (Tianjin, China). Zn(NO3)2·6H2O, tetracycline hydrochloride

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(TC), oxytetracycline dihydrate (OTC) and chlorotetracycline hydrochloride (CTC) (Fig. 1) were

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purchased from Shanghai Aladdin Chemistry Co. Ltd. (Shanghai, China). The stock solutions of

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the tetracyclines (1 mg mL-1) were prepared with MeOH as the solvent and stored at -4 oC in dark.

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Working solutions were prepared from the standard stock solutions by stepwise dilution with

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ultrapure water just before use.

2.2. Preparation of ZIF-8

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ZIF-8 was synthesized according to Yaghi et al [35]. Typically, a solid mixture of

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Zn(NO3)2·4H2O (0.210 g) and H-MeIM (0.060 g) was dissolved in 18 mL DMF in a 30-mL

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Teflon-lined bomb. The Teflon-lined bomb was sealed, placed in an oven, and kept at 140 oC for

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24 h. Colorless crystals were thus obtained and washed with DMF. The crystals were collected

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by centrifugation at 10000 rpm for 10 min. After another two cycles of DMF washing-

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centrifugation, the obtained crystals were washed with ethanol, and dried at 60 oC under reduced

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pressure for 12 h to obtain ZIF-8.

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2.3. Apparatus

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On-line SPE preconcentration was carried out on a model FIA-3100 flow injection system

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(Vital Instruments Co. Ltd, Beijing, China). Tygon pump tube (1.52 mm i.d) was used to deliver

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sample solution. Small-bore polytetrafluoroethylene (PTFE) tubes (0.5 mm i.d.) were used for all

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connections with the shortest possible length in order to minimize the dead volume. A stainless

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steel column (3 cm long × 4.6 mm i.d.) was dry-packed with 390 mg ZIF-8, and mounted on the

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HPLC injector valve to replace the sample loop for on-line SPE. Before SPE, the ZIF-8 packed

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column was conditioned with the mobile phase until the detection response reached stability.

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All HPLC separations were performed on a HPLC system fitted with a Waters 600 HPLC

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pump and a Water 2996 photodiode array detector (PAD, Milford, MA, USA), and an analytical

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reversed-phase column (C18, 25 cm long × 4.6 mm i.d., BaseLine Co. Ltd, Tianjin, China) under

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isocratic condition at 25 oC. Data acquisition and processing were carried out on an Empower

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chromatography data system. A mixture of 10% MeOH - 20% ACN - 70% 0.02 mol L-1 oxalic

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acid solution was used as the mobile phase for desorption and separation of tetracyclines at a flow

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rate of 1.0 mL min-1. The mobile phase was filtered with 0.45 µm filter films and then degassed by

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ultrasonication prior to use.

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X-ray diffraction spectrometry (XRD), scanning electron microscopy (SEM), thermal

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gravimetric analysis (TGA), and nitrogen adsorption were employed to characterize the prepared

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ZIF-8. The XRD patterns were recorded with a D/Max-2500 X-ray diffractometer (Rigaku, Japan)

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using Cu K radiation (λ=1.5418 Å) over the angular range from 3o to 80o. The TGA experiments

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were performed on a PTC-10A thermal gravimetric analyzer (Rigaku, Japan) from room

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temperature to 800 oC at a ramp rate of 10 oC min-1. The SEM images of the ZIF-8 were recorded

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on a Shimadzu SS-550 scanning electron microscope (Kyoto, Japan) at 15.0 kV. The BET surface

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area was measured on an ASAP 2010 micropore physisorption analyzer (Micromeritics, Norcross,

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GA) using nitrogen adsorption at 77 K in the range 0.02 ≤ P/P0 ≤ 0.20. Zeta potentials were

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measured on a zeta potential analyzer (Zetasizer Nano ZS90, Britain).

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2.4. Procedures for the on-line SPE and HPLC separation

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The flow manifold for the on-line SPE preconcentration coupled with HPLC for

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derermination of trace TCs in water samples is the same as that in a previous work [34]. First, a

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model FIA-3100 flow injection system was used to introduce the sample solution into the ZIF-8

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packed SPE column at a flow rate of 3.0 mL min−1 for 10 min while the HPLC injector valve was

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in the “load” position, so that the ZIF-8 packed SPE column preconcentrated the tetracyclines and

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the unwanted water went to waste. Second, the mobile phase was pumped at a flow rate of 1.0 mL

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min−1 to elute the tetracyclines adsorbed on ZIF-8 packed SPE column in the backflush mode into

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the analytical C18 column for 1 min by switching the HPLC valve from “load” to “inject” position

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to compress the sample band in the SPE column into a narrow band before entering the analytical

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column. Finally, the HPLC injector valve was switched to the “load” position for next sample

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preconcentration, and at the same time the tetracyclines were separated in the analytical C18

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column. A complete cycle of the on-line SPE and HPLC separation of the tetracyclines lasted 15

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min. The peak areas calculated from the chromatographic peaks monitored at 365 nm were used

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for data evaluation.

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2.5. Sample pretreatment

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Lake and river water samples were collected locally, filtered through a 0.45 µm water

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membrane, and stored in clean glass bottles which were washed with detergents, water, methanol,

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and ultrapure water, and dried before use. The pH of water samples was adjusted with HCl or

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NaOH to insure efficient SPE, and analyzed immediately. McIlvane/ethylendiaminetetra acetic acid (McIlvane/EDTA) solution was prepared by

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dissolving 15.0 g of disodium hydrogen phosphate dihydrate, 3.72 g of EDTA and 13.0 g of citric

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acid monohydrate in 1.00 L water (pH adjusted to 2.90 with 1.0 M phosphoric acid or 1.0 M

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sodium hydroxide) [40].

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The milk samples were purchased from a local supermarket in Tianjin, China. A 10.0 g milk

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sample was added in a 50 mL centrifuge tube with 30 mL of the McIlvane/EDTA buffer. The

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mixture was then centrifuged at 4000 rpm for 15 min. The supernatant was used for the

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subsequent SPE [40].

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3. Results and discussion

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3.1. Characterization of the synthesized ZIF-8

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The experimental XRD pattern of the synthesized ZIF-8 powder is in good agreement with

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the simulated one, showing the successful preparation of ZIF-8. In addition, the XRD pattern of

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ZIF-8 after 120 extraction cycles also matched well with the simulated one, revealing the good

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stability of ZIF-8 for on-line SPE of tetracyclines (Fig. 2A). The SEM image exhibits ZIF-8 with

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an average size of 10 μm (Fig. 2B). The TGA data reveal that the ZIF-8 is stable up to 350 oC (Fig.

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2C). The N2 adsorption result shows the BET surface area of 699 m2 g-1 for ZIF-8.

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Fig. 2. here 3.2. Optimization of the mobile phase composition

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In this work, a mixture of MeOH-ACN-H2O was used as the mobile phase. Acids such as

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phosphoric, citric or tartaric acid should be added into the mobile phase to reduce the tailing peaks

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of tetracyclines resulting from the absorption of tetracyclines on the reverse phase column [41].

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Thus, 0.02 M oxalic acid was added in the mobile phase to improve the peak symmetry of

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tetracyclines in this work. Baseline separation of tetracyclines was achieved in the mobile phase

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with different ratios of ACN and MeOH (Fig.3). In order to reduce the retention time and to

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improve the column efficiency of tetracyclines, a mobile phase of 10% MeOH - 20% ACN - 70%

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0.02 M oxalic acid solution was selected. Fig. 3. here 3.3. Factors affecting the on-line SPE of tetracyclines

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Parameters such as the pH value of sample solution, ionic strength in sample solution, sample

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loading flow rate, and sample loading time were optimized to achieve good sensitivity and

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precision for tetracyclines. The influence of pH on the on-line SPE of the tetracyclines (500 µg L-1

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each) was tested in the pH range of 2.5-11.0 at a sample flow rate of 3.0 mL min-1 for 60 s

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extraction. The maximum chromatographic peak areas of OTC, TC and CTC were obtained in the

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pH range of 2.5-9.0 (Fig. 4A). The existence of various ionization degrees on the tetracyclines

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structure were related to their acidic dissociation constants. The pKa1, pKa2, and pKa3 values of the

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tetracyclines in aqueous solution are around 3.3, 7.5 and 9.0, respectively [42]. The charge of

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these tetracyclines could be positive (pH < pKa1,), zwitterion (pKa1 < pH < pKa3) or negative (pH >

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pKa3) under different pH conditions. The positively charged tetracyclines (pH < pKa1,) are

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favorable for extraction on negatively charged ZIF-8 ( < 0) in the pH range of 2.5-3.3, suggesting

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the electrostatic interaction between ZIF-8 and tetracyclines was involved in the extraction of the

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tetracyclines on ZIF-8. There was no obvious change in the peak areas in the pH range of 3.3-9.0,

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showing the electrostatic interaction was not the only mechanisms involved in the extraction of the

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tetracyclines on ZIF-8. Beside that, the hydrophobic and π-π interaction between the aromatic

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rings of the tetracyclines and the aromatic imizole rings of the ZIF-8 should be also considered for

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the extraction of the tetracyclines on ZIF-8. Further increase in the pH values led to decreased

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chromatographic peak areas due to the electrostatic repulsion between the negatively charged

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ZIF-8 and the anionic tetracyclines.

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The effect of ionic strength was also investigated by spiking NaCl in the concentrations range

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of 0 g L-1 to 80 g L-1 at a sample flow rate of 3.0 mL min-1 for 60 s preconcentration. The

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chromatographic peak areas of the tetracyclines decreased as the NaCl concentration increased

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from 0 to 10 g L-1, then leveled off in the NaCl concentration range of 10 to 80 g L-1. Increase of

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NaCl concentration from 0 to 10 g L-1 resulted in the decrease of extraction efficiency of

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tetracyclines as a result of decreased electrostatic interaction between the tetracyclines and ZIF-8.

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However, no significant change in the extraction efficiency of tetracyclines on ZIF-8 was

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observed in the NaCl concentration range of 10 to 80 g L-1. The results suggest the hydrophobic

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and π-π interaction between tetracyclines and ZIF-8 should be the dominant mechanisms in the

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NaCl concentration range of 10 to 80 g L-1 rather than the electrostatic interaction mechanism (Fig.

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4B).

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Fig. 4. here

The sample loading flow rate was also evaluated in the range of 2.2-4.1 mL min-1 for 60 s

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extraction (Fig. 4C). The result shows that the chromatographic peak areas increased slightly upon

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the increasing of the loading flow rate from 2.2 to 4.1 mL min-1. The effect of sample loading time

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on the on-line SPE of the tetracyclines was also investigated at a sample flow rate of 3.0 mL min-1

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(Fig. 4D). The chromatographic peak areas increased as the sample loading time increased. The

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linear ranges of the sample loading time for OTC, TC and CTC are 60-900 s, 60-720 s and 60-900

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s, respectively.

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3.4. Desorption of the adsorbed tetracyclines from the ZIF-8 packed column The optimum HPLC mobile phase of 10% MeOH-20%-ACN-70%-0.02M oxalic acid

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solution was also used as the effective eluent for the desorption of absorbed tetracyclines from the

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ZIF-8-packed column. Complete desorption of the adsorbed tetracyclines from the ZIF-8-packed

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column was achieved within 0.5 min (Fig. 5). In order to ensure the complete desorption of the

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adsorbed tetracyclines from ZIF-8-packed precolumn, 1.0 min was selected as the desorption time

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of the adsorbed tetracyclines.

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Fig. 5. here 3.5. Figures of merit

The analytical characteristic data of ZIF-8-packed column for on-line SPE couple with HPLC

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for the determination of tetracyclines is summarized in Table 1. The enhancement factor (EF) was

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defined as the ratio of the sensitivity of an analyte after extraction to that before extraction. The

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use of the ZIF-8 as the sorbent for 10 min preconcentration at a sample loading flow rate of 3 mL

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min−1 (30 mL samples) gave EFs of 35-61, detection limits (S/N = 3) of 1.5-8.0 µg L−1,

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quantification limits (S/N = 10) of 5.0-26.7 µg L−1, uncertainties of 0.9-1.1 µg L−1, and a sample

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throughput of 4 samples h−1 for the tetracyclines. The linear ranges of the OTC, TC and CTC were

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5-1000 µg L-1, 10-750 µg L-1, and 25-1000 µg L-1, respectively. The interday, intraday precision

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(RSD) for nine replicate extractions of the mixture of tetracyclines (500 µg L-1 for each) was in

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the range of 1.6-3.6%, showing the high precision for the determination of tetracyclines. The

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sorption capacities of OTC, TC and CTC on ZIF-8 were 83.6 mg g-1, 29.2 mg g-1, and 12.1 mg g-1,

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respectively. ZIF-8 also gave better or comparable linear range and detection limits compared with

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other adsorbents (Table 2).

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Table 2. here

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Table 3. here

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3.6. Analysis of tetracyclines in water and milk samples

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The feasibility of the developed method was further demonstrated for the analysis of local

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environmental water samples. The recoveries of three replicate extraction obtained by spiking with

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50 µg L-1 of standard solution of tetracyclines in water and milk samples were from 70.3% to

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107.4% (Table 3), indicating the availability of the developed method for the determination of

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tetracyclines in the water samples. The loss of recovery likely resulted from the interference of the

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impurities in river, lake, and milk samples. Fig. 6 shows the chromatograms of a standard solution

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of the tetracyclines, a river water sample spiked with the tetracyclines, and a river water sample.

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Table 3. here

4. Conclusions

We have demonstrated the feasibility of ZIF-8 as a novel on-line SPE sorbent for simple,

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rapid, and sensitive determination of OTC, TC and CTC in water samples with wide linear range

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and low detection limit. The remarkable advantages, such as high surface area, permanent

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nanoscale porosity, good stability, and tunable cavities, make ZIFs promising as novel sorbents for

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on-line SPE of trace analytes in complex matrixes.

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Acknowledgements

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This work was supported by the National Natural Science Foundation of China (Grants

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20935001, 21077057), and the Fundamental Research Funds for the Central Universities.

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317 16 Page 16 of 27

317 318

Figures Captions Fig. 1. Chemical structures of the tetracyclines studied.

320

Fig. 2. (A) XRD patterns of ZIF-8, ZIF-8 after 120 extraction cycles, and simulated ZIF-8; (B)

321

SEM images of the prepared ZIF-8; (C) TGA curve of the prepared ZIF-8.

322

Fig. 3. The chromatograms of on-line SPE of OTC, TC and CTC (500 µg L-1 for each) with

323

different mobile phase (a) 10% MeOH - 20% ACN - 70% 0.02 M; (b) 12% MeOH - 18% ACN -

324

70% 0.02 M; (c) 15% MeOH - 15% ACN - 70% 0.02 M; (d) 18% MeOH - 12% ACN - 70% 0.02

325

M; (e) 20% MeOH - 10% ACN - 70% 0.02 M.

326

Fig. 4. Effects of pH (A), ionic strength (B), loading flow rate (C), and sample loading time (D) on

327

the on-line SPE of tetracyclines. The concentration of each tetracyclines was 500 µg L-1.

328

Fig. 5. Chromatograms of on-line SPE of tetracyclines with different desorption time (a) 0.5 min,

329

(b) 1.0 min, (c) 1.5 min, (d) 2.0 min, (e) 3.0 min, (f) 5.0 min, (g) 7.0 min, and (h) 10.0 min. The

330

concentration of each tetracyclines was 500 µg L-1.

331

Fig. 6. Chromatograms of (a) a standard water sample spiked with 50 µg L-1 of tetracyclines; (b) a

332

river water sample spiked with 50 µg L-1 of tetracyclines; (c) a river water sample.

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17 Page 17 of 27

Table 1. Characteristic data for the present on-line SPE of OTC, TC and CTC.

Linear range (µg L-1)

R2

OTC

5-1000

0.9999

TC

10-750

CTC

25-1000

Detection limits (µg L-1)

Quantification limits (µg L-)

Uncertainty (µg L-1)

37

1.5

5.0

0.9998

35

2.0

0.9994

61

8.0

RSD (%) (50µg L-1, n=9) Intra-day

1.1

1.6

3.6

6.7

0.9

1.8

2.5

26.7

1.0

2.0

2.7

ip t

Inter-day

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Table 2. Comparison of the different sample preconcentration and detection methods for the determination of TCs Adsorbent

Instrumental method

Linear range (µg L-1)

Detection limits (µg L-1)

Samples

Ref.

Off-line SPE

Discovery DSC-phenyl

HPLC-PAD

50-850

4-9

Honey

1

Magnetic SPE

Silica-coated magnetite particles

HPLC-UV

30-600

0.01

Off-line SPE

MIP monolithic column

HPLC-UV

50-1000

10.2-18.8

Dispersive solid-phase microextraction

Silica-based sorbents

HPLC-PAD

20-400

In-tube SMPE

MAA-EGDMA monolithic column

HPLC-PAD

ZIF-8

HPLC-PAD

us

cr

Milk

an

7.9-20.5

Milk, honey

49

Milk

610

0.7-3.2

Water

100-10,000

16-30

Fish

M

2-50

5-1000

2

711

Muscle 1.5-8.0

Water

This work

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On-line SPE

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Preconcentration method

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Table 3. Analytical results for the determination of tetracyclines in water and milk samples.

Recovery (%, n = 3) OTC

TC

CTC

0

nd.

nd.

nd.

50

84.1 ± 2.0

101.7 ± 6.3

107.4 ± 7.0

0

nd.

nd.

50

87.5 ± 1.0

86.7 ± 2.0

0

nd.

nd.

50

85.1 ± 1.2

86.4 ± 4.8

0

nd.

50

73.5 ± 2.0

0

an

Spiked concentration (µg L-1)

nd.

Milk sample 2 50

nd.

101.6 ± 4.5

nd.

nd.

70.3 ± 4.7

77.6 ± 3.8

nd.

nd.

nd.

78.6 ± 3.0

81.4 ± 5.8

79.5 ± 5.0

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Milk sample 1

104.1 ± 5.4

cr

Lake water 2

River water

ip t

Lake water 1

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Highlights

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ZIF-8 was sued as sorbent for on-line SPE of tetracyclines in water samples Efficient extraction of tetracyclines was achieved on ZIF-8 packed column The developed method offers wide linear range, good precision and low detection limit

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