A Simple, Sensitive, and Reliable Method for the Simultaneous Determination of Multiple Antibiotics in Vegetables through SPE-HPLC-MS/MS

Antibiotics, widely used in livestock breeding, enter the environment through animal manure because of incomplete absorption in animals, especially the farmland ecosystem. Therefore, antibiotics may be adsorbed by plants and even become hazardous to human health through the food chain. In this study, a simple, sensitive, and reliable method was developed for the simultaneous determination of eleven antibiotics, including four sulfonamides, two tetracyclines, three fluoroquinolones, tylosin, and chloramphenicol in different vegetable samples using SPE-HPLC-MS/MS. Vegetable samples were extracted by acetonitrile added with hydrochloric acid (125:4, v/v). The extracts were enriched by circumrotating evaporation, and then cleaned through SPE on a hydrophilic-lipophilic balance (HLB) cartridge. All compounds were determined on a C18 reverse phase column through HPLC-MS/MS. The mean recoveries of 11 antibiotics from spiked samples of vegetables ranged from 71.4% to 104.0%. The limits of detection and quantification were 0.06–1.88 μg/kg and 0.20–6.25 μg/kg, respectively. The applicability of this technique demonstrated its good selectivity, high efficiency, and convenience by the analysis of 35 vegetable samples available from a vegetable greenhouse. Antibiotic residues in vegetables have aroused wide concern from the public. Therefore, standards should be established for antibiotic residues in vegetables to ensure food safety and human health.


Introduction
Antibiotics have been widely used to treat infectious diseases and promote growth for livestock and poultry [1,2].It was estimated that about 14,600 tons of antibiotics were produced for animals in the United States in 2012 [3].In China, 52% of all antibiotics (approximately 162,000 tons) were used for veterinary medicine in 2013 [4].However, antibiotics can be weakly absorbed and incompletely metabolized in animal guts, and 30%-90% of administered antibiotics are excreted into the environment via feces and urine in an unchanged form [5].The majority of excrements containing antibiotic residues have been frequently applied in agricultural fields at concentrations of µg/kg to mg/kg levels [6,7].

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The persistence of antibiotics may have potential threats to the agro-ecological environment, especially vegetation growth and safety [8].Existing studies have found that residual antibiotics in the soil have a great negative impact on the environmental non-target organisms, such as wheat and maize [9,10].Furthermore, antibiotics can be taken up by various plants, crops, and soil animals.Residual antibiotics from the soils can be absorbed by vegetables on farmland as they grow, and even harm human health through the food chain [11,12].There is evidence that antibiotics can perform biological accumulation and become distributed in the sequence leaf > stem > root [7].Therefore, a wide concern has been aroused about vegetation safety and human health.
The characteristics of antibiotics are different because of their various chemical structures.In addition, multiple types of antibiotics are difficult to simultaneously analyze [13].Although methods have been developed for the determination of multiple antibiotic residues in samples, most of them have focused on the detection of residues in environmental media such as soil, animal manures, sludge, and sewage.Additionally, some methods for food products have centered on matrices like milk, honey, and meat (Table 1).Vegetables have just become noticed following environmental media including soil and manure; in addition, the simultaneous extraction of antibiotic residues from vegetables is more difficult than from liquid food samples due to the presence of pigments such as chlorophyll, xanthophyll, and so on.Therefore, only a few methods have been described in the literature for the analysis of multiple antibiotics in vegetables.The analytical methods in vegetables were developed with detection limits in the range of 0.021-0.092µg/kg and 0.575-1.538µg/kg using UPLC-ESI-MS/MS and HPLC-FLD, respectively, but only for quinolone antibiotics [14,15].Yu et al. [16] developed a QuEChERS-UHPLC-MS/MS for the determination of multiple antibiotics in leafy vegetables with average recoveries of only 57%-91% and limits of detection of 0.33-2.92µg/kg.Comparing the existing methods [14][15][16][17][18], the present method was developed for the simultaneous determination of multiple classes of antibiotics with lower detection limits, which was highly sensitive and conveniently operated.The objective of the present study is to develop a simple, sensitive, and reliable method to simultaneously determine eleven target antibiotics in various types of vegetables using solid phase extraction (SPE) with high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS).The selected antibiotics include sulfonamides, tetracyclines, fluoroquinolones, tylosin, and chloramphenicol.Different proportions of extracting solvents for sample extraction and several SPE cartridges for clean-up were compared.The limits of detection (LODs), the limits of quantification (LOQs), recoveries, and linearity of the method were evaluated in detail.Finally, the method was applied to determinate 15 vegetable samples from a greenhouse and to validate the feasibility.

Validation of Extracting Agents
The pH is one of the dominant factors on varying physicochemical properties of multiple-class antibiotics.For example, when the pH is between pKa1 and pKa2, tetracyclines present a zwitterion (±0), while sulfanilamides are neutral molecules [17,29,30].To obtain better and interference-less extracts, it is necessary to adjust the suitable solution pH value.Acetonitrile was reported to be a type of effective extract solution.During the extraction procedure, different proportions of the extraction solutions containing acetonitrile (ACN) and hydrochloric acid (HCl) with the volume ratio of 125:1 (M1), 125:4 (M2), and 125:8 (M3) were chosen as the extracting agents (Figure 1).Among the solvents, ACN/HCl (125:4, v/v) significantly promoted the recovery of analytes in vegetable samples.The recoveries of the target antibiotics from vegetables ranged from 60.9% to 100.7%, with relative standard deviations (RSDs) of between 1.0% and 6.8%.Extracting agents M1 and M3 gave lower recoveries with a range of 18.1%-58.2%and 24.9%-64.1%,with RSDs of 1.8%-8.2%and 0.9%-8.7%,respectively.ACN and HCl with the volume ratio of 125:4 could provide an appropriate acid base environment for the simultaneous extraction of multiple antibiotics, which can significantly improve the ionization efficiencies and recoveries of antibiotics.
The objective of the present study is to develop a simple, sensitive, and reliable method to simultaneously determine eleven target antibiotics in various types of vegetables using solid phase extraction (SPE) with high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS).The selected antibiotics include sulfonamides, tetracyclines, fluoroquinolones, tylosin, and chloramphenicol.Different proportions of extracting solvents for sample extraction and several SPE cartridges for clean-up were compared.The limits of detection (LODs), the limits of quantification (LOQs), recoveries, and linearity of the method were evaluated in detail.Finally, the method was applied to determinate 15 vegetable samples from a greenhouse and to validate the feasibility.

Validation of Extracting Agents
The pH is one of the dominant factors on varying physicochemical properties of multiple-class antibiotics.For example, when the pH is between pKa1 and pKa2, tetracyclines present a zwitterion (±0), while sulfanilamides are neutral molecules [17,29,30].To obtain better and interference-less extracts, it is necessary to adjust the suitable solution pH value.Acetonitrile was reported to be a type of effective extract solution.During the extraction procedure, different proportions of the extraction solutions containing acetonitrile (ACN) and hydrochloric acid (HCl) with the volume ratio of 125:1 (M1), 125:4 (M2), and 125:8 (M3) were chosen as the extracting agents (Figure 1).Among the solvents, ACN/HCl (125:4, v/v) significantly promoted the recovery of analytes in vegetable samples.The recoveries of the target antibiotics from vegetables ranged from 60.9% to 100.7%, with relative standard deviations (RSDs) of between 1.0% and 6.8%.Extracting agents M1 and M3 gave lower recoveries with a range of 18.1%-58.2%and 24.9%-64.1%,with RSDs of 1.8%-8.2%and 0.9%-8.7%,respectively.ACN and HCl with the volume ratio of 125:4 could provide an appropriate acid base environment for the simultaneous extraction of multiple antibiotics, which can significantly improve the ionization efficiencies and recoveries of antibiotics.The vegetable samples were determinated using SPE and HPLC-MS/MS after extraction by acetonitrile/hydrochloric acid (125:4, v/v), and the baseline of the chromatogram rose compared with the baseline of standard substances.This might be the result of antibiotic substances uniting with hydrochloric acid to form salts. Therefore, anhydrous sodium carbonate (Na 2 CO 3 ) was added to the extracts for neutralizing superfluous HCl.

Optimization of SPE Cartridges
Selective adsorption is the basis for ensuring the efficient extraction of antibiotics from vegetable media.Three kinds of common SPE cartridges including Oasis HLB (Waters, Milford, MA, USA), C 18 (Agela, Torrance, USA), and NH 2 (Agela, Torrance, USA) were used to obtain efficient clean-up of the vegetable samples.The mean recoveries for the three SPE cartridges are shown in Figure 2.Both the C 18 and NH 2 cartridges had lower recoveries than the HLB cartridge.The recoveries ranged from 28.0% to 73.6% and from 17.6% to 72.4% for the C 18 and NH 2 cartridges, respectively, while those for the HLB cartridge ranged from 60.9% to 100.9%.The difference may be due to both the C 18 and NH 2 cartridges having high selectivity of the target compounds.The HLB cartridge contained lipotropic divinyl benzene and hydrophilic N-vinyl pyrrolidone as the adsorbents in contrast to the C 18 and NH 2 cartridges, which contained silica gel.The HLB cartridge has a higher adsorption capacity for antibiotics with a high polarity.
Molecules 2018, 23, x FOR PEER REVIEW 4 of 13 The vegetable samples were determinated using SPE and HPLC-MS/MS after extraction by acetonitrile/hydrochloric acid (125:4, v/v), and the baseline of the chromatogram rose compared with the baseline of standard substances.This might be the result of antibiotic substances uniting with hydrochloric acid to form salts. Therefore, anhydrous sodium carbonate (Na2CO3) was added to the extracts for neutralizing superfluous HCl.

Optimization of SPE Cartridges
Selective adsorption is the basis for ensuring the efficient extraction of antibiotics from vegetable media.Three kinds of common SPE cartridges including Oasis HLB (Waters, Milford, MA, USA), C18 (Agela, Torrance, USA), and NH2 (Agela, Torrance, USA) were used to obtain efficient clean-up of the vegetable samples.The mean recoveries for the three SPE cartridges are shown in Figure 2.Both the C18 and NH2 cartridges had lower recoveries than the HLB cartridge.The recoveries ranged from 28.0% to 73.6% and from 17.6% to 72.4% for the C18 and NH2 cartridges, respectively, while those for the HLB cartridge ranged from 60.9% to 100.9%.The difference may be due to both the C18 and NH2 cartridges having high selectivity of the target compounds.The HLB cartridge contained lipotropic divinyl benzene and hydrophilic N-vinyl pyrrolidone as the adsorbents in contrast to the C18 and NH2 cartridges, which contained silica gel.The HLB cartridge has a higher adsorption capacity for antibiotics with a high polarity.

Method Efficiency
Each multicomponent standard contained every analyte at the same concentration.Aliquots of the 11 standard stock solutions were added to methanol to obtain 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 0.1, 0.5, 1, 5, and 10 μg/mL standard mixtures solutions to construct the calibration curves used for the quantification of target antibiotics in vegetables.All of targets showed good linearity over a range of 0.001-10 μg/mL, and the correlation coefficient (r 2 ) of all calibration curves was >0.99.The limits of detection (LOD) and the limits of quantification (LOQ) were calculated as three times and ten times the standard deviation of the measurement of control samples divided by the slope of the calibration curve, respectively (Table 2).It is noted that the present method gave very low LOD and LOQ values ranging from 0.005-0.227and 0.015-0.760μg/kg.These data were substantially lower than those (0.33-1.73 and 1.10-5.77μg/kg) obtained using QuEChERS and UHPLC-MS/MS analysis of the nineteen veterinary antibiotics in leafy

Method Efficiency
Each multicomponent standard contained every analyte at the same concentration.Aliquots of the 11 standard stock solutions were added to methanol to obtain 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 0.1, 0.5, 1, 5, and 10 µg/mL standard mixtures solutions to construct the calibration curves used for the quantification of target antibiotics in vegetables.All of targets showed good linearity over a range of 0.001-10 µg/mL, and the correlation coefficient (r 2 ) of all calibration curves was >0.99.The limits of detection (LOD) and the limits of quantification (LOQ) were calculated as three times and ten times the standard deviation of the measurement of control samples divided by the slope of the calibration curve, respectively (Table 2).It is noted that the present method gave very low LOD and LOQ values ranging from 0.005-0.227and 0.015-0.760µg/kg.These data were substantially lower than those (0.33-1.73 and 1.10-5.77µg/kg) obtained using QuEChERS and UHPLC-MS/MS analysis of the nineteen veterinary antibiotics in leafy vegetables [17].Therefore, the method developed in the present study is recommended because of the high efficiency and low costs.

Method Precision and Accuracy
For testing the precision and accuracy of the method, aliquots of standard mixtures solutions were respectively spiked into the different vegetable samples including leek, celery, lentil, carob, and cauliflower to obtain 5, 10, and 50 µg/kg of spiked samples, and they were tested four times a day for three days following the protocol described in step 3.6.The obtained data were corrected and were quantified by the established calibration curves.The accuracy was expressed in terms of recovery rates and the precision was expressed as relative standard deviation (RSD).Table 3 shows that the recovery rates when the antibiotics were added at 5, 10, and 50 µg/kg were 71.9%-100.0%,72.8%-99.2%,and 71.4%-104.0%respectively.Furthermore, the RSD of all analytes ranged from 1.2% to 13.4%.Different types of vegetables (leaf, root and stem, melon, and fruits) contained different vegetable fats, chlorophyll, and protein content.This would affect the extraction and purification of antibiotics, resulting in various recoveries of different vegetables.The results indicate that a surveillance programme for eleven veterinary compounds can be performed under the proposed chromatographic conditions.Therefore, this method can meet the requirements of different types of vegetables detection of antibiotics.According to the Decision of the European Commission 2002/657/EC [31] for the approval of a method for drug residue analysis, the average recovery of quantitative methods at analyte concentrations higher than 10 µg/kg should be 80%-110% and the RSD should not exceed 25.0%.The results presented in Table 3 indicate that all the analytes, except TYL, CIP, and CAP, meet these criteria.

Samples Analyses
Once the analytical methodology was validated, it was applied to detect the different types of vegetables.In total, 35 different vegetable samples was processed by the optimum procedures, as described in Section 3.6, with a blank sample to check and correct for possible contamination and interferences and a spiked blank at an intermediate concentration to calculate the extraction efficiency.Table 4 indicates that all target antibiotics were differently detected in the analyzed samples of scale farms.Tetracyclines were the predominant antibiotics in the different vegetables and the average residual concentration was 4.026 µg/kg.Fluoroquinolones, sulphonamides, CAP, and TYL contributed less residuals, with concentrations of 3.463, 0.123, 0.050, and 0.037 µg/kg, respectively.The results in this study are in accordance with the residual regulation of antibiotics in animal manures [20].This is explained by the fact that the antibiotics caused bioaccumulation in vegetables due to the application of animal manures to farmland.Amongst the TCs, the detection frequencies were 71% for OTC with the highest mean residual concentration of 2.578 µg/kg, the maximum residual concentration was 4.706 µg/kg, and the minimum residual concentration was below LOD.CTC was detected in all the samples and the average concentration was 1.448 µg/kg, and the maximum and minimum concentration was 4.966 and 1.043 µg/kg, respectively.FQ was detected in 34 vegetable samples.However, three FQ antibiotics were not simultaneously detected and had different levels in the samples.The detection frequency of ENR was 54%, and the average residual concentration was 0.785 µg/kg.CIP was detected in 71% of samples, with the mean concentration of 0.785 µg/kg.NOR in FQ antibiotics had the highest detection frequency (86%) and the highest average concentration (1.743 µg/kg), indicating that there is likely to be widespread use of NOR in the livestock industry because of its cheapness.Similarly, SA was only undetected in one vegetable sample, while the detection frequencies were 66% for SMN, 51% for SDMe, 63% for ST, and 71% for SMZ.The residual concentrations of SAs ranged from ND to 1.956 µg/kg, and the average concentration was 0.123 µg/kg.The average concentrations of individual antibiotics decreased in the order of ST (0.083 µg/kg), SMN (0.023 µg/kg), SMZ (0.015 µg/kg), and SDMe (0.002 µg/kg).SAs and individual SA antibiotics were found at lower levels than the first two classes of antibiotics.CAP and TYL had a relatively low detection rate in 35 samples.The detection frequency of CAP was 28%, the average concentration was 0.050 µg/kg, and the maximum concentration was 0.698 µg/kg.TYL was detected in four samples, and the average concentration was 0.037 µg/kg.
Each of the primary standards was accurately weighed (10 mg) into individual 10 mL amber volumetric flasks, dissolved, and made up to the mark with MeOH, in order to obtain individual stock standard solutions in MeOH with the concentration of 1 mg/mL, with the exception of fluoroquinolones (NOR, CIP, ENR, and NOR-D 5 ) that were prepared in methanol with 0.03% NaOH added to enhance dissolution.To obtain working standard solutions, 0.01, 0.05, 0.1, 0.5, 1, 5, 10, 50, and 100 µL aliquots of the individual stock standard solutions were added into individual 10 mL brown volumetric flasks.All the solutions were stored at 4 • C.

Vegetable Samples
For method optimization, vegetable samples were collected from an ecological farm which did not use antibiotics in Shanxi province.For method proof, Chinese chives, celery, lentils, beans, and cauliflower were collected from a greenhouse in Shandong province in August 2015.All samples were put into dark plastic bags and kept in a cooler with ice until transported to the laboratory where all samples were stored at −80 • C before analysis.

Sample Preparation
During method validation and optimization, different proportions of extract agents and various SPE cartridges were tested for analysis of the target antibiotics in vegetables.To determine antibiotic recoveries, final concentrations of 5, 10, and 50 µg/kg in vegetables were obtained by adding 1.0 mL of mixed antibiotic methanol solution, with the above respective concentrations, to 1.0 g of lyophilized vegetable sample.Each sample was mixed well and placed in the fume hood at room temperature for 24 h for complete removal of the methanol by evaporation and for interaction of the analytes with the matrix in order to approximate real conditions.Each sample was extracted and analyzed in triplicate [32,33].

Sample Extraction and Clean-Up
In the present study, the vegetable samples were lyophilized using a vacuum freeze drier and sieved through a 2 mm sieve before further handling.The extracting agents were used and extraction was assisted with ultrasound.Clean-up steps were performed using solid-phase extraction.The extraction scheme used to extract the target compounds is illustrated in Figure 3.
Molecules 2018, 23, x FOR PEER REVIEW 8 of 13 lyophilized vegetable sample.Each sample was mixed well and placed in the fume hood at room temperature for 24 h for complete removal of the methanol by evaporation and for interaction of the analytes with the matrix in order to approximate real conditions.Each sample was extracted and analyzed in triplicate [32,33].

Sample Extraction and Clean-Up
In the present study, the vegetable samples were lyophilized using a vacuum freeze drier and sieved through a 2 mm sieve before further handling.The extracting agents were used and extraction was assisted with ultrasound.Clean-up steps were performed using solid-phase extraction.The extraction scheme used to extract the target compounds is illustrated in Figure 3.

HPLC-MS/MS Analysis
The analysis was performed using an HPLC-MS/MS system consisting of an Agilent 1200 high-performance liquid chromatograph (HPLC) system and an Agilent 6410 tandem triple-quadruple mass spectrometer (MS/MS) with an electrospray ionization (ESI) interface (Agilent Technologies, Santa
For MS/MS detection, the instrument was operated in positive ion mode, with a capillary voltage of 3846 V, a drying gas temperature of 300 • C, and a drying gas flow rate of 10 L/min.Quantification of the selected substances was obtained using multiple reaction monitoring (MRM) detection.The MS/MS spectrogram and the monitored ions of the target analytes are shown in Figure 4 and Table 5, respectively.The chromatographic, interface, and MS/MS detector operating conditions are given in a detailed description in Supporting Information Table S2.For MS/MS detection, the instrument was operated in positive ion mode, with a capillary voltage of 3846 V, a drying gas temperature of 300 °C , and a drying gas flow rate of 10 L/min.Quantification of the selected substances was obtained using multiple reaction monitoring (MRM) detection.The MS/MS spectrogram and the monitored ions of the target analytes are shown in Figure 4 and Table 5, respectively.The chromatographic, interface, and MS/MS detector operating conditions are given in a detailed description in Supporting Information Table S2.

Detection of Antibiotics in Vegetables
A total of 1.0 g (±0.01 g) of vegetable sample was placed in a 50 mL centrifuge tube (Corning, New York, NY, USA) and 10 mL of acetonitrile/hydrochloric acid (125:4, v/v) was added to the tube.After vortexing (Berlin Wiggens, Berlin, Germany) the sample for 1 min, the mixture was sonicated (Ningbo Scientz, Ningbo, China) for 15 min at 4 °C and then centrifuged (Sartorius Sigma, St. Louis, MO, Germany) at 8000 g for 15 min at 4 °C.The supernatant was decanted into another 50 mL centrifuge bottle.The pellet was repeatedly extracted once with the same procedure using 10 mL of acetonitrile/hydrochloric acid (125:4, v/v), and the second supernatant was decanted into the same bottle.A total of 0.495 g of Na2CO3 was added to the supernatant for neutralizing superfluous HCl.After standing for 8 h, the extraction was centrifuged at 8000 g for 10 min at 4 °C and filtered through 0.22 μm of PVDF syringe filters (Tianjin Jinteng, Tianjin, China) into a 50 mL of round-bottom flask.The extract solution was concentrated to 3-5 mL on a rotary evaporator (70 rpm, 40 °C) (Hydrographic Guelph, Germany), and the liquid was purified and concentrated using an Oasis HLB (6 cm 3 , 500 mg) cartridge from Waters (Millford, MA, USA), which was preconditioned with 5 mL of methanol and 10 mL of DI water.The analyte was passed through the cartridge at a flow rate of 1 mL/min.After isolation, the cartridge was rinsed with 5 mL of DI water and dried under vacuum for 5 min.The analyte was eluted using 10 mL of ACN with HAc (99:1, v/v).The eluate was then evaporated to near dryness at 40 °C and redissolved in 1 mL of ACN with

Detection of Antibiotics in Vegetables
A total of 1.0 g (±0.01 g) of vegetable sample was placed in a 50 mL centrifuge tube (Corning, New York, NY, USA) and 10 mL of acetonitrile/hydrochloric acid (125:4, v/v) was added to the tube.After vortexing (Berlin Wiggens, Berlin, Germany) the sample for 1 min, the mixture was sonicated (Ningbo Scientz, Ningbo, China) for 15 min at 4 • C and then centrifuged (Sartorius Sigma, St. Louis, MO, Germany) at 8000 g for 15 min at 4 • C. The supernatant was decanted into another 50 mL centrifuge bottle.The pellet was repeatedly extracted once with the same procedure using 10 mL of acetonitrile/hydrochloric acid (125:4, v/v), and the second supernatant was decanted into the same bottle.A total of 0.495 g of Na 2 CO 3 was added to the supernatant for neutralizing superfluous HCl.After standing for 8 h, the extraction was centrifuged at 8000 g for 10 min at 4 • C and filtered through 0.22 µm of PVDF syringe filters (Tianjin Jinteng, Tianjin, China) into a 50 mL of round-bottom flask.The extract solution was concentrated to 3-5 mL on a rotary evaporator (70 rpm, 40 • C) (Hydrographic Guelph, Germany), and the liquid was purified and concentrated using an Oasis HLB (6 cm 3 , 500 mg) cartridge from Waters (Millford, MA, USA), which was preconditioned with 5 mL of methanol and 10 mL of DI water.The analyte was passed through the cartridge at a flow rate of 1 mL/min.After isolation, the cartridge was rinsed with 5 mL of DI water and dried under vacuum for 5 min.The analyte was eluted using 10 mL of ACN with HAc (99:1, v/v).The eluate was then evaporated to near dryness at 40 • C and redissolved in 1 mL of ACN with ultrapure water (20:80, v/v)

Figure 1 .
Figure 1.Comparison of 11 antibiotics recoveries from three different extraction methods for vegetables.Figure 1.Comparison of 11 antibiotics recoveries from three different extraction methods for vegetables.

Figure 1 .
Figure 1.Comparison of 11 antibiotics recoveries from three different extraction methods for vegetables.Figure 1.Comparison of 11 antibiotics recoveries from three different extraction methods for vegetables.

2 Figure 2 .
Figure 2. Comparison of 11 antibiotics recoveries from three different SPE cartridges for vegetables.

Figure 2 .
Figure 2. Comparison of 11 antibiotics recoveries from three different SPE cartridges for vegetables.

Figure 3 .
Figure 3. Procedure used for extraction of antibiotics.Optimized extraction, clean-up, and elution procedures developed in the present study are given.

Figure 3 .
Figure 3. Procedure used for extraction of antibiotics.Optimized extraction, clean-up, and elution procedures developed in the present study are given.

Table 1 .
Comparison of existing determination methods for antibiotics residues in different environments.
Molecules 2018,23,x FOR PEER REVIEW 9 of 13 Clara, CA, USA).The chromatographic separation was carried out with the use of a Waters Atlantis Sunfire C18 column (150 mm × 4.6 mm, 3.5 μm) at 35 °C.The flow rate was maintained at 0.3 mL/min, and the injection volume was 5 μL.The mobile phases A and B were 0.1% formic acid in water (A) and acetonitrile (B), respectively.The mobile phase gradient elution procedure took 28 min, as follows:

Table 5 .
Precursor masses and product ions for mass spectrometry MRM analysis of the selected antibiotics.

Table 5 .
Precursor masses and product ions for mass spectrometry MRM analysis of the selected antibiotics.