1. Introduction
Albendazole (ABZ) is a benzimidazole drug and a broad-spectrum, highly effective anthelmintic drug widely used in veterinary clinics. ABZ is metabolized into its major active metabolites albendazole sulfone (ABZ-SO
2), albendazole sulfoxide (ABZ-SO) and albendazole-2-aminosulfone (ABZ-2NH
2-SO
2) in animals. The chemical structures of ABZ, ABZ-SO
2, ABZ-SO, and ABZ-2NH
2-SO
2 are shown in
Figure 1.
With increasing large-scale breeding, various parasitic diseases are seriously hindering economic output in the animal husbandry industry [
1]. Many kinds of parasites infect animals, and parasites can parasitize various organs and tissues. ABZ has a good effect on gastrointestinal nematodes, lung nematodes, worms, and ectoparasites in pigs [
2,
3]. This drug is especially effective against
Ascaridia galli and other worms in chickens [
4]. Therefore, in many countries, ABZ is often used to treat these infections. However, some breeders do not use these drugs according to regulations, and abuse them in pursuit of greater economic benefits. These behaviors cause large amounts of these drugs to remain in animal-derived foods, such as muscle, fat, and internal organs, which can endanger the health of consumers [
5]. In addition, animal toxicological studies have shown that ABZ and its metabolites may cause malformations and embryonic lethality [
6,
7]. Therefore, it is necessary to develop efficient and rapid detection methods for residues of ABZ and its three metabolites in animal products, to avoid harming consumer health. Most countries have stipulated the maximum residue limit (MRL) of ABZ in animal products [
8,
9]. The Ministry of Agriculture and Rural Affairs of the People’s Republic of China, the Codex Alimentarius Commission (CAC), and the European Union (EU) have stipulated that the MRL of ABZ in the muscles of all edible animals should not exceed 100 μg/kg, and the limit of the ABZ residue is based on the sum of ABZ and its three metabolites. This study adopted an MRL of 100 μg/kg in animal muscle as the standard, and developed a simultaneous detection method of ABZ and its three metabolites in pig and poultry muscle.
Many quantitative methods, such as high-performance liquid chromatography-ultraviolet detection (HPLC-UV) [
10,
11], high-performance liquid chromatography-fluorescence detection (HPLC-FLD) [
12,
13,
14,
15,
16], and liquid chromatography-mass spectrometry (LC-MS) [
17,
18,
19,
20,
21], have been established for the simultaneous detection of ABZ and its metabolites in food-producing animals. Among these methods, the most widely used has been LC-MS because of its high sensitivity and accuracy. However, LC-MS instruments are very expensive, causing the detection cost to be relatively high, and high-purity reagents and highly trained operators are strictly required. The HPLC-FLD method is time-consuming and has low detection efficiency, and some HPLC-FLD studies could not quantify all metabolites of ABZ. The detection process was very difficult and complicated when gas chromatography-mass spectrometry methods were used to determine ABZ and its metabolites, due to the basic nature and low volatility of these substances. The sensitivity of HPLC-UV was also not high enough [
17]. It is very important to develop accurate, stable, and low-cost detection methods. At present, to our knowledge, there is no report on the simultaneous detection of ABZ and its three metabolites in pig or poultry muscle by ultrahigh-performance liquid chromatography-fluorescence detection (UPLC-FLD). Therefore, this study intended to develop a rapid, easy, and reliable UPLC-FLD method using liquid–liquid extraction (LLE) combined with solid-phase extraction (SPE) technology as a sample preparation technique to establish simultaneous detection of ABZ and its three metabolite residues in pig and poultry muscle (chicken, duck, goose).
2. Materials and Methods
2.1. Standards and Reagents
ABZ (CAS No. 54965–21–8, purity ≥ 98.0%), ABZ-SO2 (CAS No. 75184–71–3, purity ≥ 99.0%), and ABZ-SO (CAS No. 54029–12–8, purity ≥ 98.0%) were purchased from Sigma-Aldrich (MO, USA). ABZ-2NH2-SO2 (CAS No. 80983–34–2, purity ≥ 99.8%) was obtained from TMstandard (Changzhou, China).
HPLC-grade formic acid (purity ≥ 98%), trimethylamine (purity ≥ 99%), methanol (purity ≥ 99.9%) and acetonitrile (purity ≥ 99.9%) were purchased from Tokyo Chemical Industry (Tokyo, Japan), Thermo Fisher Scientific (MA, USA), and Merck (Darmstadt, Germany). Analytical-grade ethyl acetate, n-hexane and ammonia were supplied by Sinopharm Chemical Reagent Co. (Shanghai, China). The experimental water was ultrapure water (the resistivity reached 18.2 MΩ × cm, 25 °C).
2.2. Equipment
An ACQUITY UPLC System (Waters Corp, Milford, MA, USA) coupled to a fluorescence detector (Waters Corp, Milford, MA, USA) was used. Data acquisitions were performed by Empower 3 software.
Other equipment, such as an EnSpire multimode plate reader (PerkinElmer, Waltham, MA, USA), P300H-type ultrasonic cleaner (Elma, Munich, Germany), Vortex-Genie 2 vortex oscillator (Scientific Industries, MA, USA), N-Evap 112 nitrogen blower (Organomation, Columbus, OH, USA), 5810R high-speed refrigerated centrifuge (Eppendof, Hamburg, Germany), JJ-2/FK-A tissue homogenizer (Jiangsu Kexi Instrument Co., Ltd., Jiangsu, China), and AX205 analytical balance (Mettler-Toledo, Zurich, Switzerland), were also used in this study.
2.3. Standard Solutions
Stock standard solutions of ABZ, ABZ-SO2, ABZ-SO and ABZ-2NH2-SO2 were prepared by dissolving 2.5 mg of individual analytes in 25 mL of methanol to obtain a final concentration of 100.0 μg/mL. The stock standard solutions were stored stably for 3 months at −70 °C in the dark. The solutions required shaking for the analytes to fully dissolve, and sonication was applied to assist ABZ dissolution.
Working standard solutions (10.0, 1.0, and 0.1 μg/mL) of ABZ, ABZ-SO2, ABZ-SO and ABZ-2NH2-SO2 were prepared by diluting the stock standard solution (100.0 μg/mL) of each compound with acetonitrile and 0.2% formic acid aqueous solution containing 0.05% triethylamine (31:69, v/v). The working standard solutions were stored stably for 3 months at −34 °C in the dark.
2.4. Blank Samples and Sample Preparation
This study was conducted in accordance with the ethics requirements of the official ethical committee of our university. Muscle samples were obtained from chickens, ducks, geese, and a pig, none of which had received any medication. The samples were stored at −34 °C in a refrigerator after they were homogenized with a tissue homogenizer.
Two grams of homogeneous blank muscle were accurately weighed in 50 mL polypropylene centrifuge tubes after thawing. Samples were spiked with 15 mL of ethyl acetate, vortexed for 5 min by a vortex oscillator, ultrasonically extracted for 5 min, and centrifuged for 10 min at 12,000 rpm (4 °C). The supernatant was transferred into a new centrifuge tube after centrifugation. Subsequently, the precipitate was again extracted as before, and the supernatant was combined with that from the first extraction. The supernatants were blown to near dryness with nitrogen. The residue sample was dissolved in 5 mL of mobile phase and spiked with 15 mL of n-hexane saturated with acetonitrile for degreasing. Then, the sample was vortexed for 2 min and centrifuged for 5 min at 6000 rpm (4 °C). Subsequently, the n-hexane layer was discarded, and the supernatant was collected. After OASIS® PRiME HLB SPE cartridges (60 mg/3 mL, Waters Corp, USA) were conditioned by 3 mL of methanol and 3 mL of ultrapure water, the supernatant was purified by OASIS® PRiME HLB SPE cartridges. Then, the samples were eluted sequentially with 3 mL of mobile phase and 3 mL of 20% ammoniated methanol (ammonia water:methanol = 2:8, v/v), and the eluate solution was collected into 10 mL centrifuge tubes. The eluate solution was blown to near dryness with nitrogen. The residue sample was reconstituted with 2.0 mL of mobile phase, and the mixture was vortexed at low speed for 1 min. Finally, the samples were passed through a hydrophilic PTFE type (13 mm × 0.22 μm) needle (Thermo Fisher Scientific, USA), and the filtrate was analyzed by UPLC-FLD.
2.5. UPLC-FLD Analysis
A UPLC system equipped with a fluorescence detector was employed. Compound separation was executed on an ACQUITY UPLC® BEH C18 (2.1 mm × 100 mm, 1.7 μm) chromatographic column (Waters Corp, USA) connected to a VanGuardTM BEH C18 (2.1 mm × 5 mm, 1.7 μm) guard column (Waters Corp, USA) with an appropriate column temperature of 35 °C. Mobile phase A was acetonitrile, and mobile phase B consisted of 0.2% formic acid aqueous solution containing 0.05% trimethylamine. Mobile phases A and B were degassed quickly for 20 min by an ultrasonic cleaner before they were used. Isocratic elution was utilized in the method, and the ratio of mobile phases A and B was 31:69 (v/v). The flow rate was 0.25 mL/min, and the injection volume was 5 µL. The total run time was 6 min. The excitation wavelength and emission wavelength of the four compounds were 286 and 335 nm, respectively.
2.6. Method Validation
The procedure of the UPLC-FLD method was validated by referring to the requirements of the EU [
22]. The validation criteria involved sensitivity, linearity, recovery and precision.
2.6.1. Sensitivity
The sensitivity of the method was assessed in terms of the LODs and LOQs. When the signal-to-noise ratio (S/N) ≥ 3, the corresponding additive concentration was the LOD of the analytical method. When S/N ≥ 10, the corresponding additive concentration was the LOQ of the analytical method; at the same time, the concentration met the accuracy and precision requirements (recovery ≥ 70%, relative standard deviation (RSD) ≤ 20%). The working standard solutions of ABZ, ABZ-SO2, ABZ-SO and ABZ-2NH2-SO2 were diluted stepwise with each blank muscle matrix extract solution to give solutions of different concentrations. Then, each concentration of the four compounds when S/N ≥ 3 and S/N ≥ 10 was detected 3 times by UPLC-FLD, and the average S/N was obtained.
2.6.2. Linearity
Mobile phases A and B (31:69, v/v) were used to dilute the working standard solutions of ABZ, ABZ-SO2, ABZ-SO and ABZ-2NH2-SO2 to a series of concentrations (10.0, 20.0, 25.0, 50.0, 100.0, 200.0 and 400.0 μg/L for ABZ; 1.0, 10.0, 25.0, 50.0, 100.0, 200.0 and 400.0 μg/L for ABZ-SO2; 8.0, 10.0, 25.0, 50.0, 100.0, 200.0 and 400.0 μg/L for ABZ-SO; and 1.5, 10.0, 25.0, 50.0, 100.0, 200.0 and 400.0 μg/L for ABZ-2NH2-SO2), and these solutions were then analyzed 5 times by the optimized UPLC-FLD method. Calibration curves were prepared using the peak areas as the ordinate (Y) and the concentrations of the working solutions as the abscissa (X).
2.6.3. Recovery and Precision
The recovery and precision were determined by analyzing blank muscle samples spiked with each compound in six replicates at the LOQ, 0.5 MRL, 1.0 MRL and 2.0 MRL levels. Recoveries were determined by comparing the chromatographic peak areas of extracted analytes from calibration curves of each compound. Precision, including intraday precision and interday precision, was evaluated by RSD. The intraday RSD was determined by analyzing blank muscle samples spiked with each compound in six replicates at the four concentration levels at different times on the same day, and the interday RSD was determined by analyzing blank muscle samples spiked with each compound in six replicates at the four concentration levels on different days.