Magnesium sulfate anhydrous

Citric acid monohydrate (≥ 99%), disodium hydrogen phosphate dodecahydrate (≥ 99%), EDTA (Ethylendiaminetetraacetic acid; p.a.), sodium chloride (p.a.), magnesium sulfate anhydrous (p.a.), Sodium sulfate anhydrous (p.a.) and hydrochloric acid (35%) were purchased from Lach:ner (Czech Republic). Magnesium nitrate hexahydrate (> 99%), ammonium (25%), methanol (LC–MS grade), acetonitrile (LC–MS grade), and water (LC–MS grade) were purchased from VWR (USA). Sodium hydroxide (> 98%) was purchased from Penta. Formic acid (LC–MS grade), enrofloxacin (> 99%), ciprofloxacin (> 98%), enrofloxacin-d₅ (> 99%), and ciprofloxacin-d₈ (> 99%) were purchased from Sigma Aldrich (USA). OASIS HLB (Hydrophylic-Lipophilic Balanced) cartridges (200 mg; 6 mL; particle diameter 25–35 μm) were purchased from WATERS (USA). Nitrogen (4.7) and argon (5.0) gas were purchased from Siad (Czech Republic). Nylon syringe filters (13 mm, 0.22 μm) were purchased from Chromservis (Czech Republic). Physico-chemical properties of FQs can be found in Table S1.

The pesticide standards were purchased from Restek (Bellefonte, Pennsylvania, USA) and Lab Instruments (Castellana Grotte, Italy). HPLC-MS-grade acetonitrile, methanol, formic acid, and acetic acid were obtained from Lachner (Neratovice, Czech Republic). Sodium chloride, magnesium sulfate anhydrous, disodium hydrogen citrate sesquihydrate (C₆H₈Na₂O₈), and trisodium citrate dihydrate (C₆H₅O₇Na₃ × 2H₂O) were obtained from Lachner (Czech Republic). The graphitized carbon black (GCB), C18 sorbent, and primary and secondary amines (PSA) were supplied from Sigma-Aldrich (St Louis, Missouri, USA).
The selected citrus fruits were purchased from local markets in Belgrade, Serbia, in February 2022, and a total of 76 samples were analyzed (28 oranges, 26 lemons, 17 tangerines, and 5 grapefruits). Each fruit sample was analyzed as the whole fruit following European Union Guidelines Regulation, (EC) No. 396/2005 Annex [20 ]. During the survey, fruits were stored in fridges in labeled plastic bags at 4 °C until analysis.