Disodium hydrogen phosphate

Cadmium oxide (CdO; 99.5%), oleic acid (OA; technical grade 90%), octadecene (ODE; technical grade 90%), zinc acetate (Zn(Ac)₂; 99.99%), sulfur (S; 99.998%), d-penicillamine (DPA; 99%) and 3-mercaptopropionic acid (3-MPA; 99%) were purchased from Sigma-Aldrich (Saint Louis, MO, USA). Acetic acid (Ac, 99.7%), ethanol (99.8%), hexane (95%), hydrochloric acid (98.5%), sodium hydroxide (98.8%), potassium dihydrogen phosphate and disodium hydrogen phosphate were purchased from Chempur (Piekary Śląskie, Poland). All chemicals were used without further purification.

Oxytetracycline (OXT), chlortetracycline (CTC), tetracycline (TC), doxycycline hyclate (DC), tiamulin fumarate (TIAM), tylosine tartrate (TYL), ciprofloxacin (CIP), enrofloxacin (ENR), lincomycin (LINCO), trimethoprim (TRIM), activated carbon, EDTA disodium salt and formic acid were obtained from Sigma Aldrich (St. Louis, MO, USA). HPLC-grade methanol, acetonitrile and octadecyl C₁₈ 40 µm Prep LC sorbent were purchased from Baker (Deventer, The Netherlands). Disodium hydrogen phosphate and sodium hydroxide were from Chempur (Piekary Śląskie, Poland), citric acid was from Acros Organic (Morris Plains, NJ, USA) and PSA Bonded Silica sorbent was from Supelco (Bellefonte, PA, USA). Purified water was prepared with a Milli-Q water system from Millipore (Billerica, MA, USA).
McIlvaine-Na₂EDTA buffer was prepared by dissolving 11.406 g Na₂EDTA in 115.65 mL 0.2 M phosphate buffer and 184.65 mL 0.1 M citric acid. The pH was adjusted to 4.0 by adding 0.1 M citric acid or 0.2 M phosphate buffer. Three SPE cartridges were tested; OASIS HLB (60 mg, 3 mL) from Waters (Milford, MA, USA) and Strata-SAX (strong anion exchanger, 500 mg, 12 mL) and Strata-X-CW (weak cation mixed mode, 300 mg, 3 mL) both from Phenomenex (Torrance, CA, USA). An SPE manifold (J.T. Baker, PA, USA) and a pump as a vacuum source were used.