Milli q gradient water system

Ethephon, HEPA, fosetyl-Al, glyphosate, N-acetyl-glyphosate, AMPA, and N-acetyl-AMPA reference standards were purchased from Dr Ehrenstorfer GmbH (Schlosser, Augsburg, Germany). N-Acetyl-d3-glufosinate, used as the internal standard, was acquired from Sigma-Aldrich (Saint Louis, MO, USA). Purity of all compounds was ≥99.7%.
Stock standard solutions of each compound (1 mg mL⁻¹) were prepared by exact weighing of the solid substances and dissolved in 50 mL of solvent (methanol or a mixture of methanol:water), according to the instructions provided by EURL [7 ], and they were stored at −18 °C without being exposed to light. Then, a working standard solution (at 10 mg L⁻¹), containing the polar pesticides, was prepared in an aqueous solution (10% acetonitrile) and was stored as the stock standard solutions. The stock standard solutions were stable up to one year and working standard solutions were prepared every two months.
LC-MS grade methanol, acetonitrile and water were purchased from Honeywell (LC-MS grade, Morrison, NJ, USA) while ultrapure water was obtained by a Milli-Q water gradient system (Millipore, Bedford, MA, USA). Formic acid was purchased from Fisher Scientific (Erembodegem, Belgium).
Finally, 0.22 µm nylon syringe filters were used and they were acquired from Agilent Technologies (Santa Clara, CA, USA).

Amoxicillin was purchased from Duchefa (Holland). Ampicillin, metronidazole, cefuroxime, ciprofloxacin, trimethoprim, and sulfamethoxazole were purchased from Fluka (Denmark). Tetracycline hydrochloride was purchased from Sigma-Aldrich, and doxycycline hydrochloride from Nycomed (Denmark). The internal standard (IS) d4-sulfamethoxazole was purchased from Toronto Research Chemicals Canada), and the ISs d3-trimethoprim and d8-ciprofloxacin from Qmx Laboratories (England). Acetonitrille and methanol were of HPLC grade and purchased from Lab-Scan (Poland). Ammonium formate (≧99.0%), formic acid (98–199%), and triethylamine (≧99%) were purchased from Merck KGaA (Germany), and Sigma-Aldrich (Denmark), respectively. Pure water was produced in house with a Milli-Q water gradient system (Millipore, Bedford, MA, USA).
A standard antibiotic-mix with a concentration of 10 µg/mL was prepared by mixing a known amount of each stock solution with methanol in a 10 mL volumetric flask. Likewise an internal standard solution with a concentration 10 µg/mL for each IS was prepared. This standard antibiotic-mix was used to prepare all standard solutions used in all analysis. The standard antibiotic-mix was protected from light and stored at −18°C.