Furosemide

Pronase and methylene blue were obtained from Sigma-Aldrich (Taufkirchen, Germany). NaCl, KCl, MgSO₄, Ca(NO₃)₂, and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) were obtained from Carl Roth (Karlsruhe, Germany). Tryptophan-d₅ was obtained from Alsachim (Illkirch Graffenstaden, France). Telmisartan-d₇ was purchased from Sigma (Taufkirchen, Germany). Trimipramin-d₃, bisoprolol-d₅, furosemide, glibenclamid and hydrochlorothiazide were obtained from LGC (Wesel, Germany). Ammonium formate, ammonium acetate (both analytical grade), formic acid (LC–MS grade), d-glucose-d₇ and palmitic acid-d₃₁ were purchased from Merck (Darmstadt, Germany). Acetonitrile (ACN, LC–MS grade), methanol (MeOH, LC–MS grade), and all other chemicals and reagents (analytical grade) were from VWR (Darmstadt, Germany). Chloroform (analytical reagent grade) was from Fisher (Schwerte, Germany). Deionized water was produced by a Millipore system (18.2 Ω × cm water resistance) from Merck (Darmstadt, Germany). Reaction tubes and pipette tips were obtained from Sarstedt (Nümbrecht, Germany). 2 mL homogenizer tubes prefilled with acid washed 0.5 mm glass beads were obtained from Biozym (Hess, Germany).

The internal standards (IS) canrenone-d₄, hydrochlorothiazide (HCT)-¹³C₆, lisinopril-d₅, torasemide-d₆, and ramiprilat-d₅ were purchased from Alsachim (Illkirch Graffenstaden, France), canrenone from European Pharmacopoeia (Strasbourg, France), HCT, furosemide, and torasemide from LGC (Luckenwalde, Germany), lisinopril from Merck (Darmstadt, Germany), ramiprilat from Aventis Pharma (Strasbourg, France), and enalaprilat from Sigma Aldrich (St. Louis, USA). All other chemicals (LC-MS grade or analytical grade) were from VWR (Darmstadt, Germany). Mitra VAMS with a 10 μL absorbing tip were purchased from Neoteryx (Torrance, USA). Blank blood, stabilized with ethylenediaminetetraacetic acid (EDTA), used for development and validation of the procedure was submitted to the authors’ laboratory for regular toxicological analysis and handled in accordance with the institutional protocol and regulations concerning data privacy and sample handling. FPB loaded VAMS, matching EDTA-stabilized blood samples, and medication plans for applicability studies were collected from volunteers with hypertension as part of their regular consultation at Saarland University Hospital, Homburg, Germany. All volunteers provided written informed consent and the local ethic committees approved the study (Homburg Hypertonie Register, 142/10).