Tca solution

Normal human serum purchased from Merck (Darmstadt, Germany) was tested. Frozen human serum was thawed at room temperature. Then, 100 μL of the human serum sample was 100 times diluted in deionised water spiked with appropriate concentrations of the analyte, transferred to a centrifugal tube, mixed with 50 µL of 7.5% (w/v) TCA solution (Sigma-Aldrich, Saint Louis, MO, USA) in order to precipitate proteins, centrifuged at 4000× g for 10 min, and filtered through a 0.22 µm Millipore filter. The supernatant was analysed in triplicate by the optimised voltammetric procedure and UHPLC-ESI-MS method.

The electrochemical experiments were performed using a µAutolab analyzer (Eco Chemie, Utrecht, The Netherlands) controlled by GPES 4.9 software (CV, AdSV) or FRA 4.9 software (EIS) in a 10 mL quartz cell with a commercially available screen-printed sensor (Metrohm-DropSens, Oviedo, Spain). The sensors consisted of a carbon working electrode (diameter of 4 mm), a carbon auxiliary electrode, and a silver pseudo-reference electrode (SPCE, ref. 110).
The 1 M solution of HNO₃, CH₃COOH, and acetate buffers with a pH of 3.5, 4.0, and 4.5 were prepared from Merck (Darmstadt, Germany) reagents. Additionally, Merck standard solutions of Ca(II), Mg(II), Fe(III), and Cl(-I), as well as Sigma-Aldrich (Saint Louis, MO, USA) reagents (epinephrine, dopamine, uric acid, glucose, and ascorbic acid) were used in interference studies. The 1.0 and 0.1 mM solutions of NDIT were prepared in N,N-dimethylformamide (Sigma-Aldrich, Saint Louis, MO, USA). Normal human serum was purchased from Merck (Darmstadt, Germany). In order to precipitate proteins from serum samples, 15% (w/v) TCA solution (Sigma-Aldrich, Saint Louis, MO, USA) was used.