Digifilter

Nitric acid, hydrochloric acid and hydrogen peroxide were of Suprapur grade and were bought from Merck (Darmstadt, Germany). A standard solution of Se (1 mg/L) was bought from PlasmaCAL (SCP SCIENCE, Baie-D’Urfé, QC, Canada). The enzymes used during simulated digestion (pepsin and pancreatin) and the dialysis tubes were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sodium bicarbonate was bought from Avantor Performance Materials (POCH, Gliwice, Poland). High-purity deionized water (resistivity of 18.2 MWcm), obtained using an Ultrapure Millipore Direct-Q-R 3UV (Millipore, Bedford, MA, USA), was used throughout the analysis. Professional polypropylene recipients certified for trace elements analysis (DigiTubes and DigiFilters), which were applied for the digestion, filtration and digest storage, were purchased from SCP SCIENCE, Canada.

Nitric acid, hydrochloric acid, and hydrogen peroxide, which were used during the digestion stage and spectrometric determinations, were of Suprapur Grade and were bought from Merck (Darmstadt, Germany). The standard solution of Cr (1 mg/L) was bought from PlasmaCAL (SCP SCIENCE, Baie-D’Urfe, QC, Canada). Pepsin, pancreatin, and dialysis tubes, which were used to perform the simulated digestion stage, were purchased from Sigma-Aldrich (St. Louis, MO, USA). Sodium bicarbonate was bought from Avantor Performance Materials (POCH, Gliwice, Poland). During all determinations, high-purity deionized water (resistivity of 18.2 MWcm) obtained using an Ultrapure Millipore Direct-Q-R 3UV (Millipore, Bedford, MA, USA) was used. During the digestion stage, the DigiTubes and DigiFilters (SCP SCIENCE, Canada), certified for trace element analysis, were used.

Acid digestions were performed in a DigiPREP Jr block digestion system from SCP Science (Montreal, Quebec, Canada) equipped with a 24-position graphite heating block for 50 mL polypropylene (PE) digestion tubes (DigiTubes; SCP Science; Montreal, Quebec, Canada). The digestion procedure employed here was based on the conditions established in previous investigations by our research group [33 (link)]. The dried and powdered subsamples (0.25 g) were first placed in the digestion tubes with 5 mL of HNO₃, 2 mL of HCl, and 2 mL of nanopure water. Next, they were digested by applying a stepwise temperature increase procedure for 20 min up to 65 °C and maintaining this temperature for a total of 30 min. The second digestion was performed after a cooling step by adding 3 mL of H₂O₂, gradually increasing the temperature for 30 min up to 110 °C, and holding it for a total of 60 min. Prior to analysis, the digested samples were filtered through a 0.45-μm filter (DigiFILTER; SCP Science; Montreal, Quebec, Canada) using a –600 mbar vacuum port. They were then transferred to a clean 50-mL volumetric DigiTube, which was completed with nanopure water to 50 mL. All samples were carried out in triplicate.