Digitube tubes

The water used was 18 megohm water, purified with a Milli-Q^® Integral 10 system (Merck, Darmstadt, Germany).
A selenium standard solution of 1000 mg/L, and a multi-element standard solution IV (at the concentration of 1000 mg/L): Ag, Al, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Mg, Mn, Na, Ni, Pb, Sr, Tl, Zn, were purchased from Merck KGaA (Darmstadt, Germany).
Calibration blank (Cal Blk) and calibration standard (Cal Std) solutions were prepared at concentrations of 0 μg/L (Cal Blk), 4 μg/L (Cal Std 1), 12 μg/L (Cal Std 2), and 20 μg/L (Cal Std 3), using the selenium standard stock solution, or multi-element standard stock solution, into separate 50 mL DigiTUBE^® tubes (SCP SCIENCE, Baie-D’Urfe, QC, Canada) with the addition of 0.5 mL internal standard stock solution, 0.5 mL methanol and 5 mL concentrated nitric acid, and then diluted to the final volume with water.

Fine powder (50 mg) of freeze‐dried samples was digested in 2 ml of 60% HNO₃ at 110°C in DigiTUBE® tubes (SCP SCIENCE, Québec, Canada) for approximately 2 hr. Once the samples had cooled, 2 ml of 60% HClO was added and the samples were heated at 110°C for a further approximately 2 hr. Once digestion was completed, the samples were cooled and made up to a volume of 10 ml with ultra‐pure water. The total concentration of the following 13 elements was measured, based on selected specific wavelengths using an ICP‐OES (iCAP 7400; Thermo Fisher Scientific, Waltham, MA, USA): Al 396.152 nm, Fe 259.940 nm, sodium (Na) 589.592 nm, boron (B) 249.773 nm, P 213.618 nm, sulfur (S) 180.731 nm, silicon (Si) 251.611 nm, Ca 315.887 nm, copper (Cu) 324.754 nm, K 766.490 nm, Mg 285.213 nm, Mn 257.610 nm, and zinc (Zn) 206.200 nm.
Total carbon (C) and N were measured by dry combustion using a Vario MAX cube (Elementar, Hanau, Germany) with aspartic acid as a standard.

Calibration blank (Cal Blk) and calibration standard (Cal Std) solutions were prepared at concentrations of 0 μg L⁻¹ (Cal Blk), 4 μg L⁻¹ (Cal Std 1), 12 μg L⁻¹ (Cal Std 2), and 20 μg L⁻¹ (Cal Std 3), using the selenium standard stock solution, or multi-element standard stock solution, into separate 50 mL DigiTUBE^® tubes (SCP SCIENCE, Baie-D’Urfe, QC, Canada) with the addition of 0.5 mL internal standard stock solution, 0.5 mL methanol and 5 mL concentrated nitric acid, and then diluted to the final volume with water. A Shimadzu GFA-EX7i graphite furnace mounted on a Shimadzu model 6300 atomic absorption spectrophotometer was used with a Shimadzu ASC-6100 autosampler. Pyrolytically coated graphite tubes and background correction (D2 lamp) were utilized. Working solutions were prepared by diluting immediately before use standard solutions for absorption a spectroscopy.