Nitric 5 acid

All reagents used for metal analysis—nitric (V) acid (65%) and hydrochloric acid (36%)—were of Suprapur Grade and purchased from Merck (Darmstadt, Germany). Certified stock solutions of the investigated elements (1000 mg/L, Merck) were used to plot calibration curves for each metal. An Ultrapure Millipore Direct-Q-R 3UV system (Merck) allowed us to obtain high purity deionized water (resistivity 18.2 MWcm), which was used during all spectroscopic analysis (including the digestion process). Polypropylene tubes were used to store the digests obtained during metal analysis.
The solvents used during LC-MS determinations (acetonitrile, formic acid and water) were of spectroscopic grade and were purchased from J. T. Baker (Center Valley, PA, USA). Standards of catechins (C, EGC, EGCG, ECG, and EC), gallic acid, DPPH (2,2-diphenyl-1-picrylhydrazyl), ABTS [2,2′-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] diammonium salt) and trolox were bought from Sigma-Aldrich (St. Louis, MO, USA). Folin-Ciocalteu reagent, sodium carbonate and citric acid were bought from Stanlab (Lublin, Poland), whereas potassium persulfate (analytical grade) from POCH Polish Chemicals Reagents (Gliwice, Poland).

The standards of 6-gingerol and 10-shogaol were purchased from Sigma-Aldrich (St. Louis, MO, USA). Water, formic acid, and acetonitrile of spectroscopic grade were produced by J. T. Baker (Center Valley, PA, USA).
Nitric (V) acid (65%) and hydrochloric acid (36%) were of Suprapur Grade and were bought from Merck (Darmstadt, Germany). High purity deionized water (resistivity 18.2 MΩcm) obtained using an Ultrapure Millipore Direct-Q-R 3UV (Merck, Darmstadt, Germany) was used throughout the analysis. Quartz crucibles and polypropylene recipients were applied for the digestion and storage of digests, respectively.
A series of standard solutions of Ca, Cd, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, and Zn at a concentration of 1000 mg/L (Merck, Germany) was used to plot the calibration curve for each element.
Diethyl ether and silica gel (230–400 mesh) applied in the GC-MS analysis of the extracts were produced by Merck (Germany). A C9–C25 hydrocarbon solution used as a reference mixture for GC-MS analysis was purchased from Sigma-Aldrich.