2.1. Chemicals and samples
All reagents used were of analytical grade purity. Solvents for HPLC were filtered through 0.22 μm NL 17 filters and degassed under vacuum for at least 15 min before use. Sodium nitrate, sodium nitrite, l-ascorbate and tetrabutylammonium hydrogenosulphate were supplied by Sigma Chemicals Co. (St. Louis, MO, USA), acetonitrile Licrosolv and activated charcoal were from Merk (Darmstradt, Germany) and n-octylamine was supplied by Fluka. Sep-pak cartridges were from Waters. Standard solutions of sodium nitrite (1000 mg/L) and sodium nitrate (1000 mg/L) were prepared from NaNO3 and NaNO2 previously dried in an oven (100 °C during 1 h). More diluted standard solutions, used in the calibration curves, and were obtained from the concentrated solutions by dilution. The solutions were treated with some chloroform drops to prevent the development of microorganisms and were stored in a refrigerator.