Appropriate aliquots of the standard stock solutions of
nitrate and nitrite, together with a fixed amount of sulfite
(internal standard, IS) were transferred to separate 10-mL
volumetric flasks. The volumes were completed with
deionized water. The concentration ranges were 0.2–
2.5 mg L−1 (nitrite) and 0.5–5 mg L−1 (nitrate). Each
solution was injected in triplicate. Peak area ratios (analyte/IS)