A spectrophotometric technique is developed to simultaneously quantify nitrate and nitrite in plasma treated water. The measurement is based on examining the inflection points (wavelengths) in the derivative absorbance of the nitrate or nitrite solution. At the inflection points of the pure nitrate solution, the derivative absorbance is zero and independent of the nitrate's concentration, and thus the nitrite's concentration in a mixed nitrate and nitrite solution can be obtained by using the Beer's law at these points. The nitrate's concentration can also be achieved from the inflection points of nitrite in the same manner. The relation between the tested substance's (nitrate or nitrite) concentration and the second- or the third-order absorbances is obtained at these inflection points. Test measurements for mixed aqueous solutions of nitrate and nitrite with or without hydrogen peroxide confirm the reliability of this technique. We applied this technique to quantify the nitrate and nitrite generated in air plasma treated aqueous solutions. The results indicate that both nitrate and nitrite concentrations increase with the plasma treatment time, and the nitrite species is found to be generated prior to the nitrate species in the air plasma treated aqueous solution. Moreover, the production rate of total nitrogen species is independent of the solutions' pH value. These results are relevant to diverse applications of plasma activated solutions in materials processing, biotechnology, medicine and other fields.