Standardization of nitric oxide aqueous solutions by modified Saltzman method

Nitric oxide (NO) aqueous solutions were prepared by saturating pure NO gas and hydrolyzing 1 mM 1-hydroxy-2-oxo-3-(N-methyl-3-aminoethyl)-3-methyl-1-triazene (NOC-7), a NO donor, under anerobic conditions. The modified Saltzman method was employed for standardization of the NO aqueous solutions. NO and NO(2) in the solutions were driven with nitrogen gas stream into the first Saltzman solution to measure NO(2) and the leaked NO was driven with air stream through an oxidizing solution into the second Saltzman solution to measure NO, and NO(-)(2) and NO(-)(3) in the residual solutions were determined directly and after reduction with nitrate reductase, respectively. The concentrations of nitrogen oxide species in the NO solutions were about 1.8 mM NO/0.01 mM NO(2)/0.1 mM NO(-)(2)/0.1 mM NO(-)(3), and unchanged during keeping at 20 degrees C for 1 h under anerobic conditions but became 0.05 mM NO/0.01 mM NO(2)/1.7 mM NO(-)(2)/0.1 mM NO(-)(3) by keeping at 20 degrees C for 10 min under aerobic conditions. Instability of NO under aerobic conditions was supported by consumption of 1/4 equivalent amount of dissolved oxygen, and by loss of ability to convert 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO) to carboxy-PTI. Simultaneous quantification of nitrogen oxide species by the modified Saltzman method was found to be useful for practical standardization of NO aqueous solutions.