Working on only nitric oxide, how about nitrite?
Introducing Eicom NOx Analyzer, Model ENO-20
(NOx= nitrite and nitrate, nitrite= NO2-,Nitrate= NO3-)
More important role of nitrite
If detection of nitric oxide is important to your research, have you thought of detecting nitrite, which can bring you more insight? The detection of nitric oxide (NO) in biological liquid samples is extremely difficult due to the transient half-life of NO molecules. Some report the biological lifetime to be in the order of milliseconds. Once you remove the sample from the animal or other source, it is almost impossible to directly detect nitric oxide. Thus, detection of nitrite and nitrate, considered to be the major metabolites of NO, can be used to determine the NO levels. Moreover, if you evaluate the nitric oxide level with an in-vivo sensing procedure, it shows only one aspect of the nitric oxide profile. Monitoring nitrite level has become increasingly important in recent years. Studies have shown nitrite to be indirectly related to physiological activity and it is also a signaling molecule. The specific biological activity of nitric oxide is explained here.
Nitrite and nitrate (NOx) assay kits are not sensitive enough
The reported range of nitrite levels in the plasma is as low as 50 to 300 nM. On the market, fluorometric or colorimetric reagent kits to detect nitrite and indirectly nitrate are available following enzymatic reduction. The typical detection limit is 20 pmol which is equivalent to 1-10 µM with actual sample volumes. This lower limit of detection is not robust enough to detect nitrite in biological fluids. Moreover, those reagents are relatively specific to nitrite, but still there can be cross talks with other compounds. The ENO-20 easily provides quantitative data with enhanced specificity and sensitivity for nitrite and nitrate.
HPLC + diazo coupling, Principle of the ENO-20
The ENO-20’s high sensitivity and specificity are accomplished with the combination of a dizao coupling method and chromatography. Nitrite and nitrate are separated from other substances on a unique separation column and mobile phase. Nitrite then reacts with a compound called Griess reagent and generates diazo compounds which have a red color. The level of nitrite can be monitored with peak height or area with a retention time of 4.5 min from the injection of the sample. Nitrate is reduced to nitrite using a cadmium reduced copper column which reacts to the Griess reagent as well. The nitrate peak has an 8 min retention time. The level of diazo compound is measured by absorbance at 540 nm using a visible detector. The separation column is robust as well as the entire ENO-20. Normal lifetime of the column is at least 3 months with regular use. All separation and detection technologies are provided by Eicom for the ENO-20 including mobile phase ingredients, Greiss reagent, separation columns, and reduction columns. You can relax and detect nitrite and nitrate with 10 nM sensitivity (0.1 pmol). | 
