Product quantification requires a combination of analytical chemistry tools . Ionic species such as NO 2 -and remaining NO 3 -can be quantified by ion or liquid chromatography (IC and LC, respectively) although spectrophotometric methods also exist . NH3/NH4 + is typically quantified by spectrophotometric methods, with NMR required in cases employing isotope labeling . For some conditions and catalysts, quantification of these two products is sufficient to close the nitrogen balance and determine NO3RR FE. In alkaline conditions hydroxylamine may be formed, quantifiable by fluorescent and spectrophotometric methods and liquid chromatography-mass spectrometry (LC-MS). Gaseous products including a range of NOx products and N2 are also possible, quantifiable by MS (although distinct speciation can be challenging). Gaseous NH3