Methods and compositions are disclosed for quantifying and/or identifying a nitroderivative compound, or a residue thereof, and are applicable for assessing in vivo nitrative stress and for aiding in diagnosis of mammalian pathologies such as asthma, atherosclerosis, Alzheimer's disease, inflammation, ischemia, Parkinson's disease, and cancer. A preferred method comprises a) providing a sample containing the nitroderivative compound, or residue thereof, in the form of a diazo compound or residue thereof; b) azo-coupling the diazo compound, or residue thereof, to a target compound capable of producing a signal, to yield an azo-coupled compound; c) isolating the nitroderivative compound, or residue thereof, or the azo-coupled compound; d) quantitating and/or detecting the signal, to yield a quantitation and/or detection result; and e) determining from the quantitation and/or detection result the identity and/or concentration of the nitroderivative compound, or residue thereof, in the sample.