An adaptation of the classical "standard addition" technique has been implemented to quantitatively measure the magnitude of an air leak into the gas lines of any plasma reactor. This measurement can be run on any original equipment manufacturer plasma reactor equipped with some means for the spectral analysis of the plasma. The only additional requirement is a mass flow controller used to make the incremental air (or N-2) additions to generate an in situ calibration curve. The technique can be readily automated to provide a diagnostic of hardware integrity.