The parameters REL (restricted energy loss) and Z 2/β 2 are used to represent the energy loss of highly energetic heavy ions in nuclear emulsion. The relations between these parameters and track widths are approximately independent of the charge of the moving nucleus. It is shown that it is possible to use the two parameters for the identification of particle masses of cosmic-ray nitrogen. The identification is based on photometric measurements of track widths of carbon, nitrogen and oxygen. The accuracy of the calibration is ±0.1 amu.