Each volatile substance has its own frost line (e.g. carbon monoxide,[1]nitrogen,[2] and argon[3]), so it is important to always specify which material's frost line is referred. A tracer gas may be used for materials that are otherwise difficult to detect; for example diazenylium for carbon monoxide.
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^Dartois, E.; Engrand, C.; Brunetto, R.; Duprat, J.; Pino, T.; Quirico, E.; Remusat, L.; Bardin, N.; Briani, G.; Mostefaoui, S.; Morinaud, G.; Crane, B.; Szwec, N.; Delauche, L.; Jamme, F.; Sandt, Ch.; Dumas, P. (2013). "UltraCarbonaceous Antarctic micrometeorites, probing the Solar System beyond the nitrogen snow-line by E. Dartois, et al". Icarus. 224 (1): 243–252. Bibcode:2013Icar..224..243D. doi:10.1016/j.icarus.2013.03.002.