Limit on the cosmological variation of mp/me from the inversion spectrum of ammonia

We obtain the limit on the space-time variation of the ratio of the proton mass to the electron mass, mu=m(p)/m(e), based on comparison of quasar absorption spectra of NH3 with CO, HCO+ and HCN rotational spectra. For the inversion transition in NH3 (lambda approximate to 1.25 cm(-1)) the relative frequency shift is significantly enhanced: delta omega/omega=-4.46 delta mu/mu. This enhancement allows one to increase sensitivity to the variation of mu using NH3 spectra for high redshift objects. We use published data on microwave spectra of the object B0218+357 to place the limit delta mu/mu=(0.6 +/- 1.9)x10(-6) at redshift z=0.6847; this limit is several times better than the limits obtained by different methods and may be significantly improved. Assuming linear time dependence we obtain mu/mu=(-1 +/- 3)x10(-16) yr(-1).