Saturated Vapor Pressure and Sublimation Enthalpy of l‑serine and l‑cysteine

The crystal–vapor equilibrium for the standard
amino acids l-serine and l-cysteine was studied
by Knudsen effusion
mass spectrometry in the temperature ranges of 387–442 and
361–429 K, respectively. At these relatively low temperatures,
the surface degradation of the samples was shown to be minor. The
vapor pressure obtained by the Knudsen effusion method was used to
calibrate the mass spectrometer. The temperature dependence of the
vapor pressure in the investigated temperature range is described
by a linear equation ln(p/Pa) = −(18.227 ±
289)/T + (39.828 ± 0.593) for l-serine
and ln(p/Pa) = −(16.658 ± 270)/T + (38.295 ± 0.806) for l-cysteine. The set
of thermodynamic functions for l-serine and l-cysteine
in the ideal gas state was developed. The sublimation enthalpy was
obtained by the methods of the second and third laws of thermodynamics.
The values ΔsubH°(298.15 K)
recommended from the analysis of all available data are 151.4 ±
2.3 and 142.5 ± 1.9 kJ mol–1 for l-serine and l-cysteine, respectively.