The influence of temperature and X-ray dose on the deprotonation of lyophilized phenylalanine during X-ray photoelectron spectroscopy

Lyophilized phenylalanine (LP) samples were prepared from aqueous solutions at pH ∼ 1.3 and subsequently analysed using X-ray photoelectron spectroscopy (XPS) in combination with cryogenics. When samples are measured at temperatures above ∼0 °C deprotonation occurs, which gradually proceeds with X-ray bombardment. In addition, deprotonation scales linearly with the difference between the Cl and the Na concentration, which strongly suggests that HCl sublimates from the sample.