The total charge response curve of an Am source placed in a liquid argon ionization cell has been measured. In a pure liquid argon sample (O concentration 0.1 ppm), it can be described by a semi-empirical formula which is derived from a model formerly elaborated by Thomas and Imel. The validity of this new formula was also tested in the case of oxygen-polluted liquid argon. The agreement with the data seems to indicate that the initial ion–electron pair recombination factor is not affected when the O concentration is smaller than 7 ppm. Further use of our recombination factor parameterization has been made to analyze data of liquid argon purity monitors utilizing Am sources with a sensitivity to O concentrations of 0.1 ppm. (Elsevier)