Careful analysis is made of the accuracies possible when gaseous ion mobilities are determined from data obtained by combining electrospray ionization (ESI), high-field asymmetric waveform ion mobility spectrometry (FAIMS)at atmospheric pressure and room temperature, and mass spectrometry (MS). New results are reported for the mobilities of positive and negative amino acid ions in air at elevated values of E/N, the ratio of the electric field strength to the gas number density.