Accurate mass measurements of positive ions in the desorption chemical ionization mode

Abstract

AbstractDesorption chemical ionization mass spectrometry employing ammonia as the reagent gas has been extensively used to obtain molecular mass and structural information on a wide variety of compounds. Mass‐deficient reference standards normally used for calibration purposes in mass spectrometry do not provide adequate mass spectra under ammonia chemical ionization conditions. In order to overcome this problem a mixture of ammonia and methane as reagent gases was employed. In high‐resolution accurate mass measurement experiments, this gas mixture allows the simultaneous detection of mass spectra of perfluorokerosene adequate for calibration purposes and spectra containing molecular mass information of the analyte. A needle valve system was used to control the composition of the gas mixture introduced into the ion source. For positive‐ion accurate mass measurements of higher masses (up to m/z = 2300), Fomblin 18/8 oil was successfully used as a reference standard under ammonia, methane and isobutane desorption chemical ionization conditions.