ACCURATE MASS MEASUREMENTS OF POSITIVE-IONS IN THE DESORPTION CHEMICAL-IONIZATION MODE

Desorption 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 sp ectrometry 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 si multaneous detection of mass spectra of perfluorokerosene adequate for calibration purposes and spectra containing molecular mass informatio n of the analyte. A needle valve system was used to control the compos ition of the gas mixture introduced into the ion source. For positive- ion accurate mass measurements of higher masses (up to m/z = 2300), Fo mblin 18/8 oil was successfully used as a reference standard under amm onia, methane and isobutane desorption chemical ionization conditions.