Determination of elemental compositions by high resolution mass spectrometry without mass calibrants

Perfluorokerosene can almost always be used as the mass calibrant for ions produced through electron impact ionization of compounds introduced into a mass spectrometer in the gas phase. Unfortunately, no completely universal calibrant is available for ions created by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) of analytes introduced into a mass spectrometer in the liquid phase. ESI and APCI generally provide less sensitivity than electron impact ionization of compounds introduced in the gas phase and a portion of the weaker total signal must arise from calibrant ions. Solvent conditions must be found that provide ions from both the calibrant and analytes, or an alternative flow to the ionization region must be provided for the calibrant solution. These problems were avoided by developing a methodology to determine elemental compositions of ions without using mass calibrants. The methodology utilizes the ability of double focusing mass spectrometers to accurately measure relative abundances of ions and exact mass differences between ions. This approach should simplify analyses of environmental samples that contain mixtures of compounds not amenable to gas chromatography or volatilization from a probe. From one to five steps were used. First, from mass peak profiles of the molecular ion or protonated molecular ion, [M]+, and the [M + 1]+ and [M + 2]+ ions, abundances for the [M + 1]+ and [M + 2]+ profiles relative to the [M]+ profile were determined. The [M + 1]+ and [M + 2]+ profiles resulted from the heavier isotopes of the elements in [M]+, and the profile abundances limited the number of possible elemental compositions for [M]+. Then, to determine if [M]+ contained N atoms, the [M + 1]+ profile was observed with sufficiently high mass resolution to at least partially resolve the profiles of ions containing a 15N or a 13C atom. Next, for a prominent fragment ion, [F]+, relative abundances of the [F + 1]+ and [F + 2]+ profiles were also determined, and the [F + 1]+ profile was inspected for a profile due to 15N atoms to provide a shorter list of possible compositions for [F]+. The lists for [M]+ and [F]+ were compared, and [M]+ compositions that could not produce any possible compositions of [F]+ were rejected, as were [F]+ compositions that could not arise from any possible composition of [M]+. Fourth, exact mass differences between ions were obtained from three mass peak profiles by referencing an unknown exact mass difference against a known exact mass difference. Exact mass differences between [M]+ and [F]+ ions provided compositions of neutral losses from [M]+. Only compositions of [M]+ that could lose the observed neutral loss to provide possible compositions of [F]+ remained viable. Finally, if multiple compositions of [M]+ were still possible, profiles were obtained for [M]+ and two fragment ions resulting from known neutral losses using theoretical exact masses based on each [M]+ composition as the center masses of the three profiles. When the calibration mass option was not used in the multiple ion detector (MID) descriptor, three centered profiles were obtained only for the correct composition. This methodology was demonstrated for seven compounds with molecular weights between 159 and 318 Da. For the lowest-mass compound, only the first step was required to obtain the correct composition of [M]+ while, for the other compounds, two or more steps were needed. Copyright © 1999 John Wiley & Sons, Ltd.