Accurate mass measurements for interpreting spectra of atmospheric pressure ionization

A multireflecting time-of-flight mass spectrometer with mass resolving power over 20 000 was used for analysis of ions formed in atmospheric pressure ionization of laboratory air. Ion masses were measured with an accuracy of about 1-2 ppm and within a wide dynamic range above 10^6 relative to the entire ionic content and above 10^5 relative to major peaks in the spectra. The elemental composition was found for most of recorded ion peaks. Apparently all of them are organic ions representing various classes of polar compounds. There were also found radical ions and purely hydrocarbon ions which presumably were formed due to fragmentation in the ion transport interface. Results are conveniently displayed on the plot in the axes "mass-mass defect", where regular structures are observed due to elemental composition.