Fluorine mass defect

Accurate mass scales (or calibration curves) are generally established by measuring the mass spectrum of a reference compound simultaneously with the spectrum of the sample. The precise mass of every ion in the spectrum of the reference compound is known, so a precise mass correlation is thereby provided. Common reference materials are perfluorokerosene (PFK) and perfluorotributylamine (PFTBA), the mass spectra of which are shown in Figures 3.1 and 3.2, respectively. Since all the ions formed from these compounds contain several fluorine atoms (18.9984) and no hydrogen atoms (1.0078), they have negative mass defects and are well separated from organic ions that normally have positive mass defects. Of course, other chemicals may be used to provide reference masses, as long as the exact masses in its spectrum are known. 

Fluorine has an atomic number of 9 and a relative atomic weight of 18.9984 u. This negative mass defect leads to substantially lower monoisotopic masses of highly fluorinated compounds than the respective nominal mass. For instance, the miz ratio of the perfluorooctanoate anion is 412.9664. Other organic compounds usually have monoisotopic masses higher than the respective nominal mass, since most other elements have a positive mass defect. This difference can be taken advantage of by high-resolution MS. 

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