Determination of elemental compositions of fragment ions without a standard sample in a high resolution mass spectrum

Abstract

In recent years accurate mass measurements in mass spectra of organic compounds have been used as a reliable technique to determine their elemental compositions.Usually perfluorokerosine or heptacosa fluorotributylamine is employed as an internal standard.
In this paper a new method of measuring accurate masses without a standard sample is proposed. Thisis especially powerful for fragment ions, but in certain cases molecular ions and/or fragment ions of higher mass number may not be applicable to. The plinciple of the method is as follows. If an abundant peak is present at m/e R, peaks including heavy isotopes are also observed at m/e(R+1)and so on. Additionally these peaks are often constituted from not a nuclide combination but a few. So that mass differences among peaks in the same mass number are a criterion in selection of possible nuclide combinations. The relative abundance ratio among the main peak at m/e R and isotopic peaksat m/e(R+1)should also be examined. As an example this method is applied to determine nuclide combinations of three main fragment ions in the mass spectrum of ethyl p-arninobenzoate and the following results were obtained.
Two peaks at m/e39 have compositions C₃H₃ and C₃ ¹³CH₂, respectively. Abundunt C₅H₅ and weak C₄H₃N fragment ions correspond to two peaks at m/e65. Compositions of two peaks at m/e92 are shown to be C₆H₄O and C₆H₆N. However, for two fragment ions at m/e120 it was found that two combinations C₆H₂NO₂-C₇H₆NO and C₇H₆N0-C₈H₁₀N are both possible, and could not be differentiated because of theexperimental difficulties.